Honeycomb pattern formation on poly(vinyl chloride) films: electrically-driven microparticle trapping and the effect of drying temperature

This work presents the effect of drying temperature on the formation of poly(vinyl chloride) (PVC) honeycomb microstructures formed by the breath figure technique. Results revealed the self-assembly of honeycomb patterns with small cell diameter and thick cell walls dried at room temperature. An increase in cell diameter and a decrease in wall thickness were observed as drying temperature was increased up to 70 °C while no formation of patterns was noted at temperatures greater than or equal to 80 °C. The presence of honeycomb patterns consequently enhanced the static water contact angle of the PVC layer. Electrowetting experiments revealed more pronounced reduction in the water contact angle on honeycomb-structured PVC compared to a flat PVC layer at any given applied voltage. A proof-of-concept on the feasibility of the honeycomb structures to trap microparticles by electrically-driven droplet actuation was further demonstrated. Corresponding SEM images confirmed the entrapment of microparticles in the honeycomb cells and walls after the electrowetting experiment. These results offer new and facile strategies for tuning the morphological properties of polymeric honeycomb microstructures and its possible application in microparticle trapping and sensing.


Introduction
Generating micro-structured patterns on surfaces significantly influences the properties of polymers, leading to surprising improvements in their performance for particular applications [1]. One of the most interesting structures inspired by nature is the honeycomb pattern which can be described as interconnected and repeating units of spherical, hexagonal, or polyhedral cells [2]. Its unique properties and structure inspired the construction of light and robust aircraft and spacecraft [3], protection gear [4], panels, packaging, and cushioning [5] because of an ability to absorb impact and energy. The repeating units of a honeycomb pattern can also effectively trap particles and impurities thus making them a promising structure for air and water filters [6]. In the context of polymers, honeycomb structures can be generated on the surface to facilitate favorable cell adhesion, growth, and control since the uniform roughness of honeycomb mimics the in vivo cell environment [7]. Highly hydrophobic polymers can also be produced using this pattern [8] that could be used for antiwetting, anti-fogging, or anti-frosting applications. The formation of honeycomb surface patterns can be achieved using surface modification techniques such as lithography [9], laser ablation/ patterning [10], and imprinting [11]. Self-assembly of polymers at certain conditions which results in the formation of honeycomb patterns, commonly known as the breath figure method, has also been demonstrated in previous literature [12]. In this technique, the polymers tend to form hollow structures due to the interaction of the polymer with the Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. surrounding moisture and by the minimization of the free energy of the polymer chains after being in a nonequilibrium state [13]. Considering chemical and environmental parameters such as solvent polarity and volatility, polymer chain length, ionic strength, room humidity, pressure, and temperature is crucial in forming the desired honeycomb structure in the breath figure technique [14,15]. The interplay of these complex factors is anticipated to vary for different polymer-solvent combinations thus opening a wide opportunity for exploration and optimization in the field of polymer science.
In this work, we demonstrate the formation of honeycomb poly(vinyl chloride) (PVC) film on a conducting glass. The breath figure method was implemented through the self-assembly of polymers into honeycomb microstructures after dip-coating the substrate in a PVC solution. PVC was used as a model polymer due to its durability, excellent dielectric property, and high chemical and flame resistance that are important for commercial and industrial use. Work from Hashim and colleagues presented the fabrication of honeycomb PVC films through the addition of a Schiff base and nickel chloride [16] while Liu and colleagues reported the formation of honeycomb structures of PVC with focus on the effect of polymer concentration and relative humidity by introducing saturated salt solutions [17]. Their work, however, is limited to the morphological characterization of the produced pattern. Here, we present a facile approach in producing and tuning the morphological profile of the honeycomb structure by changing the drying temperature. We further investigate the wetting behavior of the produced structure. Furthermore, we demonstrate that the produced honeycomb structure can be utilized as membrane material for electrically-driven trapping of microparticles. To the best of our knowledge, the effect of drying temperature on the formation of honeycomb microstructures, as well as their wetting profile and applications for electrically-driven microparticle trapping, has never been previously reported. The strategies presented in this work could provide a basis for further optimization and control of microscale pattern formation of PVC and other polymers, and their potential applications.

Experimental details 2.1. Materials collection and preparation
Poly(vinyl) chloride powder (PVC, Sigma-Aldrich M W ∼43,000) was used as received with no additive and further treatment. Commercially available indium tin oxide (ITO)-coated glass sheets (50 mm × 50 mm × 1.1 mm) with a conductive layer thickness of 1850 ± 250 Å (<10Ω, Welljoin, China) were used as substrate. Anhydrous tetrahydrofuran (THF, Sigma-Aldrich, 99.9%) and anhydrous N,N-dimethyl formamide (DMF, Sigma-Aldrich, 99.8%) were used to prepare the solution. Zinc oxide powder (Sigma, particle size, <3 μm) was used as model microparticles while deionized water was used to prepare the droplets in the wetting experiment.

Substrate preparation and PVC coating
The glass slide was washed with running water, sonicated, rinsed, and dried to remove impurities and other dirt. The PVC layer was introduced to the glass slide by dip-coating the glass in 15% PVC solution with THF and DMF as solvents (v/v, 4:1) for about 10 seconds. Coated glass samples were dried under the following temperature conditions: room temperature (25°C), 40°C, 50°C, 70°C, and 80°C for at least 3 hours.

Characterization techniques and electrowetting experiment
A tabletop scanning electron microscope (SEM, Hitachi TM-1000) was used to inspect the morphology of the PVC film produced in the experiment. The wetting behavior of the films were investigated by measuring the water contact angle of a sessile drop under an optical tensiometer (Thetalite, Biolin Scientific). Electricallydriven microparticle trapping experiments were performed using a DC power supply with terminals connected to the ITO glass and droplet through a platinum wire. Droplets of deionized water and water with 10 ppm of ZnO were used in the contact angle, electrowetting, and particle trapping experiments. Figure 1 summarizes the experimental steps followed in this paper.

Results and discussion
3.1. Effect of temperature on honeycomb pattern formation Dip-coated glass slides on the PVC solution were dried under different temperatures to investigate its effect on the honeycomb pattern formation on the film. The PVC films dried under room temperature appeared as a white layer on the glass substrate. The appearance of the film eventually changed from white to transparent as the temperature was increased to more than 80°C. Scanning electron microscope images, shown in figure 2, revealed the formation of honeycomb patterns on the PVC film dried at room temperature with an average cell diameter of 6.55 ± 1.17 μm and cell wall thickness of around 2.18 ± 0.54 μm. Stacking of multiple layers of honeycomb structures was also seen. Interestingly, the average cell diameter increased and uniformity was reduced with increasing drying temperature. For films dried at 40°C (figure 2(b)), average cell diameter and cell wall thickness were measured were measured to be around 9.55 ± 1.20 μm and 2.02 ± 0.24 μm, respectively. The cell diameter further increased up to 18.15 ± 3.50 μm while the wall thickness decreased to 0.76 ± 0.17 μm as the drying temperature was increased to 70°C (figure 2(d)). For drying greater than or equal to about 80°C (figure 2(e)), a highly transparent PVC film layer was formed on the glass substrate, corresponding to the absence of a honeycomb pattern. These results demonstrate that the drying temperature could be used to tune the morphology of the resulting honeycomb patterns on the glass substrate.
Fabricating honeycomb patterns on dip-coated polymers has been demonstrated in a process called the breath figure method [13,18]. In this method, the condensed water vapor on the dip-coated substrate serves as a template for forming porous honeycomb patterns. The relatively low temperature of the solution facilitates the condensation, nucleation, and growth of the water droplets on the surface, leading to the presence of droplet arrays on the substrate [19]. It has been observed that the dissolved polymer tends to form a thin layer that envelops the condensed water droplets hence the formation of ordered cells after the solvent and water evaporate [13,15,19]. Since PVC is a hydrophobic polymer, we anticipate that it can prevent the condensed water droplets from coalescing while on the dip-coated coated substrate. Meanwhile, the low temperature of PVC dissolved in THF/DMF provides a temperature gradient that accommodates water condensation on the polymer surface. The increase in the cell size and disappearance of a honeycomb pattern with increasing drying temperature suggest that the condensation, nucleation, and growth of water vapor on the substrate are highly sensitive to the drying temperature. At temperatures of 30 to 40°C, nucleation of smaller water droplets on the PVC film was observed. The slow solvent evaporation in hydrophobic PVC further suppresses the growth of water droplets,  resulting in smaller cell size and thicker walls at lower temperatures. At higher temperatures, minimal moisture condensation and fast solvent evaporation occurred, leading to less defined formation of honeycomb structures. For temperatures between 40°C and 70°C, the solvent evaporates faster that may facilitate the growth of water droplets thereby producing larger cells. At a temperature greater than 80°C, a highly transparent PVC layer was obtained. This might indicate that higher temperature prevents water droplet condensation and honeycomb pattern formation. At temperatures greater than 80°C, the free energy of the system increases which eventually drives the polymer chains to flow towards each other. As the polymer chains with high kinetic energy flow on the glass, more homogeneous films with higher density are formed, resulting in the absence of honeycomb patterns. An analogous mechanism has been described for the formation of polymer latex films by different annealing or heating methods [20,21]. A simplified illustration of the formation of honeycomb patterns at room and elevated temperatures is presented in figure 3.
The FTIR spectra of the produced PVC honeycomb pattern, transparent PVC film, and PVC powder were obtained to investigate the changes in the chemical identity of PVC ( figure 4). Transmission bands at around 2909 cm −1 , 1428 cm −1 , and 1245 cm −1 were consistently observed among the samples which correspond to the CH 2 asymmetric stretching, aliphatic C-H bending, and C-H bending bound on Cl, respectively. The vibrational mode observed at around 1080 cm −1 is associated with the C-C vibration of the PVC backbone while weak bands at around 650 cm −1 correspond to the vibration of C-Cl bond. A dramatic increase in the intensity of the band at 1673 cm −1 corresponds to the formation and vibration of C=C bond of the vinyl during the glass transition of PVC after being dried at 80°C. These results suggest enhanced chemical bonding as the polymers  combine and become homogenized through the cells at high drying temperature. The FTIR spectra reported here is comparable to the spectra for PVC reported in [22,23].

Wettability of the PVC films
The wettability of polymeric layers is a crucial characteristic particularly for coating and membrane applications. Static contact angle (CA) of water droplets was measured using the sessile drop technique as presented in figure 5. The water droplet on ITO glass exhibited a CA of 85.81°signifying its poor hydrophobicity, as expected. A notable increase in the water contact angle (WCA), with a measured value of 126.25°, was observed with the PVC film dried at room temperature. This dramatic enhancement of WCA is due to the roughness introduced by the honeycomb structure. It can be noted that rough surfaces increase the WCA due to the formation of air pockets between the liquid droplet and irregular ridges of the substrate [24]. For PVC film dried at 50°C, WCA decreased to about 104°, suggesting an apparent drop in surface roughness of the PVC layer due to heating. The WCA significantly decreased to 88.01°for PVC as it underwent glass transition after being dried at 80°C. This is attributed to the flattening of the PVC layer as the polymer chain distance expands at a temperature of 80°C and beyond. The behavior of droplet volume vs time for the water droplets on the PVC layers was also obtained to examine the stability of the droplet. Interestingly, static droplets did not exhibit a significant decrease in volume, indicating the stability of droplets on the PVC surface. This further suggests that the droplets do not penetrate the PVC layer even with the prominent presence of porous cells in the honeycomb.

Electrically-driven microparticle trapping on PVC honeycomb patterns
A porous polymer film can potentially be used as a membrane material for water treatment by capturing impurities and particulate matter suspended in the water. Here, we investigate the feasibility of capturing microparticles suspended in water droplets within the porous region of the generated honeycomb pattern using commercial ZnO powder (10 ppm, <3 μm) as model microparticles. An electric field was applied to actuate the droplet on the polymer layer. The electrically-driven change of droplet contact angle on PVC layers dried at 80°C and room temperature is presented in figure 6, along with the respective sessile drop images.
The initial CA of a droplet with suspended microparticles on PVC dried at 80°C was found to be around 98.5°. After applying 50 V (DC), a reduced CA of about 94.1°was noted. The CA continuously decreased until reaching a value of 87.3°after applying 200 V. For the PVC dried at room temperature, an initial contact angle of 115.1°was observed which dropped to 98.7°after applying 50 V. More drastic changes in the CA, with a minimum value of 61.6°, were recorded as the voltage was increased to 200 V. The rapid decrease in CA for PVC dried at room temperature is associated with the electrically-driven penetration of liquid and microparticles into the porous area of the honeycomb structure.
We investigate the effect of the applied voltage on the morphological features of PVC films dried at room temperature and 80°C by capturing SEM images of the surface before and after applying an external voltage (0-200 V) ( figure 7). We observed no remarkable changes in the morphological features of the PVC before and after applying an electric field. This is attributed to the electrical stability and high insulating property of PVC as a dielectric polymer. In fact, PVC has been used in industries as an insulating material for electrical cables and other electronic components [25]. It has been reported that the PVC preserves its dielectric property at varying sweep frequencies, suggesting that the electric field polarity does not alter its structural features [26]. Moreover, a previous study revealed that the chemical integrity of PVC is preserved even after being exposed to varying currents of 16, 25, and 35 A [27]. This excellent insulating property of PVC supports the consistent morphological feature of the PVC film before and after the application of electric field.
Consistent morphology of the PVC with and without the application of an external voltage suggests that the material is a promising candidate as a dielectric layer for electrowetting-on-dielectric (EWOD) systems. This further indicates that the observed electrowetting behavior of the water is driven by the disturbance of the interfacial energy of the droplet caused by an applied voltage, not by changes in the structure of the dielectric layer. Since water is a polar solvent, it is anticipated to respond to any change in the electric field.
We were also able to test the feasibility of using the honeycomb structure to confine and isolate microparticles from a droplet. Since the droplet does not naturally actuate on the PVC film due to its hydrophobicity, an electric field was applied on the droplet to induce wetting over the surface. As observed in the representative SEM images (figure 8), the microparticles were distributed across the honeycomb pattern. The hydrophobic PVC walls tend to direct the water with the microparticles towards the holes of the honeycomb. After the water evaporated, microparticles were seen to reside either inside the hole or within the walls of the honeycombs. Without the application of an electric field, the microparticles form an aggregate in the droplet spot after the water has evaporated, thereby limiting the inspection and isolation of individual microparticles or small clusters. This proof-of-concept for an electrically-driven strategy to capture and trap microparticles within the polymeric honeycombs could be integrated in various substrates for sensing applications such as surface enhanced Raman spectroscopy and other microscopy techniques.

Summary
The effect of drying temperature on the formation of poly(vinyl chloride) honeycomb microstructures is examined in this work. It was revealed that room-temperature drying of dip-coated PVC results in the formation of honeycomb microstructures with smaller cell diameter and thicker cell walls. An apparent increase in the cell diameter with a decrease in cell wall thickness was observed as drying temperature was increased up to 70°C. This temperature dependence of honeycomb microstructures formation could be associated with factors such water droplet condensation and solvent evaporation. No formation of honeycomb patterns was observed at drying temperatures of 80°C and beyond. Contact angle measurements were made to investigate the wettability of the honeycomb structures wherein lower static WCA values were obtained for films dried at high temperature. Finally, electrowetting experiments were conducted to provide proof-of-concept for the feasibility of honeycomb structures as working substrates for electrically-driven microparticle trapping. The honeycomb  PVC layer exhibited more pronounced droplet actuation relative to a flat PVC layer at any given applied voltage. Corresponding SEM images revealed the presence and entrapment of model microparticles within the honeycomb cells and walls. The results presented in this work could provide additional insight on tuning and optimizing the morphological properties of polymeric honeycomb structures based on a facile and straightforward approach. This also opens the possibility of integrating polymeric honeycomb structures into substrates used for microparticle trapping, sensing, and other related applications.