Microcurrent behavior of core-shell droplet deposition in coaxial electrohydrodynamic printing

The core–shell droplets have the characteristics of stable micro-environment, small volume, large specific surface, which means wider application prospects in micro reactors, functional material synthesis and other fields. However, how to realize the controllable preparation of high flux, high resolution and small size for core–shell droplets is still the key point to be broken through. In this paper, the coaxial electrohydrodynamic printing (EHD) technology is used to combine the real-time microcurrent signal with the behavior characteristics of core–shell droplets, both achieve the interface behavior characteristics of the nozzle droplets to be analyzed and clarify the microcurrent behavior when the core–shell droplets are generated. At the same time, the influence of the inner and outer solution supply rate on the core–shell droplet was investigated. When the supply rate of inner solution changes from 10 μl/h increases to 90 μl/h, the average current increased from 0.0153 μA to 0.0652 μA, but also the droplet generation frequency raised by 14 times, moreover, the average diameter of droplets could be reduced by half. In contrast, the change of outer shell supply rate has less influence on the average current, generation frequency and diameter of droplets. This research provides a possibility for controllable high resolution of core–shell droplet EHD printing, which can be helpful significantly to improve the functional characteristics of the macroscopic and microstructure of the core–shell droplet.


Introduction
Microdroplets have increasingly application prospects in the fields of microreactors, drug encapsulation, functional material synthesis, tissue engineering, and personal-care because of small size and large specific surface area [1][2][3][4][5][6]. The isolated system can be formed by the use of core-shell to wrap microdroplets, ensuring the stability of the microenvironment but avoiding contamination caused by material infiltration between adjacent droplets [7,8]. As a typical structure of biological detection, water in oil core-shell droplets have received more and more attention [9,10].
Microemulsion technology is a common method of core-shell droplets, which uses the characteristics of oilwater interface for batch production, however, the uniformity of droplet is a problem and the location accurately is hard to be ensured [11]. Inkjet printing is helpful to be integrated, but both the diameter of droplets and the uniformity are difficult to be accepted [12]. Microfluidic technology has been chosen to generate core-shell droplets to overcome this nonuniformity, unfortunately, the design of chip channel is complex, which means the higher processing cost, and it is difficult to achieve accurate quantitative and controlled deposition [13][14][15]. The breakthrough in freestanding and spanning features are still out of reach. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Electrohydrodynamic printing (EHD) technology as a generation method of microdroplets, uses electric field force for driving the induced jet [16]. The sprayed droplets originate from the Taylor cone, and the diameter of droplets is not limited by the size of the nozzle [17,18]. EHD technology has obvious advantages of fast response to electric field, small diameter of droplets, and good compatibility of materials, which can be represented high-throughput and application of core-shell droplets [19,20]. Droplet generation frequency, average diameter and morphology of droplets are important parameters in the preparation of microdroplets, and many researchers have done a lot of research on this. Choi et al [21] designed a triangular-nozzle array for printing, which improved production efficiency, but also reduced the edge effect of the electric field for the better morphology of the droplets. Rogers et al [22] applied pulsed voltage to achieve high-frequency printing, but the micro-nozzle has a complex structure and poor compatibility. Tang et al [23] used a coaxial nozzle to prepare core-shell droplets by electrostatic atomization, but the uniformity of the droplets was a problem.
With further research, it is found that microcurrent can effectively reflect the behavior characteristics of droplets when they are generated, which means the macroscopic and microstructure of microdroplets can be detected in real time [24]. Li et al [25] studied the microcurrent characteristics of EHD printing, which provided the necessary feedback information for droplet generation. And Xiong et al [26] researched the dynamic behavior characteristics of the tuned pulsed EHD jet, and clarified the law of parameter interaction. However, the combination between real-time microcurrent signals and behavioral characteristics of core-shell microdroplets has not been investigated yet. In the previous work of our group, the self-designed microcurrent detection module has been combined with the EDW system for jet pattern recognition, and the characteristics of the EDW current was also studied, which laid the foundation for this research [27][28][29]. In this paper, the possibility for controllable high-resolution EHD printing of core-shell droplets has been proved, which is of great help in improving the macroscopic properties of core-shell droplets as well as the functional properties of local microstructures.

Materials
The inner and outer of the coaxial EHD printing are supplied with different solutions. The outer uses M8410 mineral oil (Sigma, USA) and SPAN80 surfactant (Aladdin, USA) with a mass fraction of 2% to reduce the surface tension and dynamic viscosity of the liquid, which helps to improve the rheological ability of the outer oil and promote the formation of water-in-oil core-shell droplets, and also maintain the stability of core-shell droplet structure. The deionized water is used for the inner dope, and we added mass fraction 0.2% of Fluorescein 5-isothiocyanate (F21212, Acmec, Shanghai Acmec Biochemical Co., Ltd), Rhodamine (R98641, Acmec, Shanghai Acmec Biochemical Co., Ltd), Calcein dyes (C33970, Acmec, Shanghai Acmec Biochemical Co., Ltd) for different experiences respectively, and a small amount of Dimethyl sulfoxide solution (M93920, Acmec, Shanghai Acmec Biochemical Co., Ltd) is also added to facilitate dissolution. The choose of dyes can improve observability of coaxial jet and core-shell droplets by fluorescence effect, on the other hands, it can also raise the conductivity of inner dope, promoting free charge of fluid in high voltage electric field under the effect of migration behavior, which can contribute to the formation of the Taylor cone and core-shell droplet.

System device
As shown in the figure 1, the on-demand water-in-oil core-shell droplet by coaxial EHD printing is set up. The 22G/17G coaxial nozzle is adopted (the inner diameter of the outer shell needle is 1.07 mm, while the inner diameter of the inner core needle is 0.4 mm). The precision syringe Pumps (Pump 11 Pico Plus Elite, Harvard Apparatus America, Cambridge, MA, USA) are used to supply different solutions to the inner or outer nozzles at a constant rate. The positive output of high-voltage power source (RIGOL DG1022Z, Beijing, China; HVA-502NP5, Tianjin, China) is connected with the nozzle, and the negative output is connected with the collector, forming a high voltage electric field between the nozzle and the collector. A silicon wafer is used as the collector, which is fixed on a multi-axis motion platform (REI95LM-050, Shenzhen Borui Automation Equipment Co., Ltd, Shenzhen, China). The speed and trajectory of the platform are controlled by the host computer. The distance between the nozzle and the collector is 2 mm, so that the droplets are deposited directly after ejecting from the tip of the Taylor cone, which can obtain the array and orderly distribution of core-shell droplets on the collector.

Characterization and measurements
The morphology of suspended droplets at the coaxial nozzle is captured by a high-speed CCD (UI-3130CP-C-HQ, IDS Imaging Development Systems GmbH, Obersulm, Germany) in real time. The core-shell droplets on the collector are observed with a microscope (OLYMPUS CX31, Japan) and the size are analysed by ImageJ.
The signal of microcurrent is recorded by the self-designed current detection module, as shown in figure 2. Because of the high resistance in the circuit, the current in the circuit is still only nA even if a high voltage of several thousand volts is applied at the nozzle. In consideration of safety, current display, measurement accuracy and other reasons, a special amplification circuit module is used to amplify the microcurrent signal to the range that can be detected by the measuring instrument, of which the output signal was acquired by a data acquisition card (16 Inputs, Multifunction I/O, National Instruments Corporation, Austin, TX, USA) and transmitted to the host computer in real time [27].

Results and discussion
The migration behavior of free charge is an important factor that determines the flow of solution and jet ejection in the EHD printing. In the process of jet generation, a current loop is formed between the nozzle and the collector, for which the microcurrent signal can be used to characterize the charge migration behavior in the solution.  A self-designed microcurrent detection module is used to collect the real-time current of the coaxial nozzle stably spraying water-in-oil droplets. The CCD camera collects images at a frequency of 250 frames per minute. The image of jet injection was matched with the time domain, as shown in Figure 3 five times of printing were produced within 1s. After the analysis of the preliminary experimental data, comprehensively considering the suitable droplets and stable ejecting process, the standard applied voltage of 2 kV, the distance from nozzle to collector of 2 mm, and the platform movement speed of 20 mm s −1 , both inner and outer solution rates of 50 μl/h were employed in the following studies. As can be seen that when the jet is deposited on the collector, an electrical pathway can be formed between the nozzle and the collector, resulting in a current pulse waveform with abrupt trend obtained. With the release of charge and the attenuation of the jet, the suspended droplet at the nozzle bounces back and shrinks, and the corresponding current will also have periodic changes. However, it is interesting to find that the generation of the jet is not limited to the rising edge of the current pulse waveform, but the current pulse on the falling edge is also one of the characteristic conditions, which are also presented in figure 1.
The following inference is made regarding the phenomenon that the current pulse on the falling edge is also the characteristic condition for generating jet, which is shown the vision-current matching diagram collected in figure 3. To describe the charge behavior of the system, the electrical system between the nozzle and the collector is equivalent to the RC circuit shown in figures 3(d) and (e), which includes an inductive capacitor C S , inner core-water resistors R 1 and outer shell-oil resistors R 2 . And the induction capacitor C S can be expressed as [30]:  Where 1 k is the electrical conductivity of water, 2 k is the electrical conductivity of oil, d n is the outer diameter of the core needle, D n is the outer diameter of the shell needle, h is the height of the meniscus of suspended drops.
When the jet is deposited on the collector, the shell-oil of the outer will wrap the core-water solution to form the core-shell droplet structure. However, Svideo 1 and 2 are jet images in the same group of experiments, and it can be clearly distinguished the difference between them. In Svideo 1, the obvious color difference of the solution of the inner and outer axes before the jet formation can be observed. In the process of printing, the outer oil contacts the substrate before the inner water, which corresponds to the situation in figure 3(e), and the yellow auxiliary lines in figure 3(e) is used to distinguish the chromatic boundaries between solutions. The water of the inner layer may be separated from it by the shell-oil solution. Different theoretical models are established for different situations. While in Svideo 2, the inner water is in direct contact with the substrate, corresponding to the situation in figure 3(d). Where U is the potential difference brought by high voltage field, the A represents the current detection module. When both the core and shell solutions contact the collector, it is equivalent to a parallel circuit, which means the total resistance value is smaller, leading to a larger current. In contrast, when the inner solution is enclosed in the outer and kept isolated from the collector, the whole circuit is equivalent to a series circuit, so a larger total resistance means a smaller current value will be generated. In a nutshell, the abrupt trend of current generated when the jet reaches the collector is an effective way to distinguish whether the coreshell droplets are generated.
In order to the better analysis of the interface behavior characteristics of suspended droplet, the variation of the interface behavior with voltage in a single cycle is recorded, as shown in figure 4. Taking the moment of jet generation as the starting point (0s), it will carry away part of the free charge on the liquid surface when the jet is ejected from the tip of Taylor cone to the collector, which means the free charge amount of the suspended droplet is reduced greatly. Under the action of surface tension, the suspended droplet bounces back and shrinks quickly, and the minimum surface energy state can be approached at 4 ms. With the increase of time, the free charge in the fluid continues to be moved to the tip of the Taylor cone under the effect of the space electric field force, and not only the free charge density at the tip of the Taylor cone but the component of electric field shear stress will be increased. When the electric field force is increased enough to break through the surface tension, the jet is formed at 135 ms, and the water-in-oil droplets are able to be obtained on the collector.
The droplet generation frequency and average diameter of core-shell droplets are important indicators in the manufacturing process. Therefore, the influence of solution supply rate of inner and outer dope on the generation of water-in-oil core-shell droplets are studied. The influence of different inner solution supply rate was investigated in figure 5. In the process of water-in-oil, Taylor cone is formed under the effect of electric stress at the oil-water interface, which is mainly driven by the movement of free charge of the inner dope. What's more, when the supply rate of outer oil is constant, with the increase of inner solution supply rate, the charge at the tip of the cone accumulated increased, which means the shorter time for accumulation required to achieve the critical amount of charge to creating the jet. As a result, the generation frequency of the core-shell droplets increases significantly with the increase of the inner supply rate, while the average diameter of droplet decreases accordingly. Figures 6(a)-(c) records the microcurrent situation when the outer solution supply rate is 50 μl/h while the inner solution supply rate is 10 μl/h, 50 μl/h and 90 μl/h respectively. The microcurrent shows the trend of alternating high and low levels, and the duty ratio is basically the same. When the inner solution supply rate increases from 10 μl/h to 90 μl/h, the microcurrent oscillation frequency increases 3.5 times, and the current oscillation amplitude increases 4.2 times. The microcurrent between the nozzle and the collector and the droplet generation frequency change with the inner core solution supply rate is shown in figures 6(d) and (g). With the increase of inner supply rate, the average current and the droplet generation frequency increase significantly. Not only the average current increased from 0.0153 μA to 0.0652 μA, but the droplet generation frequency raised from 0.5 Hz to 7 Hz with the increase of the inner solution supply rate from 10 μl/h to 90 μl/h. Figure 6(f) shows that the average diameter of core-shell droplet decreases with the increase of the inner solution supply rate. As the inner supply rate increases from 10 μl/h to 90 μl/h, the average diameter of droplet decreases from   1.324 mm to 0.607 mm. While it can be concluded from figures 6(g)-(i) that compared with the influence of inner solution supply rate, the affection of outer has less impact on the average current, average diameter of droplet and droplet generation frequency. These studies provide an experimental basis for the morphology control of water-in-oil droplets, and also verifies the feasibility of EHD technology in the accurate manufacturing of composite microdroplets.
In order to verify the accurate deposition of core-shell droplets based on coaxial EHD technology, core-shell droplets with different components were collected on the same substrate. Rhodamine, Fluorescein isothiocyanate, and Calcein were chosen to prepare different inner solutions according to the photoluminescence characteristics of different dyes. By comparing the excitation wavelength and emission wavelength of these dyes, it is concluded that the energy of the ultraviolet wave band can be used to make different dyes show different fluorescence effects. The filters of ultraviolet wave band were added at the light source and in front of the camera respectively, so that the morphological ch-aracteristics of core-shell droplets could be effectively recorded. In combination with the research on the microcurrent of the core-shell droplets and the controllable process parameters, different dyes of EHD experiments were carried out respectively. The core-shell droplets of different colours and sizes were arranged on the same collector by changing the parameters of inner solution supply rate, as shown in figure 7. The different core-shell droplets group was used to corresponded to a different letter respectively. The blue core-shell droplets are added with fluorescein isothiocyanate, which can display the letter 'X' through connection. The yellows are the core-shell droplets added with Rhodamine, which can indicate the letter 'M'. The greens are added with Calcein, which represents the letter 'U'. Therefore, this work is able to achieve on-demand printing of core-shell droplets effectively, but also providing the possibility for real-time control of core-shell droplets, simultaneously, the application of core-shell droplets in more fields can be promoted.

Conclusions
In this paper, the controllable preparation method of core-shell droplets printed by coaxial EHD printing is researched, but also the relationship between microcurrent signals and the behavior characteristics of core-shell droplets is explored. It is possible to realize the on-demand printing of core-shell droplets for real time controlled by combining the used of EHD printing technology, the analysis of suspended droplet interface behavior characteristics, and the definition of microcurrent behavior during the generation of core-shell droplets. At the same time, by adjusting the solution supply rate of the core and shell, the droplet generation frequency is increased by 14 times significantly, and the average diameter of the core-shell droplets is reduced to 1/2 of the original. This study provides the possibility of controllable high resolution EHD printing of core-shell droplets, which will remarkably help to improve the functional characteristics of the macroscopic and local microstructure of the core-shell droplet.