Inhibiting Oil Overflow in Electrowetting Display through Selectively Hydrophilic Modification of the Pixel Wall Based on Polydopamine and Polydopamine/Polyvinylpyrrolidone via Regional Protection Strategy

Due to the advantages of low power consumption, visual health, flexibility, and the potential of color dynamics, Electrowetting Display (EWD) has obtained rapid development in recent decades. One issue that affects the display effect and the lifetime of EWD devices is the overflow of oil across the pixel wall when a high electric field is applied. This paper adopts a method through selectively hydrophilic modification of the pixel wall based on dopamine self‐polymerization via a regional protection strategy by liquid. The static contact angle decreases from 75.4° for the original surface to 48.8° for the modified surface. Due to the limited hydrophilicity of dopamine itself, it is difficult to further improve the surface hydrophilicity of the pixel wall. Taking polydopamine as the intermediate layer, the pixel wall is further modified with polyvinylpyrrolidone (PVP) through multiple hydrogen bonds. The results show that the static contact angle decreased from 48.8° to 29.2° again, resulting in a larger wettability gradient between the pixel wall and the hydrophobic area. In addition, the phenomenon of oil overflow is significantly prevented. When the EWD device is operated at a high voltage (60 V), no oil flows over the pixel wall.


Introduction
As a means of manipulating solid-liquid wettability under the action of external fields, electrowetting technology has found wide application in display technology, [1] zoomable microlenses, [2] microfluidic laboratories, [3] phase change heat transfer, [4] energy conversion, [5] micromotor systems, [6] and other fields.As a new reflective display technology, electrowetting display technology has the advantages of low energy consumption, visual health, and flexibility of electrophoretic electronic paper display products.At the same time, it breaks through the technical bottlenecks of "color" and "video" that currently limit the application of electronic paper displays, and has achieved rapid development in recent decades.
In the electrowetting display, the competition between the electrowetting force and the interfacial tension of the liquid is used to control the shrinkage and spreading of the color oil to realize the pixel switching and the regulation of the grayscale to finally achieve the color switching effect of the display. [7]Without power, the oil film spreads in the pixel and shows the color of the oil.When a voltage is applied, the resulting electric field changes the wettability of the surface of the hydrophobic insulation layer.The water pushes the oil aside, causing it to shrink into the corner, and the pixels show the color of the substrate.Therefore, to prevent the oil from overflowing, the pixel wall material must have high hydrophilicity.However, when the photoresist is used to make the pixel wall of the electrowetting display, it has weak hydrophilicity, [8,9] which causes the oil to flow over the pixel wall under the electric field and affects the display effect.To increase the hydrophilicity of the pixel wall, various methods have been developed to modify the surface hydrophilicity of photoresists, including corona discharge modification, [10,11] heat treatment, [12,13] plasma treatment, [14,15] and ultraviolet ozone treatment. [16]However, these methods have the disadvantages of requiring high electric field, high temperature resistance, complicated operation, low stability of the modified surface, etc.Therefore, realizing a simple, stable and controllable hydrophilic modification of the pixel wall is the key to solve the problem of oil overflow across the pixel wall.One strategy is to apply hydrophilic surface modification reagents directly to the pixel wall.
With its numerous functional groups, dopamine is an excellent agent for hydrophilic surface modification.It can be selfpolymerized by oxidation and adhere to various substrates at a pH of 8.5 [17] and exhibits good stability.[25] Since the process of dopamine self-polymerization is simpler and more controllable, and the polymerization process does not require additional heating or other conditions as it can be carried out just in a weakly alkaline aqueous solution.Thus, in this work, dopamine was used for the hydrophilic modification.
Although dopamine is widely used as a modified layer, there are still some limitations to the further application of dopamine as a surface modification reagent.First, the common surface modification technology based on precision patterns can only achieve 2D modification, which cannot meet the requirements of surface modification with complex 3D microstructure.In addition, it is difficult to avoid direct contact with the unmodified area and the risk of contamination.Second, the experimental facilities for the solution lithography process are generally complicated and the precision of pattern control is quite difficult.Therefore, it is still a great challenge to achieve the selective hydrophilic modification of a confined region for dopamine.
To realize the selective hydrophilic modification of the pixel wall, the self-assembly filling method is used in the fabrication of the electrowetting display in this work.The self-assembly filling method [26] is performed in a container that contains an electrolytic solution.The oil is injected into the groove formed on the surface of the polar liquid at the three-phase boundary.When the oil comes into contact with the hydrophobic fluoride coating, it is automatically filled into the pixel by raising the surface of the polar liquid.The hydrophilic pixel wall prevents the oil from remaining on the pixel wall, protecting the hydrophobic area and restricting dopamine polymerization to a specific area (the pixel wall).In this work, under the premise that the hydrophobic surface in the pixel structure is protected by filled oil, the unprotected area (i.e., the pixel wall) is selectively modified hydrophilic based on dopamine to achieve a larger wetting gradient between the pixel wall and the hydrophobic area, which prevents the flow of the oil across the pixel wall under high voltage and further improves the stability of the electrowetting display.In addition, the modification of the domain in a limited area provides the ability to design and precisely control multi-level structural materials, which greatly improves the application range.

Results and Discussion
The structure of the electrowetting display device mainly includes upper and lower substrates, indium tin oxides (ITO) electrode, dielectric layer, hydrophobic layer, pixel wall, colored oil, polar liquid, and packaging material, as shown in Figure 1.The upper and lower substrates are made of two transparent glasses, and the conductive layer ITO is on the inner side of the substrates, and the ITO serves as a common electrode.Above the driving electrode, the dielectric layer, the hydrophobic layer, and the pixel grating structure layer are sequentially located.The pixel cells are filled with a non-polar oil liquid, and the polar liquid is filled between the upper substrate and the non-polar oil layer.The upper substrate and the lower substrate are bonded together and packed with a seal.The manufacturing process of dopamine-modified electrowetting display devices mainly consists of four parts: the fabrication of the dielectric layer and the hydrophobic insulating layer, the fabrication of the pixel wall structure, the modification of the dopamine-restricted area, and the assembly and packaging of the device as described in Experimental Section.
The hydrophilic group -OH in the molecular structure of dopamine provides the molecular basis for improving the wetting behavior of photoresists.From Figure 2, it can be seen that the photoresist (PR) exhibits weak hydrophilicity, with a static contact angle of 75.4°.The contact angle of the photoresist decreased from 75.4°to 48.8°after 24 h of modification with polydopamine (PDA), indicating that the surface of the photoresist was successfully coated with polydopamine.To investigate whether dopamine concentration affected the wettability of the photoresist, the concentration of added dopamine hydrochloride was increased from 5 to 35 g L −1 .The contact angle hardly changed, indicating that the dopamine hydrochloride concentration had no significant effect on the modification of the photoresist.Contact angle hysteresis is a very important physical phenomenon in the electrowetting effect, which has an important effect on the microdynamics based on electrowetting.The droplet on an ideal smooth surface is in thermodynamic equilibrium and has only one contact angle, the equilibrium contact angle.However, the contact angle of the actual solid surface is not just one value given in Young equation, Wenzel equation and Cassie equation, but fluctuates between two relatively stable angles, which is called contact angle hysteresis phenomenon.When the volume of the droplet increases, the droplets show Advancing Contact Angle (ACA).On the other hand, if the volume of the droplet continuously decreases, the droplets show Receding Contact Angle (RCA) at the three-phase solid-liquid-gas contact line.The difference between ACA and RCA is defined as contact angle hysteresis. [27]The non-uniformity of the chemical composition of the solid surface and the roughness of the microscopic layer are the main reasons for the phenomenon of contact angle hysteresis. [28,29]Therefore, the contact angle hysteresis of photoresist surfaces before and after dopamine modification was characterized.As shown in Figure S1, Supporting Information, compared with the photolithographic collagen film, the ACA and RCA of the polydopamine-modified photoresist decreased and the hysteresis increased.The difference in wettability between the pixel wall and the hydrophobic layer is very important to prevent oil from flowing over the wall.Therefore, a photoresist with a concentration of 5 g L −1 dopamine hydrochloride was selected as the optimal group for the electrowetting display.
The variation of surface roughness in the surface topography will affect the contact angle hysteresis, thus affecting the opening rate of the electrowetting display device.Therefore, the microstructure of the film surface was observed by atomic force microscope (AFM), and the results are shown in Figure 3.The surface of the photoresist is smooth, and the Ra is about 0.52 nm on average.After polydopamine modification, the surface roughness was improved to a certain extent, and the Ra was 1.72 nm, which was caused by the polydopamine coating on the surface of the photoresist.
A layer of polydopamine was formed on the surface of the photoresist by self-polymerization of dopamine under alkaline conditions.As shown in Figure 4a for fourier transform infrared spectroscopy (FTIR), the wide peak between 3100 and 3600 cm −1 is the stretching vibration peak of N─H/O─H.The absorption peaks at 1600 and 1506 cm −1 belong to the formant peak of benzene ring skeleton C═C and the bending vibration peak of N─H, respectively.The hydroxyl content of the photoresist surface increased significantly after dopamine modification, indicating that the polydopamine layer was successfully modified on the photoresist surface.The X-ray photoelectron spectroscopy (XPS) spectra of the photoresist before and after dopamine modification are shown in Figure 4b.The PR/PDA surface detected a new N element by energy-dispersive X-ray spectroscopy (EDS) mapping, which is derived from the polydopamine layer.The elemental analysis of the photoresist before and after dopamine modification is shown in Figure S2, Supporting Information.There was no element N on the surface of the photoresist originally while it appeared after dopamine modification.As shown in Table 1, the content of N element on the photoresist increased from 0% to 15.87% after dopamine modification, which proved that dopamine self-polymerized on the photoresist.
The morphology of the pixel wall was characterized using an optical microscope.As shown in Figure S3b, Supporting Information, the dark brown dopamine polymerized only in the white area (the pixel wall), while the pink area (the hydrophobic area) was free of dopamine (Figure S3a, Supporting Information), achieving a good domain confinement effect and a larger  wettability gradient between the pixel wall and the hydrophobic area.Figure S3c,d, Supporting Information, shows the 3D profilometer images before and after dopamine modification.A polydopamine layer was added to the pixel wall, and the structural morphology after modification is clean and clear, proving that the polydopamine layer does not affect the processing of the photoresist.
To observe the distribution of the polydopamine layer on the pixel wall, scanning electron microscope (SEM) was used to further observe the morphology of the surface before and after dopamine modification.The results are shown in Figure 5. Compared with the pixel wall before modification (Figure 5b), polydopamine particles appeared on the pixel wall after dopamine modification (Figure 5d), and the surface roughness of the pixel wall increased.Furthermore, it is evident from Figure 5c that there is no polydopamine in the unmodified region, proving that the reaction-limiting strategy is effective in this region and that oxidative self-polymerization of dopamine occurs only at the pixel wall.
The high light transmittance of the electrowetting display is very important and improving the utilization ratio of the ambient light helps to reduce the loss of the device in reflecting the light to optimize the contrast of the device and the display effect.Figure S4, Supporting Information shows the transmission spectrum and optical photos of photoresists in the visible range (400-780 nm) before and after dopamine modification.The photoresist modified with polydopamine is no longer transparent but light brown.The transmittance of the photoresist before and after dopamine modification was 98.2% and 94.3% at 550 nm, respectively.After modification, the transmittance was reduced by only 3.9% (at 550 nm).It can be seen that photoresists modified with dopamine can increase hydrophilicity to a great extent while maintaining high transmittance.
The opening rate refers to the maximum ratio between the exposed substrate area and the total pixel cell area after oil displacement in the pixel cell under the influence of the driving voltage.The test of this performance can illustrate the shrinkage of the oil.As shown in Figure 6a, the EWD devices modified with dopamine (DA) could achieve a synchronous increase in open-ing rate when the driving voltage increased, and the opening rate reached 74.02% at 60 V voltage.However, the traditional EWD devices (TD) experienced the phenomenon that oil flowed over the pixel wall at 48 V, and a stable opening rate could not be achieved at high voltage.To further observe the condition and stability of the oil, the shrinkage of the oil at different voltages was recorded by optical microscope.As shown in Figure 6b, at an operating voltage of 50 V, the oil in traditional EWD devices had already flowed over the wall and gathered together, while the oil in the dopamine-modified EWD devices still exhibited normal shrinkage.When the drive voltage increased to 60 V, the dopaminemodified EWD devices still maintained a good opening condition.
In the images of the EWD devices after several passes at high voltage, the oil in some pixel cells of the traditional EWD devices flowed over the pixel wall into neighboring pixel cells in the off state, and the overall color was blotchy and uneven (Figure 7a).However, the EWD devices after dopamine modification still showed good oil distribution (Figure 7b).
Photoelectric performance testing of traditional EWD devices typically uses an impedance analyzer to monitor the capacitancevoltage characteristics (C-V curve) and analyze the switching consistency of each pixel unit.A precision impedance analyzer was used to output the voltage and test the capacitance value.A cycle voltage of 0 to 30 V was set, and the step voltage was 1 V s −1 .The capacitance value was recorded after each voltage change.As shown in Figure 8a, the capacitance values of the two groups of samples are similar in the two switching states, which proves that the EWD devices also have good switching reversibility and stability after dopamine modification process.Response time refers to the speed at which the pixels respond to input signals, that is, the time required for the pixels to switch from dark to light (reaching a maximum optical modulation of 80%) or from light to dark.It is usually divided into two parts: On-state and Offstate.The shorter the response time, the less the user feels the drop shadow when viewing dynamic images.On the other hand, too long a response time results in residual images that do not meet video display requirements.The response time of the electrowetting display depends on the rate at which the polar liquid replaces the oil at the applied voltage, that is, the rate at which the contact angle changes.The EWD response time before and after dopamine modification is shown in Figure 8b.It can be seen that the response time of EWD before and after dopamine modification is about 20.4 and 18.9 ms, respectively, indicating that both can achieve a fast response in the open state.The capacitancevoltage curves and the response time curves show that the photoelectric properties of the EWD devices are not affected after dopamine modification.From another aspect, since the photoelectric response of the electrowetting display is very sensitive to impurities on the surface of the hydrophobic layer within the pixel, it is demonstrated that the oil protection method achieves a good limiting effect and there is no physical or chemical contamination in the non-reactive region.
Due to the limited hydrophilicity of dopamine itself, the enhanced surface hydrophilicity of the photoresist is also limited.Therefore, further hydrophilic modification of photoresist based on dopamine is considered.[32][33] After PDA is firmly deposited on the photoresist surface, since its surface contains a large number of keto groups and a small number of hydroxyl groups, it can achieve a good binding effect in the subsequent reaction with PVP through multiple hydrogen bonds. [34]ence, in the following discussion, a polydopamine intermediate layer grafted with a PVP layer was prepared to further enhance the surface hydrophilicity of the photoresist and the detailed process is described in Experimental Section.Meanwhile, its structure was characterized and the properties of the EWD devices were studied.
As shown in Figure 9, the value of static contact angle decreased from 48.8°for the photoresist/PDA layer to 29.2°for the photoresist modified by PDA/PVP hybrid layer, showing that the hydrophilicity of the photoresist/PDA/PVP surface was further improved.
To further observe the microscopic surface morphology, SEM, AFM and EDS were performed.Figure 10a,d,g,j shows that the surface morphology of the photoresist is relatively flat and smooth, while Figure 10b,e,h,k shows that after rinsing with deionized water, many nano-sized particles still adhere firmly to the surface of the photoresist after being modified with dopamine, which increases the surface of the photoresist.Figure 10c,f,i,l shows the surface morphology of photoresist after further PVP modification.On the surface, many nanoscale particles are interconnected, and the roughness is improved.These results are consistent with the AFM tests shown in Figure S5, Supporting Information.After further treat-ment with PVP, the photoresist/PDA/PVP surface roughness further improved to 2.19 nm compared to 1.72 nm for the photoresist/PDA surface.For a hydrophilic surface, increasing the surface roughness can improve the wettability of the surface.
As shown in Figure S6 and Table S1, Supporting Information, the content of C elements on the photoresist surface decreased from 69.75% to 61.93% after dopamine modification and increased to 71.48% after PVP modification.The content of O elements on the photoresist/PDA surface decreased from 22.20% to 15.11% after PVP modification.The content of N elements on the photoresist surface increased to 15.87% after dopamine modification and decreased to 13.41% after PVP modification, indicating that PVP was further modified on the surface of photoresist/PDA.
After 24 h of dopamine polymerization alone, the color of the unmodified photoresist changed from transparent to light brown.Under the same conditions, the color of the photoresist samples further modified with PVP was lighter than that of the dopamine-modified samples, indicating that PVP affected the morphology of the dopamine-modified photoresist during the modification process.The transmittance spectrum and optical photos of the photoresist before and after modification in the visible region (400-780 nm) are shown in Figure S7, Supporting Information.The transmittance of the PVP-modified photoresist was 94.4% (at 550 nm).In the visible region (400-780 nm), the transmittance of PVP was always higher than that of dopamine.This indicates that PVP can further increase the surface hydrophilicity of photoresist/PDA and improve the transmittance.
After further modification by PVP, the surface wettability of the photoresist is further improved.Therefore, the effect on the inhibition of oil flowing over pixel walls by further PVP modification are evaluated by the photoelectric properties such as the opening rate, CV curve, and response time of the EWD devices.
The test results for the opening rate are shown in Figure 11a.It can be seen that the PVP-modified electrowetting display device achieves a synchronous increase in opening rate when the control voltage increases, and the opening rate reaches 76.78% when the voltage is 60 V. Compared with the electrowetting display device modified solely with dopamine, the further modified electrowetting display device with PVP always maintains a larger opening rate between 27 and 60 V, and the maximum gap of the opening rate reaches 5.97 % at 50 V.However, the traditional electrowetting display device has the phenomenon that oil flows over the wall at 48 V, and the stable opening rate cannot be achieved at the higher voltage.To further observe the condition and stability of the oil as a function of voltage, the shrinkage of the oil at different voltages was recorded with the optical microscope.As shown in Figure 11b

Conclusion
In this work, the hydrophilic modification of the pixel wall based on dopamine was performed, which provides two viable solutions to the issue of oil flowing over the pixel wall in the electrowetting display device.The first solution is to use polydopamine as the sole modified layer.Dopamine is oxidized under alkaline conditions and self-polymerized on the photoresist surface to improve its hydrophilicity.The second option is to use the polydopamine layer as an intermediate layer, which was further grafted with PVP to improve its hydrophilicity via the multiple hydrogen bonds between PVP and PDA layer.Photoresist samples prepared by the above two methods were characterized by SEM, EDS, AFM, FTIR, and XPS.The hydrophilicity of PR/PDA and PR/PDA/PVP surface was investigated by static contact angle.The photoelectric properties of the modified EWD devices    were characterized by testing opening rate, capacitance characteristics and response time.
Through the above experiments, the following conclusions are drawn: 1) As the oil protected the hydrophobic area, dopamine oxidized and self-polymerized on the pixel wall, forming a poly-dopamine layer.There was no obvious contamination in the hydrophobic area, the selectively hydrophilic modification in a restricted area was realized, and the fabrication process was simple.The surface of the photoresist obtained by the modified method exhibits good hydrophilicity, the static contact angle decreases from 75.4°to 48.8°, and the surface roughness increases from 0.52 to 1.72 nm.
Characterization: Static contact angle and contact angle hysteresis were tested for photoresist surface wettability before and after modification using Powereach's JC-2000D1 contact angle meter.Deionized water with a drop volume of 10 μL was chosen as the test fluid.The average value of each sample was measured three times.Infrared analysis of photoresist before and after modification was performed using Nicolet 6700 Fourier transform infrared spectrometer.The scanning wavenumber ranges from 500 to 4000 cm −1 , the scanning line number is 32, and the spectral peak resolution is 4 cm −1 .Surface roughness before and after modification was characterized using Brock Multimode 8 atomic force microscope, and surface structure was analyzed using NanoScope 9.1 software.AXIS SUPRA X-ray photoelectron spectrometer from Shimazu was used to record X-ray photoelectron spectra.The transmission spectrum of glass was measured with the Perkin Elmer Lamda 950 UV-vis-NIR spectrometer.Under an accelerated voltage of 20 kV, the surface morphology and elemental distribution of the samples before and after modification were examined using a field emission scanning electron microscope and an energy dispersion spectrometer.Prior to testing, a 5-10 nm thick gold layer was sprayed onto the samples using a vacuum ion sputter coating system.Morphology before and after modification was examined using Hours CSUC-200C light microscope.Morphologies before and after the modification were characterized using Leica DCM8 3D morphologies.The TH2828 LCR impedance analyzer was used to output the voltage and value of the test capacitance.a voltage of 0 to 30 to 0 V was set, the step voltage was 1 V s −1 , and the switching consistency of each pixel unit was analyzed.Rigol DG −1032 waveform generator and Agitec's ATA-2161 dual-channel independent amplifier were used to provide waveform drivers for the instrument (below 30 V voltage with 50% duty cycle 5 Hz).The output of the waveform amplifier was connected to the PCB, and the PCB of the sample and control switch was connected through the test terminal.The response time of the switch was measured with a colorimeter.
Fabrication of Dielectric Layer and Hydrophobic Insulating Layer: ITO glass substrate was ultrasonically cleaned in pure water at 30 °C for 10 min, and then the substrate was dried with N 2 .After finishing the ultrasonic cleaning, the substrate is put into the UV-ozone device for 5 min secondary cleaning, and the contact angle of the glass surface is about 10°after cleaning.First, the dielectric layer was prepared, and the negative photoresist material HN-018N was spun onto the ITO glass substrate.Then, it was heated on the hot plate at 110 °C before drying, removing part of the solvent, and preliminary curing was performed.The photolithographic device was used to expose the whole surface for 17 s, and the exposure intensity was 17 W cm −2 .Then, the exposed substrate was placed on the hot plate at 110 °C for soft drying to reduce the expansion of the photoresist and remove the residual solvent.The photoresist in the unexposed area was dissolved with 0.4 wt% potassium hydroxide solution as developer and removed.Baked in a dust-free oven at 190 °C constant temperature for 60 min to volatilize the remaining developer and increase the structural hardness and adhesion between the substrate to obtain a thickness of about 400 nm dielectric layer.The second step was the fabrication of a hydrophobic insulating layer.The hyflon layer was deposited by a standard spin coating method, and the substrate was pre-cured on the hot plate at 85 °C for 90 s, so that the film was formed first and a large amount of solvent was volatilized.Then, the remaining solvent in the film was volatilized in a dust-free oven at 185 °C for 30 min at constant temperature.A hydrophobic insulating layer with a thickness of about 400 nm was obtained.
Fabrication of the Pixel Wall Structure: Due to the extremely low surface energy of the hydrophobic surface, it was difficult for the photoresist solution with hydrophilic energy to be applied to the surface of the hydrophobic layer.Reactive ion etching (RIE) was used to modify the hydrophobic surface by oxygen plasma to increase the hydrophilicity of the surface.Then, the negative photoresist material HN-008N was spun onto the surface of the modified hydrophobic layer.After pretreating the hot plate to remove part of the solvent, the photoresist was exposed through a mask in the photolithography process for 18 s at an exposure intensity of 17 W cm −2 , and then the hot plate was used for post-drying.Then, 0.4 wt% KOH was used as alkaline developer and rinsed with pure water for 3 min.After drying, the structure was cleaned again with a reactive ion etching machine to remove the developer and other organic impurities remaining on the surface.The resulting pixel wall height is 3.5 μm and the pixel cell size was 150 × 150 μm.To restore the original hydrophobicity of Hyflon in the pixel cell, a high-temperature reflow method was applied, and the substrate was placed in a dust-free oven for 210 °C and 1 h to minimize the interfacial energy and restore the hydrophobicity.A stable pixel wall structure with alternating hydrophilic and hydrophobic properties was obtained.
Modification of the Dopamine-Restricted Area: Oil filling was one of the most important processes in the manufacture of electrowetting display devices.EWD devices contained two kinds of non-polar organic dye oil and polar liquid, which were not soluble in each other.The oil material was usually formed by dissolving nonpolar small dye molecules in grease solvent.The oil filling process was completed by the self-assembly filling method in a pure water environment, that is, the oil was filled by water from the bottom to the top to protect the hydrophobic area.Dissolve 1 g of dopamine hydrochloride in 50 mL of water solution, prepare a 20 g L −1 DA -HCl solution, add an appropriate amount of Tris buffer, and adjust the pH of the solution to about 8.5.The substrate was taken out after standing in the reaction solution for 24 h, and the filling oil and surface contaminants were washed off to obtain the dopamine-modified pixel wall structure.
Assembly and Packaging of the Devices: Oil filling on the lower substrate again and paste encapsulation adhesive on the upper substrate.Through the UV ozone machine, the upper substrate was cleaned for 10 min to reduce the subsequent and the lower substrate when encapsulating the adhesive inside the bubble.Finally, the upper and lower substrates of the EWD devices were bonded by underwater packaging.The packaged device was dried and pressed for 18 h.
The Preparation Process of the Photoresist/PDA/PVP Sample: As shown in Figure S9, Supporting Information, first, 0.25 g of dopamine hydrochloride was dissolved in 50 mL of aqueous solution to prepare a 5 g L −1 DA -HCl solution, and an appropriate amount of Tris buffer was added to adjust the pH of the solution to ≈8.5.The photoresist substrate was removed after standing in the reaction solution for 24 h.The impurities on the packed surface and the PDA particles that were not firmly adsorbed were washed off.The resulting dopamine-modified substrate was dried in a vacuum oven to constant weight.Then, the dopamine-modified photoresist substrate was immersed in an aqueous solution of 2.0 g L −1 PVP, stirred for 24 h, then taken out and thoroughly rinsed with deionized water to remove the PVP that did not bind firmly.After drying to constant weight in a vacuum drying oven, the photoresist/PDA/PVP samples were obtained for surface morphology and chemical composition characterization.

Figure 1 .
Figure 1.a) Structure diagram of electrowetting display device; b) Process flow chart.

Figure 2 .
Figure 2. Static contact angle before and after modification.

Figure 4 .
Figure 4. a) FTIR of photoresist; b) XPS spectra before and after dopamine modification.

Figure 5 .
Figure 5. SEM images before and after dopamine modification.
, the oil in adjacent pixel cells in traditional EWD flows over the pixel wall at a drive voltage of 50 V, while the oil in PVP-modified EWD still maintains a normal shrinkage state.When the drive voltage increases to 60 V, the oil in adjacent pixel cells in traditional EWD overflows and gathers together, while PVP-modified EWD still maintains a good open state with clear pixel cell boundaries and no oil flow over the wall.The results of capacitor tracking capability with voltage are shown in Figure S8a, Supporting Information.The EWD devices modified with PVP/PDA still show good capacitor tracking capability with voltage, indicating good voltage control of the device.Moreover, the capacitance values of the two groups of samples are similar in the two switching states of the device, proving that the EWD devices fabricated by this method have good switching reversibility and stability.The results of the EWD response time before and after further PVP modification are shown in Figure S8b, Supporting Information, as 23.7 and 23.4 ms, respectively, both of which provide a fast response in the open state.

Figure 6 .
Figure 6.a) The opening rate of EWD devices before and after dopamine modification.The illustration shows the oil flowing over the wall.b) The opening state of the pixels with the voltage change.

Figure 7 .
Figure 7. EWD devices operated multiple times at high voltages in the off state a) before and b) after dopamine modification.

Figure 8 .
Figure 8. Photoelectric performance test of the EWD devices before and after dopamine modification.a) C-V curve b) response time curve.

Figure 11 .
Figure 11.a) The opening rate of EWD devices before and after PVP further modification.b) The opening state of the pixels with the voltage change.

2 )
Compared to traditional EWD devices, dopamine-modified EWD devices have better wettability boundary control.When the drive voltage is increased to 60 V, the opening rate of the EWD devices steadily increases to 73.45%, and no oil flow over the pixel wall.When the device is turned off, the oil maintains a stable propagation state.At the same time, the EWD devices modified with dopamine show good capacitance-voltage following capability, fast response in the open state, and good photoelectric performance.3) Based on the self-polymerization of dopamine intermediate layer, polyvinylpyrrolidone was further modified on the photoresist surface.The static contact angle decreased from 48.8°to 29.2°, and the surface roughness increased from 1.72 to 2.19 nm.When the drive voltage is increased to 60 V, the opening rate of the device stably increases to 76.78%, which is further improved.Further modified with PVP, EWD devices show good capacitance-voltage following capability, fast open state response and good electrical stability.

Table 1 .
Elemental Composition Determined by EDS.