High-throughput microfluidic production of carbon capture microcapsules: fundamentals, applications, and perspectives

In the last three decades, carbon dioxide (CO2) emissions have shown a significant increase from various sources. To address this pressing issue, the importance of reducing CO2 emissions has grown, leading to increased attention toward carbon capture, utilization, and storage strategies. Among these strategies, monodisperse microcapsules, produced by using droplet microfluidics, have emerged as promising tools for carbon capture, offering a potential solution to mitigate CO2 emissions. However, the limited yield of microcapsules due to the inherent low flow rate in droplet microfluidics remains a challenge. In this comprehensive review, the high-throughput production of carbon capture microcapsules using droplet microfluidics is focused on. Specifically, the detailed insights into microfluidic chip fabrication technologies, the microfluidic generation of emulsion droplets, along with the associated hydrodynamic considerations, and the generation of carbon capture microcapsules through droplet microfluidics are provided. This review highlights the substantial potential of droplet microfluidics as a promising technique for large-scale carbon capture microcapsule production, which could play a significant role in achieving carbon neutralization and emission reduction goals.

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Introduction
Carbon dioxide (CO 2 ) is a prominent greenhouse gas contributing significantly to global climate change [1].CO 2 emissions come from various substantial sources, including fossil fuel combustion [2][3][4], industrial activities [5] (e.g.steel and mining industries), land-use and agricultural practices [6], transportation [7], and building and construction [8].Over the past three decades, the mean growth rate of CO 2 has been gradually increasing, underscoring the significance of carbon capture, utilization, and storage (CCUS) as a pivotal approach to mitigate CO 2 emissions [9][10][11].Among the CCUS components, carbon capture assumes primary importance as it accounts for over 70% of the overall CCUS cost [12].Thus, it has become a focal point of extensive research efforts to enhance efficiency and reduce the cost of carbon capture.
Microcapsules have emerged as an important tool for carbon capture.Especially, in recent years, droplet microfluidics method that can precisely generate and manipulate droplets inside microchannels provides a promising approach to create the high-quality carbon capture microcapsules, as shown in figure 1.It can be seen that droplets generated from microfluidic devices are monodispersed, controllable and concentricity, which provide the necessary and versatile templates for the generation of carbon capture microcapsules.These microcapsules can be further fabricated by using interfacial polycondensation on the droplets or directly photocuring the droplets.Subsequently, the microcapsules which have absorbed CO 2 can be heated in the regeneration process to emit high-purity CO 2 that can be compressed for storage or utilization.It is worth noting that the generation performance of droplets, including size, topological structure, throughput and monodispersity, which is affected by the involving microfluidic device configuration and hydrodynamics, directly determines the characteristics of the produced microcapsules.For examples, with the same core diameter, CO 2 capture capacity increases with decreasing shell thickness [13].Additionally, the mass transfer of CO 2 and solution was also affected by the microcapsule size, and the thermal stability and CO 2 absorption capacity can be adjusted by controlling the core-shell volume ratio of the microcapsules [14,15].Over the past two decades, various approaches have been proposed to enhance the production rate of emulsion droplets, primarily focusing on including parallelized microfluidic droplet generators [16][17][18] and splitting large droplets into smaller batches [19,20].However, it is important to note that these methods necessitate precise fabrication of microfluidic devices and delicate wettability modifications of microchannels, which are currently confined to the laboratory development stage [21].Therefore, it becomes imperative to comprehensively review and summarize the available high-throughput microfluidic approaches, along with the fundamental microfluidic fabrication methods.
This review focuses on the high-throughput production of microcapsules for carbon capture using droplet microfluidics (figure 2).It begins with an overview of different microfluidic chip fabrication technologies, such as wet etching, soft lithography, hot embossing, capillary assembly, and three-dimensional (3D) printing.It then discusses the microfluidic generation of emulsion droplets and the relative hydrodynamic considerations that serve as the basis for achieving microfluidic high-throughput production.Furthermore, a comprehensive assessment of the generation of microcapsules for carbon capture via droplet microfluidics is provided, followed by a forward-looking perspective on the potential for highthroughput.By consolidating and evaluating these key aspects, this review aims to provide an informed and practical guideline for the selection of appropriate high-throughput microfluidic approaches, thereby promoting progress toward the large-scale utilization of microcapsules for carbon capture.

Microencapsulated solvents for carbon capture
Microcapsules designed for carbon capture commonly encapsulate a diverse range of solvents, including ionic liquids (ILs), metal-organic frameworks (MOFs), and chemical absorption solvents (figure 3).In the following section, an overview of the distinctive characteristics of these solvents is presented.

ILs
ILs have garnered significant attention as promising candidates for carbon capture, owing to their remarkable advantages such as negligible vapor pressure, low energy loss, and excellent thermal stability.As ionic compounds that maintain a liquid state at room temperature, ILs offer a unique capture mechanism reliant on weak interactions with CO 2 , primarily through physisorption [34,35].Notably, the recycling process of ILs entails low energy consumption, as their low thermal absorption requires only 11 kJ•mol −1 [36].Initially, ILs are applied in post-combustion capture technology.However, it should be noted that certain ILs can react with CO 2 through chemisorption, affecting CO 2 capture.Additionally, the viscosity of ILs reduces solvent losses but limits mass transfer, resulting in low absorption rates.To address these challenges, researchers have successfully synthesized task-specific ILs by incorporating suitable moieties into conventional ILs, thereby enhancing their absorption capacity.While detailed synthesis methods of ILs have been previously summarized in reviews [37][38][39], the focus of this review article will lie in exploring the diverse applications of ILs, rather than delving extensively into the synthesis procedures.
In recent studies, the utilization of ionic liquid in carbon capture has been explored with promising results.Liu et al [40] investigated the CO 2 conversion efficiency of nanosheet loading with ionic liquid and Co single atom, as shown in figure 4(a-i).It demonstrated that the presence of ionic liquid facilitates electron extraction, offering a feasible strategy for CO 2 conversion.Similarly, Vishwakarma et al [41] reported on an interfacial catalytic reaction platform for carbon capture (figure 4(a-ii)), where immobilized ionic liquid catalysts were used in a continuous-flow micro-reactor.This platform Figure 1.Schematic illustration of the application of carbon capture with microcapsules generated from microfluidic droplets.Reprinted with permission from [22].Copyright (2011) American Chemical Society.Reproduced from [23] with permission from the Royal Society of Chemistry.Reprinted with permission from [24].Copyright (2011) American Chemical Society.[13] John Wiley & Sons.© 2022 Wiley Periodicals LLC.Reproduced from [25], with permission from Springer Nature.Reprinted with permission from [26].Copyright (2011) American Chemical Society.effectively facilitated CO 2 utilization and enhanced mass transfer.For now, most of the ILs are still studied in the laboratory because of the difficulty in the synthesis of highthroughput of ILs and the high cost [37].During the industrial process, Chevron Company is a famous institution to use commercial ionic liquid for the capture of CO 2 [42].The commercial IL used in the industrial process by the Chevron Company is [bmim] [acetate] with 14 wt.% water, which is able to reduce CO 2 greatly in a complex gaseous stream.

MOFs
MOFs represent a class of crystalline porous materials constructed from multi-metallic units called secondary building units (SBUs) and organic linkers [48].The final topology structure of the material framework is predominantly determined by the SBUs, making them a critical factor in MOF design.The structure of SBUs depends on several key factors, including the types of metal ions, the structure of organic ligands, the ratio of metal ions to ligands, and the types of solvents [49].The exceptional porosity and large surface area of MOFs contribute to their high CO 2 adsorption capacity, surpassing that of disordered carbon materials which is less than 2000 m 2 g −1 .Some MOFs exhibit surface areas reaching 5000 m 2 g −1 or even higher [50].Moreover, the unsaturated metal ligands of MOFs usually have a pH above 7, further enhancing their CO 2 adsorption capacity.For example, MOF-74 demonstrates a remarkable CO 2 adsorption capacity of up to 6.25 mmol•g −1 at 25 • C and 1 bar [51].To improve their surface area, porosity, and thermal stability, the regulation of orifice shape, orifice size, and the choice of materials can be effectively employed [52].Despite these advantageous properties, the practical industrial application of MOFs is impeded by their poor stability and tendency to form powders. Therefore, a viable strategy to overcome these challenges involves combining MOFs with other materials such as metallic nanoparticles, graphene, and metal oxides, which can enhance their stability and facilitate industrial application.
Nandi et al [43] reported an ultra-microporous MOF, with a structure shown in figure 4(b-i).This MOF exhibited remarkable CO 2 /H 2 selectivity and demonstrated CO 2 is self-diffusivity.The unique ultra-microporous architecture ensured exceptional stability, even in the presence of humid gas streams.In another study, Kumar et al [44] investigated the CO 2 adsorption capabilities of five different materials (figure 4(b-ii)).The evaluation involved various techniques, such as temperature-programmed desorption, thermogravimetric analysis and mass spectrometry (MS).Among the tested materials, MOFs, HKUST-1 and Mg-MOF-74, exhibited relatively high adsorption capacities.However, these MOFs showed sensitivity to humidity, presenting a challenge for their future application in carbon capture.Furthermore, Liang et al [45] presented a Cu (II) MOF characterized by a high density of active sites (figure 4(b-iii)), which demonstrated impressive CO 2 adsorption performance, with a high volumetric uptake of 171 cm 3 •cm −3 under ambient conditions.An overview of the themes discussed in this review, from microfluidic chip fabrication technologies to applications of droplet microfluidics for carbon capture.Reproduced from [27] with permission from the Royal Society of Chemistry.Reproduced from [28], with permission from Springer Nature.Reproduced from [29] with permission from the Royal Society of Chemistry.Reproduced from [30], with permission from Springer Nature.Reprinted from [31], © 2020 Elsevier Ltd All rights reserved.Reproduced from [32] CC BY 4.0.Reproduced from [33] with permission from the Royal Society of Chemistry.Reprinted from [14] Zeolite 13X is one of the most common commercial MOFs in the industrial carbon capture application.Especially, the isostatic heat of adsorption for CO 2 by the Zeolite 13X is 37.2 kJ mol −1 , which is much higher than that of nitrogen and methane [53].

Chemical absorption solvents
The chemical reaction with an alkaline solution has proven to be an effective method for CO 2 absorption, owing to the acidic nature of CO 2 .This reaction leads to the formation of a weakly bonded intermediate compound, which can subsequently be regenerated [54].The ideal chemical solvent for CO 2 absorption exhibits several key characteristics, such as high reactivity with CO 2 , a large absorption capacity, high thermal stability, low regeneration cost, minimal environmental impact, and cost-effectiveness.Currently, commercial chemical absorption solvents used for the carbon capture are applied into the post-combustion and pre-combustion capture technologies [55].These solvents are most used in industry for carbon capture.Monoethanolamine (MEA), one most used commercial chemical absorption solvent, has a fast reaction rate with CO 2 and its removal ratio of CO 2 is able to keep high even at the low CO 2 concentration.However, the corrosivity and regeneration energy requirement of MEA are high, which is unfavorable for the industrial application [56].Another chemical solvent, Selexol, is able to capture CO 2 in a combined cycle power plant [57].In addition, piperazine (PZ)-promoted potassium carbonate (K 2 CO 3 ), and concentrated aqueous PZ are also belonged to the commercial solvents for the industrial carbon capture applications [58].
Amine-based solvents are classified into primary, secondary, or tertiary, depending on the degree of nitrogen atom substitution [59].For example, MEA represents a typical primary amine, characterized by one alkanol chain and two hydrogen atoms bonded to a nitrogen atom.Primary and secondary alkanol amines exhibit rapid reactivity with CO 2 , forming carbamates, while tertiary alkanol amines require the facilitation of the CO 2 hydrolysis reaction to yield bicarbonates [59].The heat of the reaction involved with bicarbonate formation is lower than that of carbamate formation, making tertiary amines often blended with primary or secondary amines to reduce solvent regeneration costs.Despite MEA being a relatively economical and well-established industrial technology, it faces certain challenges, including solvent degradation, high energy consumption during regeneration, and equipment corrosion [60].Therefore, exploring alternative solvents with higher CO 2 capacity and lower energy consumption is of paramount importance.
Three classes of amine-modified sorbents based on their respective preparation methods are summarized (figure 4(c-i)) [61].Class 1 sorbents are prepared through physical impregnation, while Class 2 sorbents are produced via covalent tethering using silane linkage.Generally, Class 1 sorbents exhibit higher CO 2 capacity compared to Class 2 sorbents, owing to their higher amine content.Class 3 sorbents, on the other hand, are prepared through direct covalent tethering via in-situ polymerization.In an insightful study, Gebald et al [62] introduced a novel amine-based nano-fibrillated cellulose and investigated its CO 2 absorption/desorption process.The material demonstrated exceptional stability over 100 cycles, with a reduction in CO 2 absorption capacity of less than 5%.Sculley and Zhou [46] added zirconium content to the complex amine-supported porous silica systems for carbon capture (figure 4(c-ii)), which improved desorption kinetics effectively.Furthermore, as shown in figure 4(c-iii), amine solvents were combined with MOFs to realize the chemisorption of CO 2 [47].
In summary, the unique combination of low vapor pressure and high thermal stability makes ILs highly advantageous for applications in carbon capture.MOFs exhibit high surface area and CO 2 adsorption capacities, rendering them favorable candidates for carbon capture.Chemical absorption solvents, on the other hand, are widely used in the industry due to their cost-effectiveness.As a result, these three types of solvents are commonly utilized in carbon capture processes.However, besides the advantages, these solvents also have some certain limitations, including restricted mass transfer and absorption rates, which are summarized in table 1.It is benefit to enhance mass transfer and improve absorption rates with the encapsulation of these solvents, so that droplet microfluidics is an effective tool for carbon capture.

Microfluidic chip fabrication technologies
The fabrication of microfluidic devices plays a fundamental role in droplet microfluidics.A high degree of integration, specific geometric structures, and physicochemical features, such as surface properties and basic materials, are all critical considerations in the design of microfluidic devices.When designing microfluidic devices for droplet formation, it is important to take into account the various characteristics of available fabrication methods, such as transparency, channel wettability, pressure resistance, cost, temperature resistance, and more.The important fabrication methods, including wet etching, soft lithography, hot embossing, capillary assembly and 3D printing are introduced briefly.
In wet etching method, micromachined microfluidic devices based on glass/silicon substrates are extensively utilized.These substrates possess several advantages, including high thermal stability and effective solvent resistance, making them suitable for use in high-temperature and organic environments [63][64][65].Additionally, the transparency of glass facilitates the observation of droplet formation in the microfluidic devices.However, the properties of glass and silicon also present challenges during the preparation and application process, such as brittleness and gas impermeability.The typical procedure of micromachining of glass/silicon is illustrated in figure 5(a).The resulting microchannels are quasi-twodimensional, making them easy to fabricate in parallelized microfluidic droplet generators.However, large-scale production of glass and silicon microfluidic devices also requires overcoming several impediments such as high costs and the need for clean-room equipment.
Rapid prototyping is another widely used fabrication method for the microfluidic device, with PDMS-based soft lithography being the most commonly used approach.PDMSbased soft lithography is notable for its speed and costeffectiveness, and the substrate can be reusable for mass production and industrial applications [71].The typical procedure of PDMS-based soft lithography involves several steps as shown in figure 5(b).The transparency of PDMS microfluidic devices makes them favorable for experimental visual observation as well.The entire fabrication process is carried out under mild conditions because PDMS can be cured at low temperatures and does not release hazardous substances, making it a convenient and eco-friendly option.Nevertheless, PDMS-based soft lithography also has its limitations.For examples, the wettability of PDMS needs to be modified for successful droplet formation, and the modified wettability is not always long-lasting.Additionally, the bonding part cannot withstand the high pressure in the microchannel of the device, so there is a risk of fluid leakage when using a working fluid with high viscosity.Moreover, there are limitations in the applications of the PDMS microfluidic devices for organic materials owing to the swelling of PDMS and the deformation of the microchannels [72][73][74][75][76]. Compared to the micromachining of glass/silicon, PDMS soft lithography is more widely used for water-in-oil (W/O) droplet formation because of its inherent hydrophobicity [77].It is worth noting that PDMSbased soft lithography is convenient for the mass production of droplet generation since microfluidic devices are highly replicable.
The hot embossing technology is often used to fabricate microfluidic devices based on polymethyl methacrylate (PMMA) for visualized droplet generation.PMMA is a widely used amorphous thermoplastic [78].The hot embossing process involves heating the silicon mold and PMMA sheet, pressing the mold into the PMMA sheet to transfer the patterns, cooling the pattern-carrying PMMA, and finally bonding a transparent cover onto the PMMA substrate for visualization (figure 5(c)) [79,80].Similar to PDMS, PMMA is naturally hydrophobic, making it suitable for producing W/O emulsions without the need for surface modification.There is a risk that PMMA can collapse during hot embossing, which can affect the reliability of droplet generation.In particular, if the reservoir in a step emulsification device collapses, the confinement gradient during droplet generation will be destroyed, leading to emulsification failure.Therefore, although PMMAbased hot embossing lithography is an available alternative to PDMS-based soft lithography, it is currently less popular in the laboratory.
Microfluidic devices fabricated by above methods are typically quasi-two-dimensional planar structures.Consequently, the droplets generated in these microchannels can easily contact with the channel walls, and in the worst case, collapse and stagnate due to their better wettability than the continuous phase.To avoid such problems, wettability modification of the microchannel is necessary, particularly during microfluidic droplet generation.However, selectively modifying specific parts of microchannels in these microfluidic devices remains a challenge.The 3D microchannels fabricated through the modular assembly method allow for the localized wettability treatments to be performed separately on different modules, making it easier to fabricate devices with multiple levels of droplet generators.Currently, the most widely used modular assembly method for manufacturing microfluidic devices is the glass capillary-based method, owing to its low cost, easy fabrication, and simple operation.A typical counter-flow flow-focusing microfluidic device is fabricated by nesting cylindrical glass capillary tubes within a square glass tube with matched size, which achieves good coaxial alignment for the preparation of microscale emulsion droplets and other derived functional materials (figure 5(d)) [81][82][83].These devices can typically withstand higher pressure.However, it is worth noting that the structure of the capillary tip is still usually handmade via hot drawing, which limits the reproducibility and scalability of the microfluidic devices.
The use of 3D printing has gained popularity in recent years as a technique for directly fabricating 3D structures in a single step [84][85][86].The 3D geometric information is stored in a graphic program file and automatically processed by a computer-controlled printer system, enabling accurate and high-resolution fabrication with low time consumption and cost.This technique is also environmentally friendly due to the minimal production of redundant parts during the fabrication process.Additionally, with the integration of industrialgrade user interfaces and embedded control systems, the commercialization of 3D printing technology has promising development prospects.Among various available types of 3D printing techniques, stereolithography (STL) is particularly favorable for microfluidic device fabrication due to its ability to generate small and complex channel structures that are difficult to realize using other methods [87,88].STL, an additive manufacturing process, involves slicing the 3D model of STL format and printing the photosensitive resin layer by layer using a laser beam [89][90][91].As shown in figure 5(e), a lifter moves the printing platform up and down, ensuring sufficient curing time for the photosensitive resin.It is worth noting that the cost of 3D printing increases significantly with the miniaturization and complexity of microchannels structures.Additionally, the availability of suitable photosensitive resin for 3D printing is still limited, indicating that 3D printing technology is in its infancy yet [70,92].
Herein, table 2 lists the summary and comparison of various fabrication methods for the microfluidic devices used in droplet generation, aiming to provide suggestions for appropriately performing the microfluidic device fabrication under different conditions.In general, wet etching and soft lithography have proven to be suitable for manufacturing the microfluidic devices for high-throughput droplet generation.Thus, the following sections are mainly focused on these two methods.Ultimately, the selection of a suitable fabrication method for microfluidic droplets generation should consider various factors according to the specific experimental requirements and application needs, such as cost, manufacturing difficulty, reliability, applicability, and droplet formation performance.

Single-phase droplets
Microfluidic droplet generation is a process that depends on precise flow control of the different phases, which is closely related to the microfluidic geometries used.Based on the flow characteristics of different phases, microfluidic geometries are typically classified into five geometries: co-flow, flow-focusing, T-junction, cross-flow, and step emulsification (figure 6(a)).The first four geometries rely on the viscous shear force to generate droplets, and are thus known as shearbased geometries.In contrast, step emulsification achieves droplet generation through a confinement change in the channel structure, offering a low-energy consumption approach for droplet formation.Droplets with characteristics of monodisperse, controllable and concentricity are necessary versatile templates for production of microcapsules.In order to generate the droplet templates with these characteristics, droplet microfluidics is an effective technology.Hence, it is necessary to investigate and understand the hydrodynamics of the microfluidic droplet generation process, so as to manipulate the generation of droplets.Note that, as for the understanding of the hydrodynamics underlying the microfluidic droplet generation, the hydrodynamics of single-phase droplet formation process is the base for comprehension of the double emulsion droplet formation process.Therefore, even though the majority of the microcapsules are generated from the curing of double emulsion droplets, it is meaningful to introduce the hydrodynamics of single-phase droplet formation as follows.

Conventional microfluidic generation of single-phase
droplets.In co-flow streams, the dispersed phase and continuous phase flow in the same direction through the inner and outer channels, respectively.The dispersed phase is pinched off by the continuous phase due to the viscous forces, resulting in droplet formation.The minimum droplet size formed in the co-flow stream can reach several hundred nanometers, while the most common size of droplets in references is between 200 µm-1000 µm [93][94][95][96].The generation frequency of droplets in co-flow configurations typically spans from several hundred Hz to tens of kHz.In the dripping regime, the coefficient of variation (C.V.) of droplets is typically below 3%, indicating a state of monodispersity.During the dripping regime, the viscous force imposed by the continuous flow exceeds the interfacial tension, leading to the detachment of the dispersed phase from the tip and the formation of droplets.As the flow rate of the continuous phase increases, the Capillary number of continuous phase (Ca o ) increases, resulting in a reduction in droplet size.When Ca + We ≈ 1, a transition from dripping to jetting occurs, revealing different  jetting regimes determined by the distinct dominant forces [97].At high flow rates of the continuous phase, the dominant force is the viscous force from the continuous phase, leading to the formation of a narrow jetting regime.Conversely, at high flow rates of the dispersed phase, dispersed phase inertia becomes the dominant force, resulting in a wide jetting regime.
Further research indicates that jetting occurs when We i > 1 and Re i > 1, or when the Ca i > 1 in the opposite case [98].
Similarly, in flow-focusing geometry, two immiscible fluids flow coaxially with a small orifice configured.Under the effect of hydrodynamic focusing, the dispersed phase is forced through the orifice to facilitate droplet formation.The formation of droplets is influenced by various factors, including fluid flow rates, geometrical parameters, and the fluids' interaction [99][100][101].In most 3D flow-focusing devices fabricated by the capillary assembly method, the fluids are induced in a counter-flow condition [24,[102][103][104].The fluids can also be induced concurrently, for example, Takeuchi et al [105] fabricated the symmetric flow-focusing device by the cast molding method.In quasi-2D flow-focusing devices, the fluids flow in the same direction [106][107][108][109].The minimum droplet size in a flow-focusing device can reach hundreds of nanometers, similar to those in co-flow devices [106,110].However, the common size of droplets formed in references is below 100 µm, which is smaller than that in co-flow devices with similar geometric characteristics.This is due to the strong confinement of the flow-focusing orifice, which results in most droplets being of the same magnitude as the orifice radius or smaller [106].The frequency of droplets generated in flowfocusing devices is similar to that in co-flow devices.However, the interface is unstable due to the continuous phase flowing through the focusing orifice, resulting in abrupt velocity variations.Consequently, the C.V. of droplets produced by flowfocusing devices is slightly higher than that produced by coflowing devices under the same flow conditions.The typical C.V. of droplets produced by flow-focusing is still below 5% satisfying the monodispersity requirement of most applications.In a 2D microfluidic flow-focusing device, the dispersed phase forms a spherical head due to the influence of interfacial tension.However, when the droplet size exceeds the channel depth, the spherical droplet becomes confined and takes on a disc-shaped form.As the droplet continues growing, a balance is reached between the detaching forces and the holding force acting on the droplet.Under the circumstance that detaching force surpasses the holding force, the interface of droplet collapses to form a droplet.
In the T-junction flow category, two fluids meet at a junction with a vertical angle.The buildup of pressure upstream leads to a breakup once the dispersed phase obstructs the main channel through which the continuous phase flows [111].If the stream of the continuous phase bypasses the dispersed phase, the system is shear-dominated, where the dispersed phase is sheared by the continuous phase [112].The neck then breaks into droplets, and the fluid tip retracts to the inlet due to the interfacial tension force [111].K-junction flow [113] or V-junction flow [114] are variants of the T-junction device that offer flexible flow arrangement.The common droplet size in T-junction devices ranges from 40 µm and 500 µm.In the dripping regime of T-junction cross-flow, droplet size is mainly determined by the viscous shear force, which is proportional to Ca o .
In the cross-junction flow, the dispersed phase is flanked by the continuous phase from two directions to form droplets.The cross-junction flow is regarded as the flow-focusing flow in some research, while the cross-section decreases in flowfocusing flow, which is unchangeable in the cross-junction flow [115][116][117].In cross-junction flow, the fluid tip partially plugs the exit due to the existence of a thin film of the continuous phase [118,119].The process of droplet formation is affected by a mechanism in which the competition between the shear stress and interfacial tension forces plays a dominant role.This mechanism is similar to that in the T-junction device, so this structure is difficult to classify as a flow-focusing device.The droplet size is typically over 10 µm and is mainly influenced by the channel structure, while the C.V. is below 2%.The angle between the continuous phase and the dispersed phase is variable in cross-junction flow [120][121][122].
Monodispersity is generally guaranteed by adjusting flow rates in these shear-based geometries in the dripping regime.Compared with shear-based geometries, the step emulsification device with optimized nozzles weakens the effect of fluid flow rates.As mentioned above, the formation of droplets in step emulsification is a low-energy input way, and the droplet size is insensitive to flow rates [123,124].Hence, the step emulsification device is widely used in the high-throughput production of droplets [125].
In a step emulsification process employing a wedge-shaped channel to connect a reservoir, the dispersed phase forms a tongue-shaped interface within the wedge-shaped channel [126].As the interface enters the reservoir, the tip of the bulb expands and connects with the neck through the liquid thread in the nozzle.With the continuous phase flowing back to the nozzle, the width of the neck gradually decreases until the dispersed phase breaks into droplets.The bulb's expansion is driven by the Laplace pressure difference between the high-curvature interface in the narrow channel and the lowcurvature interface in the reservoir, and the filling process is controlled by the balance between viscous forces and surface tension.The pressure difference is related to the curvature difference of the liquid-liquid interface.As the bulb fills, its curvature decreases, leading to a reduction in the internal pressure.Consequently, the pressure drop increases, resulting in an increasing flow from the neck into the bulb.When Q o exceeds the flow flowing into the nozzle Q i , the neck width shrinks to the critical value.The neck becomes unstable and collapses quickly as soon as it detaches from the walls of the nozzle.The size of the droplet is determined by the initial wetting regime, and the contact angle of the fluid plays a crucial role.

High-throughput microfluidic generation of singlephase droplets.
The production rate for a single droplet maker typically ranges from tens to hundreds of microliters per minute, rendering it unsuitable for industrial applications [127].To address this limitation, the scale-up strategies for microfluidic process, including the parallelization of microfluidic droplet generation units [128] and multiple splitting of emulsion droplets [129], have been proposed to enhance productivity.
Individual and connected microfluidic droplet generation units have been integrated into one device (figure 6(b-i)).This allows for independent regulation and minor disturbance between units, resulting in products with different compositions [130].However, the cost and complexity of this approach of integrating numerous stand-alone pumps limit the practicality of this approach.An alternative strategy involves distributing the dispersed phase in a multi-channel structure (figure 6(b-ii)).This structure is more compact and beneficial for minimizing the device.Due to the interconnection between channels, the even distribution of dispersed phases is vital in this configuration.One blocked channel will result in inhomogeneous fluid distribution, which may lead to polydisperse droplets and is unfavorable for the reliability of high-throughput droplet generation.In contrast to the parallelization strategy, multiple splitting of droplets involves generating emulsion upstream, and then stepwise splitting them through hierarchical junctions (figure 6(b-iii)) [131].This passive breakup can be repeated without a significant increase in C.V. until droplet radii decrease to approximately the same order as the channel width, resulting in higher yield and reliability [129].
Fluid distribution is critical for realizing mass production at an industrial scale, as uneven flow rates can lead to variations in residence time, mass, and heat transfer [132].The multi-channel network is specifically designed to ensure even flow rates in each unit, which is particularly important for maintaining droplet monodispersity in shear-based geometries (figure 6(b)).Two common microfluidic channel layouts are the tree-type and ladder-type networks (figure 6(c)).In the tree-type network, the number of branch channels increases by a factor of two between consecutive levels, resulting in a total number of branches equal to 2 n , where n is the level of the tree [133].Conversely, the ladder-type network consists of several parallel branches connected to a main channel, and the number of branches is arbitrary.While the design of the ladder-type network is more compact and less affected by manufacturing deviations, the tree-type network is more energy efficient.Therefore, it is important to carefully consider the choice of the network when designing microfluidic channels.
Flow-focusing generators and step-emulsification units are suitable parallelization units for droplet generation at an industrial scale.To minimize the C.V. of droplet size in parallelized flow-focusing devices, Hashimoto et al [134] investigated the intersection mechanism among four coupled flow-focusing generators, uncovering the compressibility of the dispersed phase and variations at the flow-focusing orifices under pressure.In a similar study, Li et al [135] explored the geometric coupling between integrated four parallel generators and the influence of variations in the dimensions of microchannels (figure 7(a)).Meanwhile, Kang et al [28] utilized a tree-type network with three layers of fluid distributors in the preparation of nanoparticles to prevent aggregation and clogging, which could lead to the failure of the device.Nisisako and Torii [18] presented a circular arrangement module consisting of 128 junctions, which integrated more droplet generator units on one chip, resulting in reduced consumption of the continuous phase (figure 7(b)).The devices successfully produced monodisperse droplets (C.V. < 4%) and were also applied to generate biphasic Janus droplets for scale-up.
Parallelized multiple modular microfluidic reactors have been successfully employed for the continuous generation of polymer particles.However, the interaction between parallel channels can affect the flow conditions in each unit when multiple droplet generators share one inlet.To address this issue, an additional 40 mm-long wavy channel was introduced, resulting in lower C.V. of droplets generated compared to a microreactor with a 6 mm-long straight channel at low flow rates (figure 7(c)) [136].Moreover, the microreactor with the wavy channel did not exhibit a transition to the jetting regime at an increasing flow rate, unlike the one with a straight channel.During the experiment, the productivity was up to 51.2 ml•h −1 and the C.V. was less than 5%.
To achieve the required uniformity in parallelized devices, quasi-2D devices are more suitable.Soft lithography based on PDMS is commonly used for manufacturing parallelized flowfocusing devices with a high throughput of emulsion droplets.However, these devices are limited by operating conditions such as temperature and pressure.Alternatively, a 3D etched silicon wafer with 10 260 generators has been reported to produce polycaprolactone solid microparticles with a C.V. < 5%, capable of operating at high pressure and temperature conditions when the fabrication material is silicon and glass [137].
Compared to the flow-focusing generator, the stepemulsification unit has shown superior droplet formation quality, with droplet size being insensitive to flow rates [138].Rayleigh-Plateau instability is crucial in droplet formation, making it promising to achieve high-throughput production based on the massive parallelization of multiple nozzles [139].Currently, step-emulsification devices with various geometries are available, including the terrace, straightthrough, rectangular-shaped, and wedge-shaped configurations (figure 8).
In a terrace-type step emulsification device, a terrace is located between the inlet and the cavity [144].The droplet pinch-off location is on the terrace, which promotes pinchoff due to the lower reflux resistance of the continuous phase compared to step emulsification devices without a terrace.Wang et al [145] demonstrated a relationship between droplet size and terrace width under conditions where the contact angle is less than 150 • .The straight-through type step device is derived from the membrane emulsification device.Asymmetrical channels result in the generation of smaller droplets and improve droplet monodispersity [141].However, producing droplets with a size of less than 10 µm requires narrower orifices, which leads to the inactivation of most orifices [146].
The research group led by Weitz has extensively investigated different types of step emulsification devices for largescale droplet generation [142,143].A notable example is the millipede device, which utilizes triangular nozzles to generate monodisperse droplets via static instability (figure 8(d)) [143].At a dripping regime, this device could produce 40 l of droplets with a diameter of 15 µm per hour, whereas the production rate could reach up to 800 l per hour for droplets  [135] with permission from the Royal Society of Chemistry; (b) schematic of planar microfluidic geometries for mass production: (i) magnified view of 128 cross-junctions integrated on a chip, (ii) formation of arrayed monomer droplets (iii) size distribution of the droplets, (iv) SEM image of microspheres.Reproduced from [18] with permission from the Royal Society of Chemistry; (c) a module consisting of 16 individual flow-focusing channels.Reproduced from [136] with permission from the Royal Society of Chemistry.
with a size of around 160 µm.To facilitate the transport of droplets accumulating at the outlet, an open collecting channel along with buoyancy was implemented (figure 8(c)) [142].This device could generate droplets with a wide range of 30 µm-1 000 µm and a production rate between 0.03 l h −1 and 10 l h −1 .
In addition to the parallelization of microfluidic droplet generation units, multiple splitting of emulsion droplets is also a viable approach for scale-up.Droplet splitting methods can be classified as active or passive.Passive splitting mechanisms involve breaking up droplets through deliberate fluidic designs, without the need for external forces [20].This method utilizes microfluidic devices fabricated with high precision to consecutively split monodisperse droplets into smaller ones (figure 9(a)) [129].
The traditional passive splitting with T-junction splits one large droplet into two daughter droplets, while novel multifurcating channels have been developed to enable adjustment of the number and size of daughter droplets [19].Oneto-five splitting microchannels were used to investigate the effect of mother droplet size and channel lengths in the experiment (figure 9(b)), highlighting the versatility of this tool for sample splitting applications [20].The precision of microfluidic device fabrication was critical, and bypass channels were investigated for bubble splitting [147].It demonstrated that the C.V. of bubbles generated in the device with bypass channels was significantly smaller than that in the device without bypass channels.For the application of encapsulation of single cells with high throughput, multi-step splitting structures were used to split large droplets into small ones, realizing the droplet generation frequency increasing from 59 Hz to 1 300 Hz (figure 9(c)) [148].Wu et al [149] demonstrated that bifurcating angle of 90 • was beneficial for droplet splitting in the fractal tree-shaped network.The effect of capillary number was explored for the droplet size distribution.

Double emulsion droplets 4.2.1. Conventional microfluidic generation of double emul-
sion droplets.Double emulsion droplets, which consist of three immiscible phases and exhibit diverse inner structures and morphologies, are the cornerstone of fine emulsion production technology [150,151].This technology shows great  [142] with permission from the Royal Society of Chemistry and (d) wedge-shaped step emulsification devices: (i) schematic illustration of wedge-shaped step emulsification device, (ii) optical micrograph of a section, (iii) optical micrograph of a nozzle, (iv) cross-section of a nozzle.Reproduced from [143] with permission from the Royal Society of Chemistry.potential for manufacturing highly structured materials for various industrial applications [152].Therefore, it is crucial to develop precise methods for generating double emulsion droplets [153].
Double emulsion droplets can be generated by microfluidics through the combination of the above single-phase droplet generators in a one-step or two-step process.For example, double emulsion droplets are generated by tandem co-flow and flow-focusing devices in one-step and twostep ways (figure 10).In one-step devices, the inner and middle droplets are simultaneously generated in a coupled co-flow or flow-focusing geometry, forming double emulsion droplets in a single step (figure 10(a)).In two-step devices, the inner-droplets are generated in the first-level co-flow or flowfocusing geometry, then encapsulated by the middle phase downstream into a double emulsion droplet in the secondlevel geometry (figures 10(b)-(d)).Compared with two-step devices, the one-step devices have lower requirements for channel wall wettability modification due to the consistent encapsulation of the inner phase and are particularly suitable for generating double emulsion droplets with ultra-thin shells [153].Note that, flow-focusing, co-flow, and cross-junction microfluidic geometries are feasible for generating double emulsion droplets in either one-step or two-step ways, while T-junction and step-emulsification geometries only allow for two-step generation of double emulsion droplets in most flow conditions due to the characteristics of the geometry of the device (figures 10(e)-(h)).

High-throughput microfluidic generation of double emulsion droplets.
The strategies for generating highthroughput double emulsion droplets are similar to those for single-phase droplets, involving the parallelization of microfluidic droplet generation units and multiple splitting of emulsion droplets.The appropriate parallelization units for double emulsion droplet generation include flow-focusing generators and step-emulsification units.The double emulsion droplets generated in the coaxial annular microfluidic platform are a two-step method using parallel flow-focusing generators.As shown in figure 11(a), the inner droplet in the upstream annular channel was emulsified in the downstream cross-junction to form double emulsion droplets.It is applicable for high throughput and controllable production of single and double-emulsified droplets.The annular channels were used to distribute fluids into the droplet-forming units [154].When Q i was 5 ml h −1 , Q m was 20 ml h −1 , and Q o was 500 ml h −1 , respectively, the core and shell droplet sizes were 88.7 µm and 147.2 µm, respectively, with C.V.s of 5.7% and 2%.3D distributors and collection channels were used to connect the repeating droplet generators, which enable precise control and mass production of the microfluidic double emulsion droplets.The inner droplets were immediately wrapped by the intermediate fluid at the moment of generation and emulsified into double emulsion droplets, which is a one-step method to generate double emulsion droplets as mentioned above.The device contained 15 droplet-forming units and was connected by three 3D networks of distribution networks and a collection network (figure 11(b)).It demonstrated that this device was capable of producing double emulsion droplets with a C.V. of less than 6% at a rate of 1 kg d −1 in a precisely controllable manner [155].
Although parallelized flow-focusing devices have been widely used for emulsion droplet generation, they may suffer from potential issues such as channel clogging or pressure fluctuation, which can result in deteriorated formation processes and polydispersed droplets.It is worth noting that monodispersed emulsions can be generated in step emulsification devices under steady-state conditions.In particular, tandem step emulsification devices have been fabricated to produce monodispersed double emulsion droplets, and the effects of geometrical factors on their morphology, dimensions, and structures have been investigated (figure 12) [63].By adjusting the ratio of the diameters of droplets formed at the end of the first and second-step emulsification nozzles (d 1 and d 2 , respectively), various double emulsion droplets with thin-shell, thick-shell, and multicore architectures could be produced.The d 1 /d 2 ratio was related to the step heights ratio in the devices.Tandem emulsification has also been applied in millipede devices, where the droplet diameter and core size are related to the channel height [156].Millipede droplet makers, featuring 200 nozzles in a circular array, could generate 80 drops per second, achieving a production rate of 20 ml•h −1 .
Parallelizing the synthesis of double emulsion droplets presents a significant challenge due to the complexity of the required device and the high degree of uniformity needed in manufacturing.One alternative way to realize the high throughput is by splitting large droplets into smaller ones.As   shown in figure 13, one double emulsion droplet was split three times into eight identical, smaller drops, increasing throughput by three times.Unlike the usual two-step process, the inner droplets' breakup in this one-step pinching is driven by the outer droplets [19].
Currently, devices for high-throughput double emulsion droplet generation are not widely available, and step emulsion or flow-focusing devices are commonly used in parallelized microfluidic droplet generators.Consistent droplet generator manufacturing is essential to produce high-quality, monodisperse droplets.In parallelized flow-focusing devices, the outer and middle phase flow is regulated to control double emulsion droplet generation methods, whether it is a one-step or two-step process.The two-step method is more commonly used in parallelized step emulsion devices.Additionally, bifurcation structures and size optimization are investigated for multiple droplet splitting.The symmetric breakup of double emulsion droplets in the bifurcation ensures monodispersity.The comparisons between of double emulsion droplets using high-throughput microfluidic production methods are listed in table 3.However, some challenges remain unresolved in microfluidics droplet generation as mentioned above.Concerning the single-phase droplet genprocess, the relationship between microfluidic step emulsification with different device structures and the size of generated droplets remains uncertain, necessitating further investigation through extensive experiments.Additionally, the inherent hydrodynamics of microfluidic step emulsification with different device structures introduce uncertainties that hinder the exploration of high-throughput production.Regarding the double emulsion droplets generation process, one-step microfluidic step emulsification requires higher precision in manufacturing due to the consecutive step structures.On the other hand, the two-step method for generating double emulsion droplets relies on the cooperation among multiple syringe pumps, which makes it time-consuming to explore the suitable flow rates.In summary, future studies should delve deeper into the experiments of microfluidics for droplet generation processes to address these challenges effectively.Applied for the scaled-up production of complex emulsions with well-controlled structural parameters [156] Two-step method using millipede device 64-87 µm 90 µm --20 ml•h −1 Applied for the scaled-up production of complex emulsions with well-controlled structural parameters [156] Two-step method using millipede device 30 µm 70 µm --50 ml h −1 Applied for the high-throughput encapsulation of a wide variety of chemicals while providing the exquisite control achievable through microfluidics. [ Multiple splitting 28 µm 43 µm 6% 6% 5 × faster than a conventional drop maker Applied for the production of small double emulsion droplets. [19]

Influencing factors for microfluidic droplet generation
Overall, the size, productivity and composition of the singlephase droplets are manipulated by the following influenced parameters.For the generation of single-phase droplet, the flow rates of continuous phase and dispersed phase are vital in shear-based microfluidic geometries.In the shearbased microfluidic geometries, droplet size decreases with the increasing flow rate of continuous phase, which leads to an increment in the viscous shear and squeezing action.The increase in the dispersed phase flow rate results in the large front of the droplet head and larger pressure inside the droplet, leading to the increment of droplet size [119].However, the variations of droplet size with dispersed phase flow rate may be different because of the viscosities of dispersed phase and continuous phase [98].For now, the droplet size prediction in shear-based microfluidic geometries is mainly based on the balance between viscous and interfacial tension forces, while in microfluidic step emulsification device, interfacial tension plays a dominant role [157].Therefore, droplets formation process in the microfluidic step emulsification device is mainly influenced by the wall wetting effects, so that the microfluidic device channel structure and contact angle of the fluid on the channel wall play a crucial role in the droplet size [158].
Compared with single-phase droplets, three phases are involved in the generation of double emulsion droplets.Hence, the parameters metioned above need to consider the properties of three phases.For instances, the success rate for encapsulation of double emulsion droplets, the cores number, and the droplet size is determined by the synthetically regulation of flow rates of three phases [100].Similarly, the generation of double emulsion droplets is effected by viscous and interfacial tension forces, so that the size of double emulsion droplets increases with viscosity ratio of middle phase and outer phase [100].
On the other hand, it is worth noting that some non-Newtonian fluids are usually used to produce the microcapsules via the droplet microfluidic method [159], which do not follow Newton's law of viscosity and have variable viscosity dependent on stress [160].As mentioned above, fluid viscosity is a typical influencing factor to determine the characteristic parameters of the microfluidic droplets generation.Therefore, non-Newtonian fluids effects have also received attention in the microfluidic droplets generation.For examples, it has proved that elasticity of the non-Newtonian dispersed phase is able to extend the thin thread neck of the dispersed phase during the droplet formation in a microfluidic T-junction device.As a result, the generation of droplet will be delayed or even resisted when compared with the Nowtonian fluids [161].In addition, with the increasing We, the non-Newtonian rheology changes the neck geometry in the jet regime, leading to the increment of generated satellite droplets.Hence, the generated droplets with non-Newtonian fluids are polydispersed and smaller on average compared with those generated with Newtonian fluids [162].In general, although non-Newtonian fluids effects have been demonstrated to have a non-negligible influence on the performance of the microfluidic droplets generation, there is still a long way to systematically clarify this influence for the decision-making of droplets and the consequent microcapsules.

Applications of droplet microfluidics for carbon capture
The core-shell structure is a fundamental morphological characteristic of the microcapsule, which can encapsulate either a single core or multiple cores, referred to as mononuclear or polynuclear, respectively.Highly permeable microencapsulated carbon sorbents have been developed for carbon capture applications (figure 14).Compared to neat liquid sorbents, the mass transfer from the shell is only slightly lower, but the absorption rate is increased by an order-of-magnitude due to the enhancement of the surface area [25].Fixed bed and fluidized bed are the most commonly used reactor configurations in gas-solid reactions.Therefore, high-throughput generation of microcapsules is employed for industrial scale-up under operating conditions.

Ionic liquid microcapsules
ILs are a class of organic salts that exist in liquid form at room temperature.Their low vapor pressure and high thermal stability make them highly attractive to carbon capture applications.However, their high viscosity and sensitivity to water have limited their large-scale utilization.Encapsulation of ILs in the microcapsule is an efficient method to address these challenges, as the ILs can be effectively confined with loading efficiencies exceeding 80% and their performance can be significantly improved in the applications of CO 2 handling, microreactors, and heavy metal extraction (figure 15(a)) [163].
Compared with the neat ILs, the properties, including CO 2 solubility, CO 2 sorption, CO 2 selectivity, recycling capacity and regeneration, of encapsulated ILs are enhanced.With the increasing surface area of encapsulated ILs, the solubility of ILs increases.What's more, the viscosity of the IL was reduced due to the use of encapsulated ILs, which improves the CO 2 solubility.Hiraga et al [166] have demonstrated that the CO 2 solubility is enhanced due to a coactive effect existing between the molecular interaction and the ILs.It is effective to investigate the adsorption isotherm of microcapsules so as to evaluate absorption performance.Lemusa et al [167] reported that high CO 2 solubility of encapsulated ILs is related to the lowest temperature, which is the main factor influencing the CO 2 solubility.In addition, Kaviani et al [168] developed a singlestep non-solvent induced phase separation method to produce microcapsules, resulting in higher specific surface area and improved absorption rate and CO 2 solubility.It was noted that CO 2 sorption process is evaluated by kinetics and described by the pseudo-second-order model.The encapsulated ILs used in fixed bed absorption were able to overcome the mass transfer restriction of physical absorption.Wang et al [169] investigated the sorption kinetic performance of encapsulated ILs in carbon capture, proving that the sorption rate of IL microcapsules is much faster than neat ILs.In-depth investigations into the mass transfer mechanism of encapsulated ILs lead to the development of a kinetic model for CO 2 uptake.Remarkably, the CO 2 capture kinetics of encapsulated ILs exhibit a 50-fold increase compared to the neat liquid [170].
High CO 2 selectivity is a worldwide research topic on exploration of solvents and materials for carbon capture.It was proven that CO 2 selectivity is higher with the increasing IL content in the microcapsules.Mohamed et al [171] investigated the selectivity performance through encapsulated ILs and computational study.The results indicated that CO 2 selectivity increases with the high IL content in the zeolitic imidazolate framework-8 (ZIF-8).Furthermore, the effect of viscosity and the mechanism of CO 2 /CH 4 selectivity using encapsulated ILs was investigated by the calculations and simulations.In terms of recycling capacity of encapsulated ILs, it is meaningful to check the compatibility and reversibility [172].For instance, Song et al [165] demonstrated that the encapsulated ILs and phase-change ionic liquids (PCILs) have higher CO 2 capacities and superior stability compared to free ILs or PCILs (figure 15(a)).Similarly, Santiago et al [164] focused on the potential of encapsulated amino-acid-based ILs (aa-ILs) in CO 2 capture, given their low cost, abundant availability, and non-toxic biodegradability.Encapsulated aa-ILs exhibited high CO 2 absorption capacities and rates, making them promising materials for future CO 2 capture applications (figure 15(b)).Notably, the capture-desorption of CO 2 cycles is accomplished without a significant decrease in successive productivity.Infrared analysis revealed enhanced CO 2 sorption capacity and CO 2 /N 2 selectivity of the encapsulated ILs.However, it is observed that the capacity slightly reduced by about 0.104 wt% after eight recycling cycles [173].Besides, the CO 2 absorption capacity grows up to a certain value and then declines with the increasing recyclability cycles [172].Stolaroff et al [33] produced microcapsules using three promising solvent/shell pairs with a microfluidic flow-focusing capillary device, demonstrating a 3.5-fold enhancement in CO 2 absorption rate compared to the liquid film of the solvent (figure 15(c)).Lemus et al [167] conducted a study on the CO 2 absorption of encapsulated ILs at 0.1 MPa and 303 K, reporting a CO 2 absorption capacity of 0.036 g g −1 IL.The modest conditions were suitable for the regeneration of encapsulated ILs, and it turned out that the successive capture-desorption CO 2 cycles are obtained without operation productivity loss.Furthermore, the regeneration of ILs is easy because of the low regeneration energy requirement [167].It is worth noting that, in recent research, various IL materials have been encapsulated  Encapsulate ILs with droplet microfluidics: (a) encapsulation of ILs: (i) multiple advanced applications of encapsulate ILs, (ii) the improvement of CO 2 capacity, (iii) comparison of CO 2 uptake capacity between encapsulated IL, agitated IL, and still IL.Reprinted with permission from [163].Copyright (2020) American Chemical Society.Reprinted with permission from [164].Copyright (2018) American Chemical Society; (b) encapsulation of aa-ILs: (i) SEM and (ii) TEM images of the carbon capsules to obtain aa-ILs, (iii) the improvement of CO 2 uptake in aa-ENIL compared to neat aa-ILs.Reprinted with permission from [165].Copyright (2019) American Chemical Society; (c) schematic of the microfluidic device for encapsulation of ILs: (i) microcapsules generated via the microfluidic flow-focusing device, (ii) the droplet formation process, (iii) micrograph of generated microcapsules.Reproduced from [33] with permission from the Royal Society of Chemistry.for carbon capture applications, opening up new avenues for improving the efficiency and sustainability of carbon capture technologies [15,[174][175][176].

MOF microcapsules
The MOFs are highly porous materials with a large specific surface area, formed by the coordination of metal clusters with organic ligands [177].Due to their high capture capacity and selectivity, MOFs have become a popular choice for CO 2 capture applications (figure 16(a)).For now, there have been some researches devoted to the fabrication of hollow structured MOF materials.For instance, Zhang et al [178] investigated a hollow structured zeolitic imidazolate framework (ZIF-8-H) nanosphere.It proved that the capacity of ZIF-8-H is favorable for binding various sized substrates and it can be recovered and reused without loss of catalytic activity.Furthermore, spray-drying is a versatile method assemble nanoMOFs into spherical hollow structures [179].Besides, most MOF adsorption mechanisms are based on physical adsorption, MOF synthesis is a time-consuming process and not easily scalable for large-scale production [180].To overcome this challenge, a novel MOF-based hybrid sorbent was developed and encapsulated within a glass capillary device (figure 16(b)) [14].These microcapsules exhibit good recyclability and stability, opening up a new approach for the research of controlled hybrid sorbents.For instance, Hsieh et al [181] quantitatively studied the effects of ligand concentration, synthesis time, and temperature on MOF synthesis, which could be beneficial for the development of MOF-based CO 2 capture applications.Additionally, Ameloot et al [182] investigated the selective permeability of MOF layers in the shape of hollow microcapsules, and found that the micropore size of MOF crystallites was related to their selective permeability (figure 16(c)).These hollow MOF capsules were generated in a co-flowing stream and it was found that the monodispersity of the generated microcapsules was critical to their performance.Microcapsules encapsulating MOFs exhibit small sizes, facilitating minimum defects and maximum interfacial area for efficient carbon capture.Yu et al [14] studied the synthesis of a novel MOF-based hybrid sorbent, wherein they investigated the characteristics of microcapsules containing MOFs for carbon capture applicaitons.By employing a microfluidic strategy, a series of MOFs with hierarchial hollow structures were successfully generated with tunable sizes [183].
The characteristics, including CO 2 uptake, CO 2 selectivity, adsorption capacity, are used to evaluate the performance of MOFs microcapsules on carbon capture.The selectivity of CO 2 /CH 4 separation was improved by functionalizing MOFs with amino groups [184].The equilibrium selectivity for selective adsorption of CO 2 over CH 4 can be over 10 based on the MOFs with various unsaturated sites [185].The commercially available Cu 3 (BTC) 2 MOF showed a selectivity of 4 ∼ 6 at 0.1 ∼ 3 bar [186].Compared with a silica with high surface area, a MOF replete with open magnesium site has an exceptional adsorption capacity up to 23.6 wt% CO 2 at 0.1 bar and 296 K [187].Among all the commercial adsorbents, zeolite 13X is the most effective adsorbent for CO 2 separation [53].Compared with zeolite 13X, the adsorption capacities are higher in a magnesium-based MOF [51].Furthermore, various sites, such as open metal sites and Lewis basic sites, are introduced into MOFs to improve the adsorption capacity.Liang et al [45] synthesized a Cu-MOF with high CO 2 uptake and selectivity, which is able to be reused without loss of adsorption capacity.The results demonstrated that these microcapsules achieved higher efficiency and catalyst recyclability, presenting a promising avenue for future applications in assembling MOFs with novel hierarchical structures.

Chemical solvent encapsulation
Chemical absorbents, such as ethanolamine (MEA), piperazine, and carbonate solutions, have been encapsulated for carbon capture.Encapsulation of these absorbents in microcapsules has been shown to increase the uptake rate of CO 2 and mass transfer when compared with the use of neat fluid [188,189].It is worth noting that, MEA, which is most used in a number of plants, is corrosive and it will degrade during repeated regeneration cycles [190].Hence, relying on the characteristics of abundance and eco-friendly, carbonate solutions are studied as an alternative to MEA solutions in carbon capture.For instance, Vericella et al [25] employed a core-shell motif to encapsulate potassium carbonate (K 2 CO 3 ) or sodium carbonate (Na 2 CO 3 ) (figure 17(a)).The microcapsules demonstrated higher permeability than gas separation membranes, and the CO 2 absorption rate was increased by an order of magnitude.The process of CO 2 absorption-desorption could be visually monitored with a pH indicator.It turned out that the CO 2 sorption capacity of encapsulated sodium carbonate is similar to that of MEA.Zhao et al [13] produced stable microcapsules with CO 2 solvents using flow-focusing microfluidics (figure 17(b)).These microcapsules withstand temperatures up to 300 • C and could be used for multiple regeneration cycles.Wang et al [191] utilized a 3D flowfocusing microfluidic device to produce microcapsules for carbon capture, in which nanoparticle organic hybrid materials were encapsulated.The CO 2 capture rate was enhanced due to the specific surface area and enhanced mass transfer of   microcapsules.Additionally, Shi et al [192] utilized KHCO 3 as the form of carbon spheres for CO 2 capture, which exhibited superior CO 2 capture performance with a CO 2 uptake of up to 4.26 mmol•g −1 25 • C and 1 bar.Rama et al [193] used 2-amino-2-methyl-1-propanol (AMP) as the core sorbent in microcapsules, demonstrating its high CO 2 sorption capacity and its potential applicability in industrial settings.Finn and Galvin [194] proposed a mechanistic model for mass transfer and chemical reaction to predict microcapsule performance under different scenarios.The model's predictions indicated that microcapsules containing sodium carbonate solution show strong sensitivity and high CO 2 sorption capacity.The comparisons of performance parameters of carbon capture microcapsules fabricated via the droplet microfluidics method are listed in table 4.

Conclusions and perspectives
The increasing emission of CO 2 from various sources is drawing attention toward CCUS strategies.Therefore, it is promising to explore the generation of carbon capture microcapsules via droplet microfluidics.This review has provided a comprehensive overview of the progress and limitations of the high-throughput production of carbon capture microcapsules by microfluidics.The advancements in microfluidic device fabrication, methods and hydrodynamics for droplet generation, and the application of high-throughput emulsion for carbon capture have been discussed.The main fabrication methods of microfluidic devices are wet etching using glass/silicon substrates, soft lithography using PDMS, hot embossing on PMMA, capillary assembly, and 3D printing technology.The methods and hydrodynamics of co-flow, T-junction, crossjunction, and flow-focusing microfluidic droplets generation have been studied in depth, including flow modes transition and monodispersity of droplets in dripping mode.In addition, the hydrodynamics of step emulsification for droplet generation have been demonstrated to have a close association with the device geometries.This fundamental understanding of the hydrodynamics of the microfluidic droplet generation process is helpful for high-throughput droplet generation.The review emphasizes the importance of both parallelized microfluidic droplet generators and multiple splitting of droplet approaches for achieving high-throughput production.
In the recent years, the carbon capture microcapsules generated via droplet microfluidics have aroused considerable interset.However, there remain challenges and opportunities to address for the future applications.Therefore, herein, the most prominent research directions for microcapsules for carbon capture based on the droplet microfluidics are provided on three significant aspects: (i) high-throughput production of microcapsules; (ii) selectivity of materials for carbon capture microcapsules; (iii) quality of microcapsules.

High-throughput production of microcapsules
Generally, microcapsules are synthesized using interfacial polycondensation or photocuring droplets.As the templates of microcapsules, double emulsion droplets generated from microfluidic devices are monodispersed, controllable and concentricity, which are superior to those generated with bulk emulsification.Although plenty of microencapsulated carbon sorbents via droplet microfluidics have been proven to be effective for the carbon capture applications, the output of microcapsules is not enough to satisfy the industrial application.In order to increase the throughput of microcapsules based on the droplet microfluidics, parallelization of microfluidic droplet generation units and multiple splitting of emulsion droplets are effective.However, considering various materials can be applied for carbon capture, the fabrication factors of microfluidic devices for high-throughput microfluidic production of the microcapsules, such as cost, manipulation, and stability, are taken into account.In general, until now, the relevant researches involving the high-throughput production of microcapsules for carbon capture have not been really started yet.

Selectivity of materials for carbon capture microcapsules
The materials selectivity of shell and core of microcapsules is essential for the applications of carbon capture.The shell of microcapsules presents a primary protective function against environments.Furthermore, selective permeability, waterproofing and biocompatibility of the shell are also needed to be considered.The core of microcapsules is the essential element because of its key role to make carbon capture functional in the application, and it will be isolated from the external agents.Focusing on the application of carbon capture, the ideal chemical solvents enclosed into the microcapsules require the following characteristics: (i) high reactivity to CO 2 ; (ii) low regeneration cost requirements; (iii) high absorption capacity; (iv) high thermal stability; (v) reduced solvent degradation; (vi) low environmental impact; (vii) low solvent costs.For now, ILs, MOFs and chemical solvent have been selected as the core materials.It is meaningful to explore other materials, whose absorption kinetics will be increased due to the encapsulation or the inherent characteristics such as corrosive or highly viscous.The permeability of CO 2 through the cured shell material is also needed to be taken into account so that the microcapsules can be used in cycles.

Quality of microcapsules
In order to evaluate the quality of microcapsules, the numbers of C.V., shell diameter, and core diameter are used and illustrated in the available researches.For examples, Vericella et al [25] generated microcapsules with a highly uniform diameter of (600 ± 6) µm, wall thickness of (31 ± 1) µm and centricity.Wang et al [173] generated spherical microcapsules with diameter of 38 µm.Yew et al [195] produced functional coreshell microcapsules of uniform sizes with 600-720 µm, and the C.V. with 0.97% ∼ 3%.Especially, the monodispersity and centricity of microcapsules are demonstrated to be beneficial for the increment of surface area.However, the relationship between the quality of microcapsules and the CO 2 absorption properties has been far from being fully understood.It is also wondering that whether the size of microcapsules will influence the CO 2 absorption properties.
This review can trigger novel ideas for other applications such as high-throughput of microcapsules for carbon capture, which is beneficial for the application of microfluidic droplet technology, particularly in the industrial fields.In the future, one challenge in mass production is the design regulations and tactics for non-Newtonian fluid.The embodiment of quality control in parallelized droplet generators for the long-term future also needs to be investigated further.In conclusion, the generation of high-throughput droplets is an important link between theoretical research and industrial applications.

Figure 2 .
Figure 2.An overview of the themes discussed in this review, from microfluidic chip fabrication technologies to applications of droplet microfluidics for carbon capture.Reproduced from[27] with permission from the Royal Society of Chemistry.Reproduced from[28], with permission from Springer Nature.Reproduced from[29] with permission from the Royal Society of Chemistry.Reproduced from[30], with permission from Springer Nature.Reprinted from[31], © 2020 Elsevier Ltd All rights reserved.Reproduced from[32] CC BY 4.0.Reproduced from[33] with permission from the Royal Society of Chemistry.Reprinted from[14], © 2021 Institute of Process Engineering, Chinese Academy of Sciences.Publishing services by Elsevier B.V. on behalf of KeAi Communications Co., Ltd.[13] John Wiley & Sons.© 2022 Wiley Periodicals LLC.
Figure 2.An overview of the themes discussed in this review, from microfluidic chip fabrication technologies to applications of droplet microfluidics for carbon capture.Reproduced from[27] with permission from the Royal Society of Chemistry.Reproduced from[28], with permission from Springer Nature.Reproduced from[29] with permission from the Royal Society of Chemistry.Reproduced from[30], with permission from Springer Nature.Reprinted from[31], © 2020 Elsevier Ltd All rights reserved.Reproduced from[32] CC BY 4.0.Reproduced from[33] with permission from the Royal Society of Chemistry.Reprinted from[14], © 2021 Institute of Process Engineering, Chinese Academy of Sciences.Publishing services by Elsevier B.V. on behalf of KeAi Communications Co., Ltd.[13] John Wiley & Sons.© 2022 Wiley Periodicals LLC.

Figure 5 .
Figure 5. Schematic illustration of the process for fabricating glass microfluidic devices by (a) wet etching.Reproduced from [27] with permission from the Royal Society of Chemistry.[66] John Wiley & Sons.© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim; (b) soft lithography.Reproduced from [67].CC BY 4.0; (c) hot embossing process: (i) conventional and (ii) roller to roller.Reprinted from [68], © 2019 Elsevier Ltd All rights reserved.Selection and peer-review under responsibility of the scientific committee of the 10th International Conference of Materials Processing and Characterization, Reproduced from [69].© IOP Publishing Ltd All rights reserved; (d) capillary assembly: (i) capillaries with different structures; (ii) photo of capillary microfluidics assembly.Reproduced from [30], with permission from Springer Nature.(e) 3D printing.Reproduced from [70].© The Author(s).Published by IOP Publishing Ltd CC BY 4.0.

Figure 6 .
Figure 6.Schematic illustration of microfluidic characteristics: (a) flow behaviors in different droplet generators: (i) co-flow, (ii) flow-focusing, (iii) T-junction, (iv) cross-junction, (v) step emulsification.(b) The scale-up strategies for microfluidic process: (i) parallel numbering-up the nozzles for dispersed phase and continuous phase; (ii) parallel numbering-up the nozzles for dispersed phase; (iii) splitting the emulsion droplets.(c) The fluids distribution network in two common layouts: (i) ladder network and (ii) tree network.

Figure 7 .
Figure 7. Schematic of parallelization of flow-focusing devices: (a) droplets formed in four integrated flow-focusing devices.Reproduced from[135] with permission from the Royal Society of Chemistry; (b) schematic of planar microfluidic geometries for mass production: (i) magnified view of 128 cross-junctions integrated on a chip, (ii) formation of arrayed monomer droplets (iii) size distribution of the droplets, (iv) SEM image of microspheres.Reproduced from[18] with permission from the Royal Society of Chemistry; (c) a module consisting of 16 individual flow-focusing channels.Reproduced from[136] with permission from the Royal Society of Chemistry.

Figure 8 .
Figure 8. Schematic of parallelization of step-emulsification devices: (a) terrace step-emulsification device.Reprinted from [140], © 2017 Elsevier B.V. All rights reserved; (b) straight-through step emulsification device.Reprinted from [141], Copyright © 2007 Elsevier B.V. All rights reserved; (c) rectangular-shaped step emulsification device.Reproduced from[142] with permission from the Royal Society of Chemistry and (d) wedge-shaped step emulsification devices: (i) schematic illustration of wedge-shaped step emulsification device, (ii) optical micrograph of a section, (iii) optical micrograph of a nozzle, (iv) cross-section of a nozzle.Reproduced from[143] with permission from the Royal Society of Chemistry.

Figure 9 .
Figure 9. Schematic of multiple splitting of emulsion droplets: (a) droplets formed at high dispersed phase volume fractions.Reprinted figure with permission from [129], Copyright (2004) by the American Physical Society; (b) multiple splitting of droplets with multi-furcating microfluidic channels: (i) droplets splitting set up, (ii) droplets split into two, three, and five daughter droplets.Reprinted from [20], with the permission of AIP Publishing; (c) droplets consecutive splitting via multiple junctions.Reprinted from [148], Copyright © 2015 Zhang-Run Xu.Published by Elsevier B.V. All rights reserved.

Figure 10 .
Figure 10.The flow conditions of three phases in (a) one-step co-flow and flow-focusing device, (b) two-step co-flow and flow-focusing device, (c) two-step co-flow device, (d) two-step flow-focusing device, (e) one-step cross-junction device, (f) two-step cross-junction device, (g) two-step T-junction device, (h) two-step step-emulsification device to generate compound droplets.

Figure 11 .
Figure11.Schematic illustrations of (a) microfluidic chips with annular channels: (i) 144 cross-flow junctions, (ii) 72 droplet generators to produce Janus droplets, (iii) 32 triple emulsion droplet generators.Reproduced from[154] with permission from the Royal Society of Chemistry; (b) schematic of the design for microfluidic double emulsification device: (i) a microfluidic double emulsion droplets-maker unit, (ii) parallelized double emulsion chip, (iii) micrograph of a unit, (iv) actual device.Reproduced from[155] with permission from the Royal Society of Chemistry.

Figure 13 .
Figure 13.Splitting three times in the emulsion device.Reproduced from [19] with permission from the Royal Society of Chemistry.

Figure 15 .
Figure 15.Encapsulate ILs with droplet microfluidics: (a) encapsulation of ILs: (i) multiple advanced applications of encapsulate ILs, (ii) the improvement of CO 2 capacity, (iii) comparison of CO 2 uptake capacity between encapsulated IL, agitated IL, and still IL.Reprinted with permission from[163].Copyright (2020) American Chemical Society.Reprinted with permission from[164].Copyright (2018) American Chemical Society; (b) encapsulation of aa-ILs: (i) SEM and (ii) TEM images of the carbon capsules to obtain aa-ILs, (iii) the improvement of CO 2 uptake in aa-ENIL compared to neat aa-ILs.Reprinted with permission from[165].Copyright (2019) American Chemical Society; (c) schematic of the microfluidic device for encapsulation of ILs: (i) microcapsules generated via the microfluidic flow-focusing device, (ii) the droplet formation process, (iii) micrograph of generated microcapsules.Reproduced from[33] with permission from the Royal Society of Chemistry.

Figure 16 .
Figure 16.Encapsulate MOFs with droplet microfluidics: (a) the concept of encapsulating MOFs.Reprinted from [14], © 2021 Institute of Process Engineering, Chinese Academy of Sciences.Publishing services by Elsevier B.V. on behalf of KeAi Communications Co., Ltd; (b) encapsulation of MOFs with flow-focusing device: (i) schematic of the microfluidic system for encapsulated synthesis of MOFs, (ii) micrograph of generated microcapsules via microfluidics, (iii) micrograph of generated microcapsules via the conventional method.Reprinted from [14], © 2021 Institute of Process Engineering, Chinese Academy of Sciences.Publishing services by Elsevier B.V. on behalf of KeAi Communications Co., Ltd; (c) the concepts in the synthesis of MOF films and preparation of hollow capsules.Adapted from [182], with permission from Springer Nature.

Figure 17 .
Figure 17.Encapsulate chemical solvents with droplet microfluidics: (a) encapsulation of chemical solvents for carbon capture: (i) schematic illustration of the encapsulated liquid carbon capture process via the microfluidic flow-focusing device, (ii) optical image of cured silicon (iii) optical images of microcapsules in flowing CO 2 environment.Reproduced from [25], with permission from Springer Nature; (b) encapsulation of K 2 CO 3 for carbon capture: (i) schematic illustration of the encapsulated K 2 CO 3 via the microfluidic flow-focusing device, (ii) the color change of microcapsules before and after carbon capture, (iii) SEM images of capsules within different shell materials.[13] John Wiley & Sons.© 2022 Wiley Periodicals LLC.

Table 1 .
The summary and comparison of microencapsulated solvents for carbon capture.

Table 2 .
The summary and comparison of various fabrication methods.

Table 3 .
The comparisons of double emulsion droplets using high-throughput microfluidic production methods.

Table 4 .
The comparisons of performance parameters of carbon capture microcapsules fabricated via the droplet microfluidics method.