Electrically modulated adhesive hydrophobicity for patterning various microstructures

Accepted Manuscript Electrically modulated adhesive hydrophobicity for patterning various microstructures

Li Xiangmeng, Wei Huifen, Zhu Xijing PII: DOI: Reference:

S0167-9317(19)30034-6 https://doi.org/10.1016/j.mee.2019.02.008 MEE 10914

To appear in:

Microelectronic Engineering

Received date: Revised date: Accepted date:

12 October 2018 21 January 2019 21 February 2019

Please cite this article as: L. Xiangmeng, W. Huifen and Z. Xijing, Electrically modulated adhesive hydrophobicity for patterning various microstructures, Microelectronic Engineering, https://doi.org/10.1016/j.mee.2019.02.008

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Essential Title Page Information Electrically Modulated Adhesive Patterning Various Microstructures

Hydrophobicity

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Li Xiangmeng1,2,1,* [email protected], Wei Huifen3,1, Zhu Xijing1,2

Shanxi Provincial Key Laboratory of Advanced Manufacturing Technology, North

School of Mechanical Engineering, North University of China, Taiyuan 030051,

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School of Instrument and Electronics, North University of China, Taiyuan 030051,

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China *

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Corresponding author.

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University of China, Taiyuan 030051, China

These authors contribute to the work equally.

ACCEPTED MANUSCRIPT Abstract In this study, we present an approach for patterning various microstructures using modulated electric-field. Experimentally, electrical potential was applied to the water droplet on the micropillar or microhole array textured dielectric surfaces, contact state transition could occur from Cassie-Baxter mode to Wenzel mode, and therefore enhance the adhesive hydrophobic property. The liquid droplet would spread over the

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microstructure dielectric surface and fill up the asperities around the micropillar or microhole array structures, which was indicated clearly by UV-curable polymer

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deformation under electric-field and solidification under UV irradiation. By this

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means, microstructures of UV-curable polymer with various morphologies including microlens, microbump and micropillar array structures could be generated by tuning

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the applied voltage.

Keywords: Electrowetting on Dielectric; Microstructure; Adhesive Hydrophobicity;

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Wetting Transition

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1. Introduction Electrowetting on dielectric (EWOD) is a typical digital microfluidic actuation approach where a droplet on dielectric layer surface will reduce the contact angle of the droplet by applying an electric field [1-6]. When the voltage increases, the contact angle decreases until a saturation occurs at a higher enough electric field. Efforts have

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been made to improve the rapid response and reversible superhydrophobicity of the EWOD for applications in lab-on-a-chip or liquid lenses [2-5]. In the last decade,

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experimental and theoretical work have been done by researchers to reveal the mechanism for controlling over the EWOD effect on both natural and artificial

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superhydrophobic surfaces [7-11]. It was reported that droplets tend to recover from contact angles on the surface of an ideal smooth dielectric layer after the electric field

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was removed. Mugele et al. have provided review and discussion on the phenomena and mechanism for electrowetting with alternating voltage [12].

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In addition to the reversible EWOD, irreversible EWOD effect on the rough micro or nanotexture decorated surfaces was also investigated [13-17]. For instance, Krupenkin

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et al. reported that wetting state could be changed during the electrowetting on the

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silicon nanostructure surface, indicating that UV-curable polymer could be driven into the asperities through electric field [15]. Choi et al. conducted an electrowetting experiment on the surface of a polymeric microlens array dielectric layer and found

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that when the wetting properties would be reversible under low voltage, while it would become irreversible at higher voltages [16]. Liu et al. reported that ultralow

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voltage irreversible electrowetting dynamics could occur for an aqueous drop on a stainless steel surface, and the droplet contact angle on the rough surfaces had a certain degree of irreversibility due to energy barrier after the voltage was removed [17]. For dielectric surfaces decorated with regular microstructures, the contact angle hysteresis was significant, so that the energy barriers and energy loss might restrain the recovery characteristics [18-20]. Moreover, electrowetting effect have been widely employed to manipulate the liquid wettability. Shao et al. developed electrically triggered nanoimprinting methods to pattern micro and nanostructures. By using a modulated electric-field, microstructures

ACCEPTED MANUSCRIPT including aspherical microlens array, plano-convex microlenses, micropillars array with varying aspect ratio for cell-cultivation, and large area nanocavities for light emission diodes, etc., could be obtained [21-24]. Recently, a novel approach was reported to decrease the saturated contact angle in EWOD through control over the charge trapping at liquid-solid interfaces [25], and supercapacitors of extremely high performance were developed employing electrowetting effect in preparing the solid

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gel electrolyte [26].

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In our previous work, microlenses with controllable numerical aperture on SU-8 microbowl array structures and nanocone array structures were obtained [27, 28]. To

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the best of our knowledge, few researchers have studied on the fabrication of various microstructures by taking advantage of the irreversible electrowetting effect. Herein,

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we present further experimental work to investigate the wetting transition on the micropillar and microhole array structured surfaces, and demonstrate the results of

the applied electric-field.

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2. Materials and Methods

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microlens array, microbumps, and micropillars in a controllable manner via tailoring

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2.1 Fabrication of microtextured dielectric layer Silicon micropillar arrays of the dielectric layer were prepared by UV photolithography and thereafter by dry etching using Bosch process. The square-

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shaped micropillars structures have a periodicity of 100 μm in one orientation, and 40 μm in another orientation, a size of 30 μm×30 μm, and a depth of 50 μm. Besides,

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circular microholes with size of 100 μm, depth of 30 μm, and spatial ratio of 2:1 were fabricated by the similar lithography and etching process. Then, plasma-enhanced chemical vapor deposition (PECVD) was conducted to deposit a 500 nm thick silicon dioxide dielectric layer on the silicon structures. After that, a fluorocarbon (C4F8) thin film of 20 nm thick was also deposited on the thermally-oxidized layer of the microstructures using PECVD in order to enhance the hydrophobicity. 2.2 Characterization The contact angle measurement was conducted on a platform of Dataphysics (OCA20, Germany) with real time observation. Then, the behavior of 20 μl water droplets and

ACCEPTED MANUSCRIPT the decrease of contact angles were observed during applying voltage. The wetting properties were characterized with filter paper which could absorb the water droplet on the microstructure surface. The morphologies of microstructures were characterized by field-emission scanning electron microscope (FESEM, Hitach, Japan). The laser scanning confocal microscope was used to observe the surface

2.3 Observation of Irreversible Electrowetting Behavior

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morphology of the microstructures.

In order to further verify the irreversible transition characteristics of the wetting state

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on the microtextured surface during applying electric field, we investigated the

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behavior of the UV-curable polymer droplets on the contact angle platform (OCA20, Germany). The frequency of the applied voltage was a square wave of 10 Hz, the

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peak-to-peak value was slowly turned up from 0 to 38 V. Upon removing the electrode insertion and applying UV irradiation, the polymer droplet would be

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solidified. By tuning the voltage, three kinds of microstructures, including microlenses, microbumps and micropillar array structures were fabricated with different densities of several tens of micrometers.

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3. Results and discussion

Fig. 1 shows the electrowetting behavior of water droplet on the micropillar array. At the beginning, the water droplet sessile on the silicon micro-pattern surface with

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thermal dioxide layer depicted a contact angle of larger than 150°, indicating superhydrophobic Cassie-Baxter contact mode, and a sliding angle smaller than 5°

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(Fig.1a). As seen in Fig. 1b, the contact area of water droplet would keep in Cassie-Baxter contact mode partially for applied voltage of 20 V, and it would hesitate to wet into the asperities of the micropillar array. As soon as the applied voltage was high enough, fully wetting of water would occur gradually downwards to the asperities among the micropillars, until the water contact with the root interface of the pillar structures. Then, the contact state transition would become Wenzel state from Cassie-Baxter contact mode. During applying voltage of 20-35 V, the contact line of water droplets could hardly move laterally. The sliding angle would be increased with transition occurred of the contact state. As the voltage reached 35 V, the water droplets

ACCEPTED MANUSCRIPT was found to spread laterally in a sudden, with the contact area increased to larger and the contact angle decreased to 114°. Upon removing the electric field, the wetting state retained in Wenzel mode, because no recovery motion of droplet could be observed. We believe that the water droplets have completely wetted the pillar wall surface and overcome the surface energy barriers of the bottom surface under the

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electric field.

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Fig. 1.Wetting transition from Cassie-Baxter state to Wenzel state on the micropillar array textured

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surface under applied voltage from (a) 0 V to (b) 20 V and (c) 35 V. Scale bars are all 1 mm.

Fig. 2. Comparison of wetting properties of the micropillar texture surface (a) before and (b) after applying voltage, demonstrating the filter paper absorbing the water droplet on both cases. Scale bars are all 1 mm.

ACCEPTED MANUSCRIPT Fig. 2 shows the experimental results that water droplets were absorbed from the microtextured surface by the filter paper. Fig. 2a shows that the filter paper could easily take away the water droplet without residual on the textured surface without EWOD influence. Comparatively, when a voltage of 35 V was applied, the filter paper could not pick up the whole water droplet, leaving some water on the patterned surface (Fig. 2b). Therefore, the wetting transition from Cassie-Baxter to Wenzel

droplet and micropillars, as schematically shown in Fig. S1.

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would increase the surface wetting area and the adhesive interaction between the

In order to explain the mechanism for the abovementioned phenomena, we conducted

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a comparative experiment and investigated the difference of electrowetting properties

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between the smooth dielectric layer and the microtextured surface. Fig. 3a and 3b show a nearly reversible wetting properties on the smooth dielectric layer during

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increasing and decreasing the applying voltage, indicating a narrow loop shape change of contact angles and contact length in respect of the applied voltage. In comparison, an irreversible electrowetting effect was indicated on the microtextured

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surface, as seen in Fig. 3c and 3d. Therefore, the contact angle and contact length could be both retained for higher enough applied voltage, so that it might be possible

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to control the wetting state by applying various voltage.

ACCEPTED MANUSCRIPT Fig.3. Comparison of electrowetting properties smooth dielectric surface and the microtextured surface. (a) and (b) the contact angle and contact line related to the applied voltage on the smooth surface, indicating a nearly reversible electrowetting. (c) and (d) the contact angle and contact line related to the applied voltage on the rough microtextured surface, indicating a thoroughly irreversible electrowetting properties.

To further illustrate the wetting state of the microscopic condition, experiments were performed using UV-curable NOA61 droplets. Fig. S2 shows that NOA61 droplets

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could fully wet the micropillar array surface even at applied voltage of 20 V, which was indicated by the confocal microscopy image of the NOA61 replica after UV

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solidification and peeling off. Distinct from the thoroughly wetting of NOA61

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polymer on the aforementioned micropillar array texture surface, circular microhole arrays silicon micro-texture coated with thermal dioxide layer would show electric field tailored wetting properties (Fig. 4). Fig. 4a shows the confocal microscope image

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of the microholes array texture of template. Fig. 4b schematically illustrates the experimental set-up with the electrode moving laterally in a distance of about 200 μm.

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For experimental process, a blade-shape electrode was used to scan over the microhole array template by dragging the NOA61 droplet. The wetting state of polymer on the micro-textured surface must have been changed as observed from the

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SEM images. Such gradual wetting properties resembled to that of the water droplet

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on the micropillar array textured surface, and could be manifested by the difference in the height of the UV-cured polymer structure (Fig. 4c). When the voltage was 0 V, the NOA61 liquid could hardly fill inside the mold structure, so that we obtained a replica

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of NOA61 structure as microlenses after releasing the solidified NOA61 lens (Fig. 4e). When the voltage was increased to 19 V, the NOA61 liquids could partially wet into

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the microholes, leading to a replica in a shape of microbump array structures (Fig. 4f). However, when the voltage was further increased to 38 V, the NOA61 liquid could completely wet the mold structure, confirmed by morphological characterization (Fig. 4d). The results indicated a kind of thorough irreversibility, i.e., once the wetting occurred, the liquid would not recover even partially back to the initial wetting state which was similar to the micropillar structure template. By this means, various microstructure could be obtained by controlling over the applied voltage. As a matter of fact, the patterns obtained by the same applied voltage should be uniform. However, they are not uniform over the whole sample surface area, since we deliberately applied various voltage to different area of the same NOA61 droplet on a single

ACCEPTED MANUSCRIPT microtextured sample. Fig. 5 demonstrates the result of microlens array obtained by zero-voltage, and a fully-filled replica of micropillar array structure obtained by applying voltage of 38 V. The area of the patterned surface was dependent upon the contact interface between the NOA61 droplet and the microtextured dielectric surface. The contact area was a circle of about 3 mm in diameter for the non-electrowetting-induced microlens array

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surface. Meanwhile, it was also a circle of about 3.5 mm in diameter for that of the electrowetting-resulted micropillar array textured surface. Due to the morphological

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similarity, the nanotexture surface should have similar transition of wetting

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characteristics as such microstructures.

Fig. 4. (a) Confocal microscopy of the morphology of silicon template microhole array. (b) Schematic illustration of electrowetting effect on the microhole array dielectric surface. (c) SEM image of the UV-curable polymer structure replicated by electrowetting driven filling with scanning applied voltage. (d-f) show the magnified SEM images of different regions of micropillars, microlenses, and microbumps obtained by voltage of (d) 38 V, (e) 0 V, and (f) 19 V, respectively.

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Fig. 5. Demonstration of varied microstructures over a large area obtained by applying voltage of (a) 0 V and (c) 38 V. (b) and (d) are magnified images of (a) and (c), respectively.

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4. Conclusion

In conclusion, we have experimentally observed the effect of EWOD during

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manipulating the liquid wetting state on the microtextured dielectric surfaces so as to produce microstructure replica. It was clearly shown that water droplets could be removed easily using a piece of filter paper without residual on the surface before

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applying voltage, while it could be only partially removed for a voltage of 35 V,

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indicating a completely irreversible transition occurred with electric field application. The applied electric field could promote complete wetting of liquid droplets,

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confirmed indirectly from electrowetting and solidification of the UV-curable polymer replica. Moreover, various microstructures including microlenses, microbumps and micropillars, could be obtained by varying the applied voltage, facilitating

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controllable adhesive wetting properties. Such electric-field tailored adhesive wetting properties would find applications in micro- or nanofabrication and other devices. Acknowledgments This work is supported by National Natural Science Foundation of China (Grant no. 51705479); Science Foundation of Shanxi province for Youths (Grant nos. 201701D221128, 201601D011061); Scientific Research foundation of North University of China (Grant no. 2017005); and Foundation of Shanxi Key Laboratory of Advanced Manufacturing Technology (XJZZ201706). Conflicts of Interest: The authors declare no conflict of interest.

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ACCEPTED MANUSCRIPT Highlights

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Irreversible electrowetting effect was observed on microtextured dielectric. Transition of contact state could be well controlled by applying varied voltage. Contact transition occurs from Cassie-Baxter regime to fully Wenzel regime. Various polymeric micropatterns could be obtained by tuning applied voltage.

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Graphical Abstract

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