Magnetically Responsive Superhydrophobic Surface: In Situ

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Surfaces, Interfaces, and Applications

Magnetically-Responsive Superhydrophobic Surface: In Situ Reversible Switching of Water Droplet Wettability and Adhesion for Droplet Manipulation Chao Yang, Lei Wu, and Gang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04190 • Publication Date (Web): 28 May 2018 Downloaded from http://pubs.acs.org on May 28, 2018

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ACS Applied Materials & Interfaces

Magnetically-Responsive Superhydrophobic Surface: In Situ Reversible Switching of Water Droplet Wettability and Adhesion for Droplet Manipulation †

‡

†

Chao Yang , Lei Wu , and Gang Li * †

Defense Key Disciplines Lab of Novel Micro-Nano Devices and System Technology, Key

Laboratory of Optoelectronic Technology and Systems, Ministry of Education, Chongqing University, Chongqing 400044, China ‡

State Key Lab of Transducer Technology, Shanghai Institute of Microsystem and Information

Technology, Chinese Academy of Sciences, Shanghai 200050, China KEYWORDS: magnetorheological elastomer micropillars, field-stiffening effect, wettability and adhesion switching, reversible, superhydrophobic surfaces ABSTRACT: A smart, magnetically-responsive superhydrophobic surface was facilely prepared by combining spray-coating and magnetic-field-directed self-assembly. The surface comprised a dense array of magnetorheological elastomer micropillars (MREMPs). Benefitting from the magnetic field-stiffening effect of the MREMPs, the surface exhibited reversible switching of the wettability and adhesion that was responsive to an on/off magnetic field. The wettability and adhesion properties of the surfaces with MREMPs were investigated under different magnetic

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fields. The results revealed that the adhesion force and sliding behaviors of these surfaces were strongly dependent on the intensity of the applied magnetic field and the mixing ratio of polydimethylsiloxane (PDMS), iron particles, and solvent (in solution) used for preparation of the magnetically-responsive superhydrophobic surfaces. The adhesion transition was attributed to the tunable mechanical properties of the MREMPs which was easily controlled by an external magnetic field. It was also demonstrated that the magnetically-responsive superhydrophobic surface can be used as a “mechanical hand” for no-loss liquid droplet transportation. This magnetically-responsive superhydrophobic surface not only provides a novel interface for microfluidic control and droplet transportation, but also opens up new avenues for achieving smart liquid-repellent skin, programmable fluid collection

and

transport,

and

smart

microfluidic devices.

1. INTRODUCTION In the past decade, smart surfaces with reversibly switchable wettability and liquid-solid adhesion have aroused great interest because of their great potential in a wide range of scientific and

industrial

applications,

including

droplet-based

microfluidics,1

no-loss

droplet

transportation,2, 3 cell adhesion,4, 5 oil-water separation,6, 7 and biotechnology.8, 9 Such reversible switching can be achieved through an externally applied stimulus such as photo-irradiation,10-12 temperature,13, 14 an electrical field,15-17 pH,18-20 mechanical strain,21-24 or a magnetic field.25-34 Among these stimuli-responsive strategies, magnetically-responsive switching has received increasing attention because of its convenience, robustness, instantaneous response, remote controllability, and good biocompatibility. Recently, several groups have attempted to alter the surface wettability and adhesion properties of various substrates by applying a magnetic field.


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Jiang et al. developed a tunable adhesive superhydrophobic iron surface for no-loss transportation of superparamagnetic droplets by applying a magnetic field.27,

28

This surface

worked only when the transported droplet contained magnetic particles, which might interfere with samples or reagent. Stroeve et al. demonstrated a method of magnetically modulating the wetting behavior of water droplets by inducing a change in the conformation of the magnetic nanopillars on a substrate.29 Minko et al. also developed a magnetic-field-based approach for remote control of the wetting behavior, enabling transition from a superomniphobic to an omniphilic wetting state via alternation of the re-entrant curvature of a microstructured surface.30 However, these two micro-nano structured surfaces involved complicated and expensive fabrication, and switching of liquid adhesion on these surfaces was not reversible. To realize reversible switching of the surface adhesion, several novel magnetically-driven surfaces have been developed. Drotlef et al. designed arrays of flexible magnetic micropillars that undergo reversible geometrical changes upon application of a magnetic field, leading to a reversible switch between high and low adhesion states.31, 32 However, the patterned microstructures also required complicated preparation processes, which are time-consuming and costly. Jeon et al. fabricated a hydrophobic magnetorheological elastomer film and used a magnetic field to change its surface morphology, and thus the wettability, via changes in the alignment of the iron particles along the magnetic field lines.33 Though this magnetically-driven approach for altering the surface wettability and adhesion properties is facile and completely reversible, it requires uncured polydimethylsiloxane (PDMS) as a surface matrix, where this matrix is flowable and sticky, which significantly limits its applicability. More recently, Jiang et al. demonstrated the reversible switching of water-droplet adhesion on magnetic soft composite films.34 However, this approach can only yield a narrow adjustment of the water contact angle, not exceeding 15°, and


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can only provide quasi-“no-loss” water droplet transportation with ~3% loss of weight rather than absolute “no-loss” droplet transportation. Therefore, simple, inexpensive magneticallydriven methods for reversible switching of the surface adhesion to achieve absolute “no-loss” droplet transportation or collection still presents a challenge. Herein, we report the facile fabrication of a magneto-switchable superhydrophobic surface, where the wettability and adhesion can be reversibly switched between the water-repellent and water-adhesive states by on/off switching of an external magnetic field. This surface consists of a dense array of magnetorheological elastomer micropillars (MREMPs) comprising polymer/ironparticle hybrid soft materials and tends to be stiffer under a magnetic field. Taking advantage of their tunable stiffness, the MREMPs can be alternated between the collapsed and erect states by application of an external magnetic field, and thus the surface with MREMPs exhibits a unique switching between the water-repellent and water-adhesive states. In the absence of a magnetic field, the soft MREMPs are prone to buckling under droplet loads. These collapsed MREMPs create a large contact area between the sessile droplet and the surface, which results in high adhesion and a water-adhesive state for the surface. When an external magnetic field is applied, the MREMPs are stiffened and oriented vertically so that the sessile liquid droplet only touches the tips of the hydrophobic micropillars. These vertically oriented MREMPs significantly reduce the contact area between the surface and the water droplet and make the surface superhydrophobic. This magnetically responsive surface exhibits reliable wettability and adhesion switching capability with immediate field responses and non-contact control, which opens up opportunities for smart microfluidic devices. Using this novel magnetically responsive surface, we demonstrate no-loss water droplet transportation in an active and programmable manner using only an electromagnet.


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2. RESULTS AND DISCUSSION The magnetically-responsive superhydrophobic surface was simply prepared via spray selfassembly of a solution comprising PDMS prepolymers and carbonyl iron particles (CIPs) under a magnetic field. Figure 1a presents a detailed illustration of the procedure for preparation of the magnetically responsive superhydrophobic surface. To fabricate the magnetically responsive surface, we first mixed toluene solution with PDMS prepolymer containing 10% of curing agents with vigorous stirring. The CIPs were then added to the mixture, and the mixture containing the CIPs was vigorously stirred for 20 min to obtain a homogeneous solution. Thereafter, the prepared solution was sprayed onto a substrate that was placed on a neodymium magnet (20 mm in diameter, 30 mm thicknesses, and maximum ﬂux density: ~4.5 T) using an airbrush. Due to the effect of the magnetic field, the mixture of ferromagnetic CIPs and uncured PDMS spontaneously arranged along the direction of the magnetic field and formed an ordered array of pillar-like microstructures (the diameter of the obtained micropillar array ~15 mm). This fieldinduced self-assembly is driven by free-energy gradients to reach a minimum free-energy state wherein the ordered structure appears. Compared to other self-assembly systems, magnetic particles have an additional magnetostatic force, which favors the formation of magnetically aligned chains of magnetic dipoles. It is because of the strong anisotropy of dipolar forces induced by the magnetic field that the atomized droplets containing CIPs prefer a head-to-tail arrangement, resulting in the formation of chain-like clusters. Subsequent thermal curing (70ºC for 1.5h) solidified the PDMS to fix shape of the field-aligned pillar, resulting in PDMS/CIP composite micropillar arrays on the substrate. The magnetic particles in the micropillars enabled on-demand tuning of the mechanical properties, whereas the polymeric matrix defined the


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structural geometry. A schematic of the reversible switching process is presented in Figure 1b. In the absence of a magnetic field, the soft MREMPs are prone to buckling under the droplet load. In this case, the contact mode between the droplet and the surface is considered as “line contact”, which has a large contact area and results in high adhesion (water-adhesive state). Under a magnetic field, the MREMPs are stiffened and their effective compressive stiffness increases significantly35. As a result, the stiffened MREMPs are rigid enough to withstand the droplet loads, and become oriented vertically along the magnetic line of force so that the water droplet sitting on the surface only touches the tips of the MREMPs. Therefore, the contact mode between the sessile droplet and the surface shifts from “line contact” to “point contact”. According to the wetting theory, liquid-solid adhesion is mainly attributed to the van der Waals force, which is proportional to the contact area. In the “point contact” mode, the effective contact area between the sessile droplet and the surface decreases significantly, leading to a low adhesive state (water-repellent state). The optical microscope images display the tunability of the stiffness of the MREMPs under an external magnet field, where the micropillars are transformed from the collapsed morphology to the fully up-right position with on/off switching of a magnetic field (Figure 1c). This confirms that the mechanical properties and orientations of the soft micropillar arrays on the surface can be remotely controlled using an external magnetic field. This magnetic field-induced stiffening effect offers a route to tuning the wettability and adhesion properties of MREMP-based surfaces. To demonstrate that the developed magneto-switchable surface can exhibit both waterrepellent and water-adhesive properties and can switch between these two states, static contact angle (CA) and sliding angle (SA) measurements were carried out. In the absence of a magnetic field, the water droplet sitting on the MREMP-based surface that was prepared with 15 g CIPs,


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10 g PDMS, and 30 mL organic solvent exhibited a static CA of 105.6°±2° and did not roll down even at a tilt angle of 180° (Figure 2a). In contrast, when a 0.45 T magnetic field was applied, the water droplet sitting on the same MREMP-based surface exhibited a very high static CA (i.e., >150°) with low contact angle hysteresis, and could roll off the surface at low tilting angles (

Magnetically Responsive Superhydrophobic Surface: In Situ

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