Inorganic surface coating with fast wetting-dewetting transitions for

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Inorganic surface coating with fast wettingdewetting transitions for liquids manipulations Yang Yajie, Liaoliao Zhang, Jue Wang, Xinwei Wang, Libing Duan, Nan Wang, Fajun Xiao, Yanbo Xie, and Jianlin Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02537 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Applied Materials & Interfaces

Inorganic surface coating with fast wetting-dewetting transitions for liquids manipulations Yajie Yanga,b,＃, Liaoliao Zhanga,b,＃, Jue Wangc, Xinwei Wangc, Libing Duanb, Nan Wangb, Fajun Xiaob,d, Yanbo Xiea,b* and Jianlin Zhaob,d

a. International Joint laboratory of Nanofluidics and Interfaces, School of Science, Northwestern Polytechnical University, Xi’an 710072, China b.MOE Key Laboratory of Material Physics and Chemistry under Extraodinary Conditions, School of Science, Northwestern Polytechnical University, Xi’an 710072, China c.School of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen 518055, China d.Shaanxi Key Laboratory of Optical Information Technology, School of Science, Northwestern Polytechnical University, Xi’an 710072, China ＃

These authors contribute equally to this work

*Corresponding author: [email protected] Supporting Information ABSTRACT: Liquids manipulation is a fundamental issue for microfluidics and miniaturized sensors. Fast wetting states transitions by optical methods have proven being efficient for liquids manipulations by organic surface coatings, however rarely been achieved by using inorganic coatings. Here we report a fast optical induced wetting states transition surface achieved by inorganic coatings, enabling tens of seconds transitions for a wettingdewetting cycle, shortened from an hour as typically reported. Here we demonstrate a gravity-driven microfluidic reactor and switched it to a mixer after a second step exposure within minimum within 80 seconds of UV exposure. The fast wettingdewetting transition surfaces enable the fast switchable or erasable smart surfaces for water collection, miniature chemical reaction or sensing systems by using inorganic surface coatings. KEYWORDS: Atomic Layer deposition, microfluidics, photoresponsive wetting transition, liquid manipulations, ZnO

INTRODUCTION Microfluidics have presented the great potentials in chemical synthesis, point of care and many other applications due to its high integrity and low sample consumption by miniaturizing the physical, chemical and biological functionality in a chip. 1-2 Liquids manipulation is a fundamental issue of microfluidics that continuously attracts people’s attentions, since the new methods or materials can create more opportunities for the advanced applications.3-5 Among various methods of liquids handling, it has been pointed out that the optical methods are elegant for microfluidics, since most of chips (Glass, PMMA and PDMS) have good transmission factor for the visible light, thus having more freedom to move liquids without breaking any micro/nano structures. 6-7 Photo-induced wetting states transition materials are excellent options for the liquids manipulations in microscale where the surface tension and capillary forces are the dominant factors. Inspired by biosystems from nature, many photo-responsive wetting states transition materials have been invented or investigated. The

wetting states can be reversibly switched between (super)hydrophilic and (super)hydrophobic, such as ZnO, TiO2, V2O5 and organic surface monolayer by Azobenzene groups.8-16 Organic coatings have demonstrated its fast responses for the wettingdewetting transitions 17-19, however few reports on the inorganic surface coatings, which greatly constraints the applications in microfluidics as the evaporation becomes a serious problem when dealing with small volume of liquids. One of the most popular methods to prepare such surfaces are by growing of the relevant materials on a flat substrate. Taking the “Sol-gel” method as an example, by spraying or spin-coating a thin layer of the photoresponsive colloids on surface, it is possible to produce a wetting transition surface due to the intrinsic properties of particles, in addition the surface roughness formed by assembled particles or growth of crystals enhanced the wetting states to superhydrophobicity. 20-21 The wetting states transition for inorganic coatings are due to the absence/presence of valences in metal oxide, which has higher binding energy of OH- group in water. Take ZnO as an example, the UV light excited the valence band of ZnO to the conduction band, inducing higher surface energy thus hydrophilic surfaces for aqueous water.22 Either by heating, visible light exposure or dark storage, the defective sites can be recovered to the initial states causing reversible transition from hydrophilic to the hydrophobic state. 23-24 As one method in tool box of liquids handling for microfluidics, the transition-duration is critical for practical applications. However, this usually costs over at least an hour by heating 8, 25-26 , or several days in dark storage to achieve a wetting transition cycle according to previous report.8 In this paper, we report a fast-reversible photo-induced wetting states transition surfaces, fabricated by atomic layer deposition of ZnO on micro-pillared glass substrate surfaces. We found the initial hydrophobic surface transit to hydrophilic surfaces within 80 seconds with appropriate operations, and minimum only 30 seconds for the reversed transition process by

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Figure 1. (a) the fabrication process of the chip. (vi) schematic presents the contact angle measurement, not any wetting states. (b) the structures of pillared surface under scanning electronic microscope, (c) the duration of wetting transition of ALD pillared surface, with sol-gel prepared surface as a control group. (d) the typical snapshot of contact angel measurement under hydrophobic state (top) and hydrophilic state (bottom). placing on a hot plate. The fast responses enable the real time manipulation of fluids, complex chemical reactions and detections on open surfaces without chip bonding. As we demonstrated here, under a UV exposure with a photomask, we could produce a microfluidic reactor on this open surface. A second exposure process can make it switched to a typical microfluidic mixer within 80 seconds.

EXPERIMENTS AND RESULTS Fabrications and measurements The fabrication process of the chips including microfabrication and surface coating was schematically shown in Fig. 1a. The flat glass wafers (Pyrex) with thickness of 0.55mm were first cleaned by dipping in phosphoric acid (purity 99%) for over 5 min. After washing and drying, the glass wafers were developed by standard photo-lithography and etching process, including cleaning, spin coating, developing and finally etched by Reactive Ion Etching. Here, we fabricated 4µm depth micro-pillar arrays on the glass substrate with the inter distance of 10 µm, as can be seen from the SEM image of Fig. 1b. Nano peaks were fabricated during the etching of glass, which might help to increase the wetting states according to the previous results. Finally, the micro-pillared glass surface was deposited by 40nm thick ZnO layer, which can be critical for wettabilities.27-29 (The deposition process can be seen in the Supporting Information S1) The photo-responsive materials ZnO are much thinner than film prepared by sol-gel method and homogeneously distributed on the pillared surface. The pillars provide the micro/nano surface roughness enhancing the surface hydrophobicity, instead of the assembling of ZnO materials themselves. This is one of the possible reasons that induces the fast

responses of wetting transitions, where we have detailed discussions in the following sections. The chips were carefully cleaned DI water by ultrasonic cleaner. After blowing the residual water by N2 gas, we characterized the wettability by measuring the contact angles of water drops on the surfaces. To investigate the surface wettability, we measured the contact angle of a water drop about every 10 to 60 seconds when under thermo-treatment or UV exposure, depending on the speed of responses. After measuring the contact angle, the chip was cleaned by N2 gas again and treated for the next step thermo-treatment and UV exposure. The time of UV treatment/heating is counted as the summation of the chip placed under UV light or on the hotplate respectively. The contact angle of water drops on ZnO coated pillared surface rapidly decreases to about 20-30 degree within 80 seconds under the UV exposure with power density of 145 mW/cm2 with 3cm distance from the sample (blue square solid dots in Fig. 1c, EPILEDS).. We then investigated the reversed wetting transition process (from hydrophilic to hydrophobic), by placing the hydrophilic state pillared chip on a hot plate with keeping the temperatures at 180 oC, the surface transit to hydrophobic state (CA=120 degree) within 30 seconds. It seems that the time of a wetting state transition cycle is much faster comparing to the previous reported results. Although the temperatures are recorded as the temperature on the hotplate, we present in the Supporting Information S2 that the temperature of top surface increases rapidly and get saturated within 0.6 seconds, as the chip is thin (550µm). So, the temperature increasing process at top surface can be ignored.

RESULTS Fast transition of wettability As a control group, we prepared a ZnO coated surface by solgel method - spraying colloidal solution of ZnO particles and then dehydrated on the surface.30 (The fabrication process can be seen as Supporting Information S3) The sol-gel chip was placed under the same operational conditions such as UV power density as described above. The transition from initial state (hydrophobic) to hydrophilic took about 6 mins (red square dots in Fig. 1c), over 4 times longer than the ALD coated pillared chip. 31 For a reversed process, the sol-gel chip was placed on the hot plate keeping at temperatures of 180 oC, however took an hour recovering to hydrophobic state as seen from the contact angle. The typical contact angle of water drop can be seen by the snapshot of a camera performed by CCD of contact angle analyzer (JC2000C), which represents the hydrophilic and hydrophobic states of the water drop (Fig. 1d). The Fig. 1d is a schematic picture of contact angle characterization, not indicating any wetting states of the water drop, although previous studies implies the Wenzel states on the similar structures. 32-33 We first investigated the reversibility of the wetting states for ALD ZnO coated pillared surface. As shown in Fig. 2a, we can switch the wetting state between hydrophobic and hydrophilic with excellent repeatability of (can be more than) 10 cycles. All of the experiments were operated under the same operational conditions (180 oC and 145mW/cm2 for heating and UV irradiation respectively).


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Figure 2. (a) the wetting states transition were excellent repeated under heating and UV exposure process. (b) the wetting transition from hydrophilic to hydrophobic state by heating samples under various temperatures. The typical transition time is concluded in the inset figure as a function of temperature. (c) the hydrophobic to hydrophilic transition with two different UV illumination power density. To figure out the optimal working conditions of these two transition processes, the intuitive idea is to investigate the time of transition from hydrophilic to hydrophobic state under various heating temperatures, and the power density of UV light for the process of hydrophobic to hydrophilic. Each experiment was individually operated by keeping the samples at a constant temperatures ranging from 160 to 260 oC, shown in Fig. 2b. The time of transition at 160 oC is about 6 mins, close to the sol-gel prepared samples. 34 After the UV exposure, the pillared surface become hydrophilic again, so that we can repeat the heating experiment with gradually increasing the temperatures on hot plate. The colour of dots in Fig. 2b from warm to cold indicates the operational setting temperatures of individual experiments. The time of wetting states transition was rapidly shortened to minimum 30 seconds from 180 to 220 oC (inset figure of Fig, 2b). When the temperature further increases over 240 oC , the time of states transition starts to increase again, besides the reversibility of the surface wettability becomes less reliable. This possibly due to the over-heating of samples destroys the homogeneity of the atomic layer deposition of ZnO, inducing the less stable reversibility and wettability. Similar results were reported previously that the annealing of ZnO occurs at these temperatures causing the morphology changes of the ZnO film. To illustrate the non-recovery phenomena when working at the temperatures over 240 oC , we investigated the surface morphology of the different thermo-treatment surface discussed in the following sections.23 To figure out the reason of fast responses from hydrophobic to hydrophilic, we studied this transition process under different UV light power. The results in Fig. 2c indicates that higher UV light power accelerate the wetting state transition speed. The time of state transition under 145mW/cm2 UV exposure is 80 seconds, apparently much faster than the transition under 15.3mW/cm2 UV exposure (15minutes, UVP Blak-Ray, B100AP). With fast wetting transitions, it is might caught interested for the real-time liquids handling in the surface microfluidics.35-37 Fast switchable microfluidics on open surface The fast responses of the wetting states transition on this pillared surface make it a promising method to control the fluidic flow within short time on an open surface, avoiding the evaporation of liquids. A photomask printed by inkjet printer on a plastic foil was placed on top of the chip. Locally UV exposed area becomes hydrophilic with patterns corresponding to the layout of

photomask design. Thus, we can achieve microfluidic channels on an open surface from macroscale to microscale wetting states transitions. Since any flow patterns can be produced and erased on the surface, the optical stimulated wetting transition creates variety and complexity of the flows on surface. Here we demonstrated the wetting patterns of abbreviation of our University “NPU” on the surface (Fig. 3a). Dusts, pillar defects and use of noncollimation UV light decreases the quality of the hydrophilic patterns. In principles, the resolution of hydrophilic patterns should be as precise as photolithography.38 However, in practical, the resolution is much lower as we use un-collimated light source. Besides, more importantly, since the water was repelled by the interspacing of pillars at hydrophobic-hydrophilic boundary, the resolution of hydrophilic channels was determined by the unit distance of pillars. Different widths of microfluidic channels - hydrophilic can be produced on the surface after UV exposure with a mask. With placing a water drop on one side of these strips, aqueous water was automatically sucked in the exposed area by capillary forces and got wetted (Fig. 3b). We took the finest water strip of 100µm width microchannel as an example shown in Fig. 3c, where we found the width of water filled channels excellent matched to the design. The dots arrays in the snapshot demonstrate the pillar arrays on the surface. More details and experimental results can be found in the Supporting Information Figure S2. Ghosh et.al. has demonstrated that working at small scales can has some unique advantages for liquids handling, for instance the liquids can be driven antigravity by surface tension and capillary forces which were dominant factors in microscale.39 On the contrary, the gravity can be also driven forces to the liquid flow. Hereby, we presented that a gravity-driven microfluidic reactor created on the surface by first step UV exposure. Taking advantages of the fast response, we could make a second step exposure and switching the microfluidic reactor to a mixer within tens of seconds. (Fig. 3d) We demonstrate a microfluidic mixer on an open surface with layout of a Y-filter (Fig. 3e, f, g), where the fluids driven by joint actions of capillary forces and gravity.40 After the first step exposure with a photomask placed between the surface and UV light source (uncollimated light source to make all process simple and low cost), the exposed area becomes hydrophilic where we could use as a continuous flow channel(schematically shown in Fig.3h).



Figure 3. (a) presents hydrophilic patterns of “NPU” were produced after UV exposure. (b). the hydrophilic strips produced under UV exposure with a photomask and (c) the snapshot of the finest hydrophilic strip perfectly matched with the photomask (100µm width). (d). the schematic picture of the microfluidics on an open surface. After 1st step UV exposure with a Y-filter photo-mask, the exposed area becomes hydrophilic and available for the flows. (e) schematically presents the principle. (f).and (g) experimentally presents the gravity driven microfluidics from the side view and front view respectively. (h). the second step exposure switched the flow patterns to micromixer with well alignment of the existing patterns. (i). The principles of a surface mixer and (j) the experimental flow patterns. Red and green inks were dripped at the ends of channels as shown in the schematic picture of Fig. 3i. Keeping the chip tilting with about 45 degrees, two types of aqueous components continuously flow via the channels and start mixing/reaction at the merged area, driven by gravity and confined by the surface tension for flowing within a confined area. Thus, we could form a surface microfluidic reactor with maximum flow rate of 1mL/h on the surface. However, this is still far from the maximum flow rate that can be operated. We theoretically estimate the maximum flow rate can be achieved by gravity driven flow is about 7.6L/h (see Supporting Information S4). We had demonstrated the fast responses of the wetting state transitions, which can be useful for fast switching the functionality of microfluidic devices (Fig. 3e, i). We made a second step UV exposure with well aligned to the first step exposure, by an inverted “V” shape opening on the photomask introducing two branch channels on the Y-filter. By a second step exposure, we could switch a microfluidic reactor to a simplified version of a microfluidic mixer on the open surfaces. The schematic picture presents the alignment of 1st (blue) and 2nd step exposure (purple) shown in Fig. 3h. As the syringe pump dripping inks on the surface, the different mixed ratio of these two different components were collected in three outlets at bottom of the chips along the horizontal directions, experimentally demonstrated in Fig. 3j. Although we demonstrated some basic functions of the microfluidic devices as the early stage, the open surface microfluidics definitely has more applications in pre-treatment of chemical samples, molecules separations and so on.35 The variety of chip layout and fast responses possibly brings more possibilities and broad interests for the microfluidics on surface.

DISCUSSION Mechanisms of fast transitions

The wetting states transitions of the ALD samples are surprisingly fast compared to the classical inorganic surface coatings, such as prepared by sol-gel method. We suspect the mechanism is possibly relevant to the coating technologies used in this work, besides using strong UV illumination. According to the wetting theories on a rough surface, the micro/nanoscale surface roughness increases or decreases the contact angle, when the original flat surface material is hydrophobic or hydrophilic respectively.4142 For instance, the super-hydrophobic surface can be achieved by the joint actions of surface roughness and hydrophobic coatings. To produce highly rough surface for wide wettability transitions, thick layer coatings need to be prepared by using classical coating techniques such as sol-gel, which usually also needs to coat a micrometer thick photo-responsive materials.30, 34 However, the thick coatings possibly slow down the speed of wetting transitions, for the fully absence/presence of valences in ZnO film. Properly increasing of UV illumination power density and heating temperatures can help to increase the transition speed as we observed in experiments (Fig. 2c). In this work, we coated the atomic layer thick ZnO on a pillared surface, where pillars provide the micro/nano scale roughness enhancing the wetting properties and super thin ZnO film was suspected for reason of the fast wetting states transitions. We characterized the Zn ion spectra of ALD ZnO film on a smooth surface of glass by X-ray photoelectron spectroscopy. The results demonstrate the binding energy of Zn ion become slightly wider after exposed by UV light for 80 seconds(145mW/m2), while the surface turns to hydrophilic (contact angle 85 degree on a plane surface).43-44 This possibly indicates more types of Zn ion exists on the surface. More details can be seen in the Supporting Information Figure S4. After 50 seconds heating of the samples, the peak of Zn becomes narrow again, corresponding to the return of hydrophobic state with contact angles of 93 degree. The XPS results seem to support our hypothesis. The wetting states can transit between hydrophobic and hydrophilic with tens of seconds


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interest for the high throughput applications that liquids manipulations in microscale is not necessary. High power density UV light exposure accelerates the transition speed from hydrophobic to hydrophilic state as shown in Fig. 2c. However, the light exposure also has the side effects of thermo-treatment due to light absorption by the substrate, which can possibly overheat the ZnO layer causing poor reversibility of wetting transitions as we described above. Thus, laser or other types of heating with extreme high power density are not recommended for these experiments. However, once the surface temperatures can be well controlled, high power density light sources might be able to further accelerate the wetting states transitions. Although we presented a wide range of contact angle transitions (from 20-130 degree), it is still possible to further extend this ranges and produce a super-hydrophobic surface, which help to constrain the liquids in hydrophilic areas thus creating high quality flows. This is possible by optimizing the micro/nano structures on the surface, as previous studies.48 Figure 4. The surface morphology of the ALD ZnO on flat glass surface with different thermo-treatment. (a). before thermo annealing. (b). After thermo annealing at 120 oC of 45 min, (c). 200 oC heating for 1 min (d). 260 oC heating for 1 min. UV exposure and thermo-heating. Thus, this possibly creates a simple paradigm to construct fast wetting states transition surfaces, by coating atomic layer thin photo-responsive materials on a pillared surface. Thermo-effects As can be seen from the Fig. 2b, the transition duration from hydrophilic to hydrophobic state strongly relies on the heating temperatures. Increasing the temperatures helps to shorten the transition durations, however the temperatures over 240 oC can cause slower responsive and poor reversibility of the surface wettability. We suspect this is due to the overheating and annealing processes occurred, damaging the inhomogeneity of the ZnO layer distributed on surface thus decreasing the contact angle and poor reversibility. We characterize the surface morphology of the plane glass surface, coated with same thick of ALD ZnO layer, under different thermo-treatment process. As can be seen from the Fig. 4a, the surface is smooth after the atomic layer deposited ZnO (Fig. 4a). After 120 oC annealing of ZnO layer, the smooth surface become more rough with average 15nm surface roughness existing on the surface (Fig. 4b). With heated up to 200 oC, the surface still has an average roughness of 12nm at the scanning area, keeping excellent reversibility of wetting states transition (Fig. 4c). However, once the surface was heated up to 260 oC, the surface become smooth again, possibly due to the bumpy coatings exposing bare glass surface. The decreasing of coating homogeneity might be the reason of poor reversibility and smaller contact angle than working at lower temperatures (Fig. 4d).45 More rough surface leads to better wettability, matched the trends of wetting states transitions after different thermo-treatment process. 46-47 Since the wetting state transition is operated by the UV exposure with a photomask, the resolution of the flow patterns was majorly determined by the UV light source– for example optics collimation, comparable to the resolution of photo-lithography. However, the evaporation of liquids is still a problem when working at the open air, as it becomes more serious as it scales down to the limits of the photo-lithography. Besides, the gravity driven microfluidic devices are not recommended to use microchannel, considering the high fluidic resistance. However, this might be of

CONCLUSION In this paper, we demonstrated a fast and reversible wetting transition surface by atomic layer deposition of ZnO on a micropillared surface. The time for a cycle of hydrophobic and hydrophilic transition was shortened within tens of seconds instead of at least an hour by UV exposure and heating process respectively. These fast responses enable the real-time functionality switching of the microfluidics on an open surface. Hereby, we demonstrated a microfluidic reactor by inserting a corresponding photomask between the UV light source and chip. After a second step UV exposure, the microfluidic reactor can be switched to a microfluidic mixer within 80 seconds. This fast responsive surface possibly can provide a microfluidic platform in an open surface without chip bonding.

ASSOCIATED CONTENTS Supporting Information The supporting information is free of charge on the ACS Publications websites at DOI: Atomic layer deposition process of ZnO on pillared surfaces; The simulation of temperature increases at top surface of chip; Preparation of ZnO deposition by Sol-gel; The hydrophilic patterns after UV exposure on pillared surface; Maximum operational flow rate on chip; Zn ion spectra from X-ray photoelectron spectroscopy. Corresponding Authors *E-mail: [email protected] ORCID Yanbo Xie: 0000-0002-0358-9426 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge the support from Analytics and Testing Centre in NPU, and financial support from NSCF (Grant No. U1730133, U1732143, 21602176), Natural Science Basic Research Plan in Shanxi Province of China (Program No. 2017JQ2039) and the Fundamental Research Funds for the Cen-



tral Universities (Grant No. 3102017jc01001). We acknowledge the support from Analytical and Testing center of Northwestern Polytechnical University in Xi’an.

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