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Black Silicon/Elastomer Composite Surface with Switchable Wettability and Adhesion between Lotus and Rose Petal Effects by Mechanical Strain Jun Kyu Park, Zining Yang, and Seok Kim* University of Illinois at Urbana−Champaign, 1206 W. Green Street, Urbana, Illinois 61801, United States S Supporting Information *
ABSTRACT: Although many recent studies demonstrate surfaces with switchable wettability under various external stimuli, a deliberate effort to self-propel liquid droplets utilizing a surface wetting mode switch between slippery lotus and adhesive rose petal states via a mechanical strain has not been made yet, which would otherwise further benefit microfluidic applications. In this work, we present a black silicon/elastomer (bSi/ elastomer) composite surface which shows switchable wettability and adhesion across the two wetting modes by mechanical stretching. The composite surface is composed of a scale-like nanostructured silicon platelet array that covers an elastomer surface. The gap between the neighboring silicon platelets is reversibly changeable as a function of a mechanical strain, leading to the transition between the two wetting modes. Moreover, the composite surface is highly flexible although its wetting properties primarily originate from superhydrophobic bSi platelets. Different wetting characteristics of the composite surface in various mechanical strains are studied, and droplet manipulation such as droplet self-propulsion and pick-and-place using the composite surface is demonstrated, which highlights its potentials for microfluidic applications. KEYWORDS: lotus leaf, rose petal, superhydrophobic, switchable wettability, droplet manipulation
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INTRODUCTION Stimuli-responsive surfaces with switchable wettability have recently gained much attention owing to their great potentials for applications in various fields including microfluidics,1,2 oil− water separation,3,4 and biotechnology.5,6 Common stimuli to switch surface wettability include temperature control,2,5,7,8 pH change,3,6,9,10 electrical bias,4,11,12 counterion exchange,13,14 and light illumination.1,15,16 While applying these stimuli is usually time-consuming or requires special apparatus, applying a mechanical strain is relatively simple and facile to switch surface wettability without extensive expenses. Several studies have reported mechanical-strain-responsive surfaces with switchable wettability via film wrinkling or buckling.17−20 Lin and Yang demonstrated a smart surface with dual-scale roughness by depositing silica particles on a stretched polydimethylsiloxane (PDMS) surface and relaxing it to cause surface buckling.17 Similarly, Goel et al. fabricated a PDMS surface with dual-scale roughness using silver nanorods on a buckled PDMS surface.18 Wong et al. introduced a flat PDMS surface with buckled electrospun nanofibers that can modulate wettability as well as adhesion.19 Cao et al. also utilized a buckled gold-coated graphene on an acrylic elastomer substrate to achieve tunable wettability and adhesion characteristics.20 In the meanwhile, other mechanical-strain-responsive surfaces with switchable wettability have also been demonstrated without film buckling. Huang et al. reported a hydrogel and © XXXX American Chemical Society
silica particle composite that can switch its wettability between superhydrophobic and superhydrophilic states.21 Wang et al. also fabricated an reentrant structured surface and showed droplet pick-and-place using its tunable wettability and adhesion.22 Although the above-mentioned surfaces have successfully demonstrated the wettability transition upon a mechanical strain with or without buckling, they have not achieved the wettability variation within a superhydrophobic region which is large enough to self-propel liquid droplets. More practical techniques that enable both droplet selfpropulsion and pick-and-place still remain challenging to demonstrate. In this work, we present a black silicon/elastomer (bSi/ elastomer) composite surface which exhibits either the lotus effect or the rose petal effect as a function of a mechanical strain to manipulate water droplets. Two modes of wetting observed in a lotus leaf and a rose petal indicate different adhesion between the surface and a water droplet. The lotus leaf is superhydrophobic and renowned for its exceptional water repellency because it is covered by hierarchical micro- and nanoscale structures coated with hydrophobic wax.23−28 In contrast to the lotus leaf, the rose petal has a superhydrophobic Received: July 27, 2017 Accepted: August 28, 2017
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DOI: 10.1021/acsami.7b11143 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Reversible switching between two different wetting modes on a bSi/elastomer composite surface upon a biaxial mechanical strain. Schematic illustration and optical images of a droplet in the lotus state when the surface is relaxed (a) and in the rose petal state when the surface is stretched (b).
through photolithography with a photoresist (SPR220) and a successive deep reactive-ion etching (DRIE, STS Pegasus) (Figure 2a). Wet etching with hydrofluoric acid (HF) removes the buried oxide layer with undercut trench below the rim of each square-patterned device layer. Subsequently, spin-coating of a photoresist (AZ5214), flood exposure, and development leave the photoresist only inside the undercut trench (Figure
yet adhesive surface, which causes the water droplet to be stuck on the surface even when the surface is upside down.29−34 Paradoxically, the rose petal surface is also hierarchically structured and superhydrophobic similar to the lotus leaf. On a lotus leaf, the droplet rolls off the surface because it does not sink either between the microscale structures or between the nanoscale structures because they are closely packed. However, on a rose petal, the droplet sinks between the microscale structures without wetting the nanoscale structures completely because the microscale structures are sparsely distributed.31−35 Inspired by these two different effects, we designed a hierarchically structured bSi/elastomer composite surface where the gap between the microscale silicon (Si) platelets is modified through a mechanical strain to achieve the transition between the two different wetting modes related to lotus and rose petal effects (Figure 1), respectively. To fabricate the bSi/elastomer composite surface, a transferprinting method is adopted, which transfers prefabricated solid parts called “ink” onto a separate receiving substrate. A square array of Si inks is transfer-printed on an elastomer surface to create microscale structures on the surface. Subsequently, forming Si nanocones on the Si inks and coating the entire surface with a low surface energy layer complete the fabrication of the superhydrophobic bSi/elastomer composite surface with hierarchical surface structures. The hydrophobicity of the composite surface is switchable by mechanical stretching within a wide range of contact angles between 170° and 110°. Owing to this widely switchable hydrophobicity, a self-propelling droplet is possibly realized by inducing a mechanical strain gradient and thus a spatial wettability gradient over the surface. In addition, the adhesion between a droplet and the composite surface is also switchable by mechanical stretching, which is verified through the droplet pick-and-place experiment. Last, a superhydrophobic slippery composite surface is demonstrated in a curved configuration to highlight its easy use on nonflat platforms.
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Figure 2. Fabrication of a bSi/elastomer composite surface. (a) Device layer of an SOI wafer is patterned by DRIE with a photoresist-masking layer. (b) Patterned Si inks are tethered to a handle layer via thin photoresist anchors.36−39 (c,d) Fabricated Si ink array makes contact with an elastomer surface on a glass slide and is transferred to the elastomer surface. (e) bSi/elastomer composite surface is peeled from the glass slide. (f) Nanocones are formed on the Si inks by RIE. (g) SEM image of the bSi/elastomer composite surface. (h) Side-view SEM image of the nanocones formed on the top surfaces of the Si inks.
RESULTS AND DISCUSSION
The fabrication procedure of a bSi/elastomer composite surface starts with the preparation of donor and receiving substrates. A donor substrate is prepared from a silicon-on-insulator (SOI) wafer with a 50 μm thick device layer and a 1 μm thick buried oxide layer. The device layer of an SOI wafer is patterned B
DOI: 10.1021/acsami.7b11143 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces 2b). Finally, the buried oxide layer below the patterned device layer is completely etched away through the second HF etching. After this step, square-patterned Si platelets, called Si inks, from the device layer are ready for the following transferprinting step. A receiving substrate is composed of a platinumcatalyzed PDMS (Ecoflex 0020, Smooth-On Inc.). This elastomer is tough and easily stretchable such that it is stretched several times its original dimension without tearing and recovers its original shape without plastic deformation. After sufficient mixing and degassing, the elastomer precursor is poured on a microscope glass slide and is partially cured on a hot plate at 50 °C for 25 min. Then, the glass slide with the elastomer is gently placed on the prefabricated Si ink array on the donor substrate and fully cured on a hot plate at 50 °C for 3 h, which forms a strong bond between the Si ink array and the elastomer.39 The donor substrate is peeled from the receiving substrate, leaving the Si ink array on the elastomer of the receiving substrate (Figure 2c,d). This assembled Si/elastomer surface is placed on another glass slide to cover the top surfaces of the Si inks with Si nanocones by a three-step bSi process using RIE (Plasma-Therm ICP RIE)40,41 (Figure 2e,f). The process starts with an O2 plasma treatment to form a thin SiO2 film on the top surfaces of the Si inks. Successive CHF3 plasma partially etches the thin SiO2 film and leaves randomly scattered SiO2 islands. Last, Cl2 and Ar plasmas create Si nanocones with the SiO2 island layer as an etch mask. After removing the remaining SiO2 islands with buffered oxide etchant, heptadecafluoro-tetrahydrodecyl-trichlorosilane (FDTS) is deposited on the fabricated bSi/elastomer composite surface in a molecular vapor deposition chamber to make the surface superhydrophobic (Figure 2f). Figure 2g is a scanning electron microscopy (SEM) image of the fabricated bSi/elastomer composite surface. More SEM images are available in Figure S1. The lateral dimension of the individual square Si inks is 500 μm, and the spacing between the Si inks is 50 μm. It is confirmed from the image that the Si inks are approximately 10 μm deep embedded into the elastomer surface, which offers a more robust bond between the Si inks and the elastomer surface. Figure 2h is the side-view SEM image of the nanocones on the Si inks. Figure 3 summarizes the contact angle change of a water droplet on the bSi/elastomer composite surface. Because the Si inks are closely packed in the relaxed state, the droplet does not sink between the Si inks (Figure 1a). Therefore, the droplet exhibits a very high contact angle of approximately 170°, and this mode of wetting is called the Cassie state (or the lotus state). In the lotus state, the liquid/solid contact area is significantly small and the droplet can effortlessly roll off from the surface. The contact angle of a droplet in this state is theoretically predicted using eq 1. θ* is an apparent contact angle on the composite surface, and θY is an intrinsic contact angle on an FDTS-coated flat Si surface. f1 and f 2 are the fraction of the solid surface and air in contact with water, respectively.42 cos θ* = f1 ·cos θY − f2
Figure 3. Experimentally characterized switchable wettability of the composite surface upon a mechanical strain. (a) Contact angles and sliding angles of water droplets on the composite surface versus biaxial strains. (b) Series of contact angles of a water droplet on the composite surface during 10 stretch−relax cycles.
state). In the rose petal state, a small droplet (
