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Salvinia-Effect-Inspired “Sticky” Superhydrophobic Surfaces by Meniscus-Confined Electrodeposition Deyin Zheng, Youhua Jiang, Wentao Yu, Xiufen Jiang, Xin Zhao, Chang-Hwan Choi, and Guangyi Sun Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03014 • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 3, 2017

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Salvinia-Effect-Inspired “Sticky” Superhydrophobic Surfaces by Meniscus-Confined Electrodeposition

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Deyin Zheng1, Youhua Jiang2, Wentao Yu1, Xiufen Jiang1, Xin Zhao1, Chang-Hwan Choi2, and

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Guangyi Sun*1

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Institute of Robotics and Automatic Information System &Tianjin Key Laboratory of Intelligent

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Robotics, Nankai University, Tianjin, 300071, People’s Republic of China

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Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, New Jersey

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07030, USA

ABSTRACT

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Inspired by the Salvinia effect, we report the fabrication and characterization of a novel

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“sticky” superhydrophobic surface sustaining a Cassie-Baxter wetting state for water droplets with

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high contact angles but strong solid-liquid retention. Unlike superhydrophobic surfaces mimicking

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the Lotus or Petal effect, whose hydrophobicity and droplet retention are typically regulated by

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hierarchical micro- and nanostructures made of a homogeneous material with the same surface

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energy, our superhydrophobic surface merely requires singular microstructures covered with a

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hydrophobic coating, but creatively coupled with hydrophilic tips with different surface energy.

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Hydrophilic tips are selectively formed by meniscus-confined electrodeposition of a metal (e.g.,

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nickel) layer on top of hydrophobic microstructures. During the electrodeposition process, the

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superhydrophobic surface retains its plastron so that the electrolyte cannot penetrate into the cavity

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of hydrophobic microstructures, consequently making the electrochemical reaction between solid

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and electrolyte occur only on the tip. In contrast to typical superhydrophobic surfaces where

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droplets are highly mobile, the “sticky” superhydrophobic surface allows a water droplet to have

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strong local pinning and solid-liquid retention on the hydrophilic tips, which is of great

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significance in many droplet behaviors such as evaporation.

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Keywords: Salvinia effect, superhydrophobic surface, electrodeposition, contact angle hysteresis,

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droplet retention, evaporation

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INTRODUCTION

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Superhydrophobic surfaces have attracted tremendous attentions for the past couple of

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decades due to their unique properties, such as water repellent [1-3], self-cleaning [4-7], anti-

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fouling [8, 9], anti-corrosion [10-12], anti-icing [13-16], and enhanced heat and mass transfer [17-

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21] properties. More recently, it has also been experimentally proven that a well-designed

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superhydrophobic surface can produce significant drag reduction in liquid flows [22-24]. Such

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unique properties result from the air layer entrained between the hydrophobic structures on the

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surface. Thus, the stability and longevity of the entrapped air layer are important for such

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applications [25]. Although various superhydrophobic surfaces have been proposed and fabricated


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using different methods [26-29], the surfaces can generally be classified into slippery or sticky

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superhydrophobic surfaces based on the dynamic mobility of a droplet on the surfaces [26, 30],

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compared to that on a flat surface. They are also commonly referred to as the Lotus or Petal effect

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[27, 31-33], respectively, according to the significance of contact angle hysteresis or droplet

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retention [34-36]. In such surface systems, hierarchical or dual-scale (e.g., micrometer and

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nanometer) structures play an important role in determining the different wetting and adhesion

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properties [33, 37]. On the other hand, the wetting and adhesion behaviors are not only dependent

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on the geometric structures of the surface, but also on the chemical composition of the surface, i.e.,

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surface energy [33, 35]. To date most researches on artificial superhydrophobic surfaces with high

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droplet retention have focused on the geometrical influences. As a result, it has been widely

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accepted that the coexistence of high contact angle and high contact angle hysteresis must be

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obtained by mimicking the rose-petal-like hierarchical structures, where the cavity of

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microstructures must be wetted while the nanostructures hierarchically added on the

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microstructures remain non-wetted [38, 39]. Notably, most rose-petal-like artificial

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superhydrophobic surfaces are either made of or coated with a homogenous hydrophobic material,

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which has almost become a technical standard in the field. However, if one looks back to the old

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question of what the origin of contact angle hysteresis should be, one would realize that a simple

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truth has long been neglected, which is that contact angle hysteresis originates from hydrophilic

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chemical defects [35, 36, 40] and the inhomogeneity of surface energy.

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Recently, in the field of biology, a few reports have shown that for some plants and vegetables,

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e.g., Salvinia and allium, their leaves support droplets with high contact angle as well as high


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contact angle hysteresis [33], which is not because of the geometrical factor, but because of the

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presence of “adhesive” pins with different surface energy. Such a heterogeneous superhydrophobic

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surface with hydrophilic pins can provide unique wetting properties and applicability, even

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superior to the typical superhydrophobic surfaces with homogeneous surface energy. For example,

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it has been demonstrated that hydrophilic pins of fern Salvinia can help stabilize the air layer by

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pinning the air-water interface under water, preventing the loss of air from the surface especially

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in a turbulent flow condition [22, 33]. This provides an intriguing perspective for developing a

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long-term air-retaining superhydrophobic surface, which is especially crucial for underwater

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applications. Moreover, the heterogeneous superhydrophobic surfaces with hydrophilic pins are of

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high scientific interest for enhanced heat transfer [17]. Despite the promising properties, the

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fabrication of artificial superhydrophobic surfaces with hydrophilic pins has been rarely reported,

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mainly due to the difficulties of making such inhomogeneous surface structures with regulated

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surface energy on a micrometer or nanometer scale [41-43].

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In this work, we propose a feasible method to fabricate such a heterogeneous

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superhydrophobic surface textured with microstructures that are selectively decorated with

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hydrophilic tips. We investigate a novel meniscus-confined electrodeposition process to grow a

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thin layer of hydrophilic metal only on the top of the hydrophobic microstructures, while keeping

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the rest part of the microstructures intact. The key innovative idea is to utilize the meniscus-

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confined liquid-solid contact on the superhydrophobic surface to selectively localize the

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electrochemical reaction during electrodeposition process. Even fully immersed in the electrolyte,

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the superhydrophobic surface retains its plastron, and the electrolyte cannot penetrate into the


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cavity of microstructures. Consequently, the electrochemical reaction between solid structure and

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electrolyte occurs only on the tip. This method requires only one additional fabrication step of a

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regular electrodeposition process to form hydrophilic tips for an existing superhydrophobic surface.

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Whereas a localized electrodeposition process based on a meniscus shape had previously been

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reported [44-46], in those studies the guidance of electrodeposition relied mostly on the usage of

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a single freestanding micrometer scale metal (e.g., platinum) wire integrated with precisely-

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controlled X-Y-Z stages and an electrolyte dispensing system, making them not feasible for the

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fabrication of large-area dense arrays. Here, we use the similar principle of the meniscus-confined

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solid-liquid interface which is the inherent property of a pre-existing superhydrophobic surface.

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However, the novelty of our scheme is not to use such additional systems as the nozzle of

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micropipette but to realize the meniscus-confined solid-liquid interface using the autonomous gas-

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retaining mechanism of an immersed superhydrophobic surface itself. To demonstrate the

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feasibility of the newly-developed heterogeneous superhydrophobic surface with hydrophilic tips

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and characterize its uniqueness compared to typical (e.g., homogeneous) superhydrophobic

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surfaces, underwater wetting property, apparent contact angles, and the retentions of laterally

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sliding and evaporating droplets on such surfaces are investigated.

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FABRICATION SCHEME

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Figure 1 shows the overall fabrication scheme of the proposed heterogeneous

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superhydrophobic surface. First, micropost patterns of silicon are formed by using conventional


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photolithography and deep reactive ion etching (DRIE) processes (Figure 1a). In order to make the

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surface hydrophobic and retain air layer between the microstructures to support the Cassie-Baxter

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superhydrophobic state, Teflon solution (0.05 wt% Teflon® AF1600 in FC 40) is applied on the

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DRIE-etched silicon surface (Figure 1b). During the electrodeposition process (Figure 1c), the

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superhydrophobic surface retains its gas plastron so that the electrolyte makes the contact with

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only the top surface of the microposts. The electrolyte cannot penetrate into the cavity of

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microstructures due to the entrained gas plastron and consequently make the electrochemical

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reaction between solid and electrolyte occur only on the tips. Figure 1d shows the schematic of the

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processed sample.


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(a)

Si

Si

Hydrophobic coating

Hydrophobic coating

(b) Si Si

Electrolyte

Electrolyte

(c) Si Si

Hydrophilic pins

Hydrophilic pins Hydrophobic coating

(d) Si Si 1 2

Figure 1. Fabrication process of a heterogeneous superhydrophobic surface with hydrophilic tips,

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drawn schematically but not to scale. (a) Fabrication of micropost structures of silicon using

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conventional photolithography and deep reactive ion etching (DRIE). (b) Hydrophobic coating on


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microstructures. (c) Meniscus-confined electrodeposition. (d) Final superhydrophobic surfaces

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with hydrophilic tips.

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EXPERIMENTAL SECTION

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Fabrication of micropost structures. As for the silicon substrate, silicon wafer with a

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relative low resistivity was employed to allow a wide-range plating current in the electrodeposition

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step. Single-side polished, 4-in, 500±5 μm-thick, silicon wafers of p-type with resistivity

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less than 0.01 ohm-cm were used as the substrates. First, the wafer was cleaned using a standard

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piranha (mixture of H2SO4 and H2O2, 4:1 in volume) and buffered oxide etch (BOE) solutions to

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remove the organic residue and native oxide layer, respectively. After the cleaning,

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Hexamethyldisilazane (HMDS) was coated on the surface using a vapor deposition method to

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serve as an adhesion promote layer for photoresist polymer. Then, 1.5-μm thick protective layer

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of photoresist (A Z5214, Microchemicals) was spin-coated on the wafer (4000 rpm for 30 s). The

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wafer was then soft-baked on a hot plate at 95 ºC for 90 s. To pattern microstructure arrays on the

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photoresist layer, the photoresist layer was exposed to ultra violet (UV) light through a photomask

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by using a mask aligner (MA6, Karl Suss, Germany) in a proximity contact mode and then

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developed by immersing the wafer in the developer (RZX-3038, SUZHOU RUIHONG CO.,

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LTD.). The developed wafer was then hard-baked on a hot plate at 110 ºC for 2 min. The micropost

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structures were formed by a DRIE process using an inductively coupled plasma etching machine

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(Surface Technologies Systems USA Inc., Redwood, CA). The detailed recipe of the DRIE process


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is shown in Table S1 in the Supporting Information. After the DRIE step, the photoresist and

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polymer residues were removed by piranha and the native oxide layer on the silicon wafer was

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further cleaned by BOE.

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Hydrophobic coating. Teflon was used for the hydrophobic coating of the microstructured

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surface. Teflon® AF 1600 was purchased from Dupon Company. Teflon® AF 1600 can be

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dissolved in perfluorinated solvents for the coating of highly uniform and thin films. In this study,

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Teflon solution (0.05 wt%) was prepared by mixing the AF1600 powders in FC-40 solution

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(Sigma-Aldrich). The mixture was stirred with a magnetic stirrer for 12 h to completely dissolve

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the Teflon power to the perfluorinated solvent. Teflon solution (3 μL) was applied the

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microstructured silicon substrate (1×1 cm2) using a micro pipette, giving the Teflon coating

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thickness of ~5 nm. Then the substrate was baked on a hot plate at 170 ºC for 10 min, followed by

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another baking step at 330 ºC for 1 h.

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Meniscus-confined electrodeposition. The schematic illustration of the meniscus-confined

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electrodeposition process is shown in Figure 2. The micropatterend superhydrophobic silicon

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substrate was cut into small rectangular chips of 1.5×1.5 cm2, where the effective area of the

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microstructures is 1×1 cm2. Then, the edge and backside of the chip were covered with a

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protective tape (1712#, 3M, USA) to prevent the electrodeposition in those areas. Electroplating

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solution (Watts Nickel, 25% NiSO4; 5% NiCl2; 5% H3BO3; 65% H2O) was purchased from

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TRANSENE Company (Denvers, MA, USA). Keithley’s Model 2425 Sourcemeter was used as

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the current source. During the electrodeposition, a nickel sheet with the same footprint size to the

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chip was used as anode and placed in parallel with the silicon chip (cathode) to ensure a uniform


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current density on the surface. The applied bias current between the anode and cathode is a key

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factor of the electrodeposition process. Because of the dielectric property of the Teflon, the Teflon

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film works as an insulating layer between the electrolyte and silicon substrate. The dielectric

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constant of Teflon® AF is the lowest of any known solid polymers, ranging from 1.89 to 1.93, and

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the breakdown voltage is ~20 V/µm for AF 1600 at 23 ºC [47]. At the start of the electrodeposition,

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the bias current value was set at a relative high level of 120 mA (i.e., 120 mA/cm2 as a current

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density over the 1×1 cm2 surface area) to breakdown the Teflon layer on the structural tops, and

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applied until the actual current value reached the set value (120 mA). Considering the coating

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thickness of Teflon (~5nm) and the breakdown voltage (~20 V/µm), the Teflon® AF 1600 layer is

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easy to break down by the actuating voltage. There is effectively no electric current to the sidewalls

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due to the entrapped air. Thus, the breakdown of the Teflon layer should be occurred only at the

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silicon-electrolyte interface. In general, it takes less than 5 s for the current value to reach the set

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value from zero, and the final values of current and voltage at this phase are 120 mA and 8.3 V.

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According to the Faraday’s laws of electrolysis, the amount of mass changes produced by the bias

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current is proportional to the quantity of the electricity that passes. Therefore, the mass of the

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deposited nickel depends on the duration of electrodeposition and the current value. Then the

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current was set at a lower level of ~20 mA (i.e., 20 mA/cm2 over the surface area) and applied for

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5 min to ensure the uniform density and smoothness of the electroplated metal layer. After the

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electrodeposition process, the sample was rinsed with deionized water and then dried by nitrogen

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gas.

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Electrolyte Anode

Cathode

Air

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Si

Teflon (Hydrophobic) (a)

Metal (Hydrophilic) (b)

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Figure 2. Schematic illustration of meniscus-confined electrodeposition. (a) Before

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electrodeposition. (b) After electrodeposition.

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Measurement of surface morphology and chemical composition. The morphology of

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fabricated microstructures of the surfaces were analyzed by using field-emission scanning electron

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microscope (S-4800 FE-SEM, Hitachi). The chemical composition of the microstructures was

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analyzed by using the energy dispersive spectrometer (EDS, JSM-7500F, JEOL).

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Underwater immersion test. The underwater test was carried out by immersing the

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microstructured samples in water. First, the samples were partially immersed in water for the

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visualization of the light reflection of the air cushion entrained on the samples’ surfaces. The

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images were taken at ~45° to the surface of the samples. Then the samples were fully immersed in


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water (~3 cm deep), and air bubbles were continuously blown to the samples’ surfaces by a dropper

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to examine the stability of the air cushion and the wetting transition.

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Measurement of contact angles. Contact angles were measured by using an in-house contact

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angle meter with image processing software (Digimizer) at room condition (air temperature, 25 ºC;

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air humidity, 54%; air pressure, 100 kPa). A deionized water droplet of 4 μL was taken into contact

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with the surfaces to measure contact angles. Two different kinds of surfaces were prepared and

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tested for comparison: one is the superhydrophobic surface without the decoration of hydrophilic

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tips and the other is the superhydrophobic surface decorated with hydrophilic tips. The contact

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angles were measured at ten different locations over the surface of the given same sample to get

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their average and standard deviation values. The contact angle hysteresis was measured using a

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droplet dragging method [48]. The surfaces were mounted on a moving stage, while the needle

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was fixed within the view of the CCD camera. As the stage moved, the droplets were dragged by

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the needle. During the stage moving, the advancing and receding angles of the droplets were

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measured to estimate the contact angle hysteresis. The sliding angles were measured by mounting

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the surfaces on a tilting stage. The sliding angles were obtained by tilting the stage until the droplet

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just began to move. The droplet volume used to measure sliding angles was ~4 μL.

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Droplet evaporation test. To see the effects of the hydrophilic tips of the heterogeneous

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superhydrophobic surface compared to the homogeneous superhydrophobic surface with no

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decoration with the hydrophilic tips, the droplet evaporation tests were also performed on the

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surfaces. During the evaporation, the evaporation kinetics was measured by analyzing the changes

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of droplet profile, volume, contact angles, and contact diameters. At the start of the experiments,


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4 μL deionized water droplets were placed on the surfaces. Then, the droplet profiles were recorded

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with a CCD camera by taking images every 2 min to measure the droplet volumes, contact angles,

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and contact diameters, until the droplets were dried out. All the experiments were performed under

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the same room condition (air temperature, 25 ºC; air humidity, 54%; air pressure, 100 kPa).

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RESULTS AND DISCUSSION

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Microstructures fabricated by the meniscus-confined electrodeposition. Figure 3 shows

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the optical and SEM images of the heterogeneous superhydrophobic surfaces with hydrophilic tips

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of nickel fabricated by the meniscus-confined electrodeposition process. Figure 3 shows that nickel

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tips are formed on the tips of the microposts. The electrodeposited nickel is thicker around the

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edge than in the center, and has a flat bottom in the overhang part, indicating the proposed

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meniscus-confined electrodeposition method successfully form the nickel tips on the top of the

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microposts. In the Cassie-Baxter superhydrophobic wetting state, the geometry of the electrolyte

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meniscus is not a regular 2D plane so that the current does not distribute uniformly over the

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interface. In the beginning of the electrodeposition, the current is inclined to concentrate at the

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sharp edges and pass more readily through these regions than the others [49], so that the deposition

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generally starts from the sharp edges (see Figure S1 in the Supporting Information). Thus, the

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nickel gets deposited over the top surface in an anisotropic way and the thickness of the nickel

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layer is uniform over the surface. As shown in Figure 3b (see also Figure S2 in the Supporting


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Information), the electrodeposited nickel is thicker around the edge than in the center, and has a

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flat bottom in the overhang part.

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(a) 50μm

20μm

(b)

20μm

100μm 4 5

Figure 3. Morphology of the hydrophilic metal (nickel) tips fabricated on the hydrophobic

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micropost structures. (a) Optical images (top views) of the microposts. (b) Scanning electron

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microscope (SEM) images (side views) of the microposts.

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For the selective local deposition of the hydrophilic nickel tips on the structural tops, the

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Cassie-Baxter wetting state is crucial to the process. In the Cassie-Baxter wetting state, the

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electrolyte only contacts with the upper part of the micropost structures of the superhydrophobic

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surface with trapped air cushion underneath. The localized silicon-electrolyte interface ensures


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that the nickel layer deposits only on the upper part of the microposts. The hydrophobic coating of

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Teflon supports and sustains the Cassie-Baxter wetting state for the electrolyte solution during the

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electroplating process. The electrolyte solution is water-based aqueous solution and the contact

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angles of the electrolyte droplets on the Teflon-coated micropost surfaces were almost similar to

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those of water droplets (see Figure S3 in the Supporting Information), indicating the Cassie-Baxter

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superhydrophobic wetting state should be maintained with the electrolyte. It was also confirmed

7

by the light reflection from the surface when the samples were immersed in the electrolyte solution.

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Even when the Teflon-coated microstructured substrate was fully immersed in the electrolyte, the

9

air cushion was visible due to the mirror-like light reflection at the liquid-air interface, verifying

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the Cassie-Baxter wetting state [50-52]. If the electrolyte penetrates into the cavities of the

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microstructure, the light reflection decreases. Once the cavities are fully filled with the electrolyte

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(i.e., Wenzel state), the original color of the substrate is visible. In the Wenzel state, the liquid-air

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meniscus no longer exists and the electrolyte contacts with the entire surface of the micropost

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structures. For comparison, the electrodepositing process was also applied in the Wenzel state,

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which was achieved by adding surfactant (Photo-Flo 200, Kodak) to the electrolyte solution. As

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shown in Figure S4 in the Supporting Information, nickel get deposited onto sidewalls as well as

17

the top surfaces, which indicates the collapse of the Cassie-Baxter wetting state.

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As a further verification to the localized electrodeposition, qualitative elemental analysis was

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performed using an energy dispersive spectroscopy (EDS) technique. Figure 4 shows the SEM and

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EDS mapping images of the heterogeneous superhydrophobic surface selectively decorated with

21

the nickel hydrophilic tips. Figure 4b and 4d show that the nickel only exists on the tips of the


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microposts, demonstrating that the proposed meniscus-confined method is an effective way to

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localize the electrodeposition process. Moreover, Figure 4e shows that fluorine (F) is distributed

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only on the sidewalls and bottoms of the microstructure, indicating the presence of the Teflon

4

coating. It also suggests that the Teflon coating keeps good adhesion to the surfaces, which do not

5

have any direct contact with the electrolyte or rinsing water during the fabrication processes due

6

to the entrained air layer supported by the Cassie-Baxter superhydrophobic wetting state.

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(a)

(c)

(b)

(d)

(e) Si K Ni K FK 8 9

Figure 4. EDS images of the superhydrophobic surface with hydrophilic tips. (a) SEM image,

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where the ellipse indicates the nickel tips of the post array behind the post in the front. (b) Layered

11

image of element distribution. (c) Distribution of Si. (d) Distribution of nickel, where element

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distribution inside the ellipse comes from the posts behind. (e) Distribution of fluorine. (F). All the

13

images are in the same field of view. Scale bar in each image is 50 µm.


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Underwater wetting properties. Underwater immersion test was performed to investigate

3

the effect of the hydrophilic-tips on the underwater wetting properties. Figure 5 shows the images

4

of the superhydrophobic surfaces without and with hydrophilic tips, respectively, partially

5

immersed in water. The bright reflection of the underwater part of the surfaces indicates the total

6

internal reflection of light that results from the gas layer retained on the superhydrophobic surfaces.

7

The results show that there is no significant change in the appearance of the light reflection

8

between the homogeneous superhydrophobic surface and the heterogeneous superhydrophobic

9

surface selectively decorated with hydrophilic tips, suggesting that the Cassie-Baxter wetting state

10

is also well maintained on the heterogeneous superhydrophobic surfaces decorated with the

11

hydrophilic tips. It also indirectly indicates that the hydrophilic metal tips are present only on the

12

tips of microposts. The sidewalls and the bottom of the microposts are still hydrophobic with the

13

presence of Teflon coating, making these areas un-wetted by water and support the Cassie-Baxter

14

wetting state. This is a significant difference from the conventional “sticky” or Wenzel-state

15

superhydrophobic surfaces. Similar to the case of electrolyte, for a superhydrophobic surface in

16

the Cassie-Baxter state, the trapped air on the surface is visible due to the total internal reflection

17

of light at the water-air interface. When the suspended liquid-gas interface collapses, the wetting

18

state transit from the Cassie-Baxter state to the Wenzel state, in which the internal reflection of

19

light disappears and no mirror-like reflection is observed. To evaluate the underwater stability of

20

the air cushion, we also compared the surfaces without and with hydrophilic tips by blowing air

21

bubbles to the surfaces under full immersion. The Cassie-Baxter state is easy to collapse with flow


17

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1

disturbance and shows wetting transition on the homogeneous superhydrophobic surface (see

2

Movie 1 in the Supporting Information), while the Cassie-Baxter state shows better stability on

3

the heterogeneous superhydrophobic surface with the hydrophilic tops (see Movie 2 in the

4

Supporting Information). The results agree with the previous report that the hydrophilic pins of

5

could help stabilize the air layer by pinning the air-water interface under water [33].

6

(a)

(b)

Air cushion 7 8

Figure 5. Underwater immersion test. (a) Superhydrophobic surface without the decoration of

9

hydrophilic tips. (b) Superhydrophobic surface decorated with hydrophilic tips.

10 11

Contact angles and droplet mobilities. Figure 6 shows the profiles of water droplets on the

12

superhydrophobic surfaces without and with hydrophilic tips, measured for the apparent contact

13

angles and sliding angles. The measured values are also summarized in Table 1. The static contact

14

angles (Figure 6a and 6b) were measured as soon as the droplets were placed on the surfaces using

15

a needle and they were 146±2º and 148±2º on the surfaces without and with hydrophilic tips,

16

respectively. Even with the hydrophilic tips, no significant difference was observed in the static


18

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contact angles. However, the surfaces show different properties in the measurements of

2

advancing/receding contact angles in the dragging experiment (Figure 6c and 6d). The advancing

3

and receding contact angles measured on the superhydrophobic surface with no decoration of

4

hydrophilic tips were 147±2º and 133±2º respectively, whereas those on the superhydrophobic

5

surface decorated with hydrophilic tips were 154±2º and 103±2º respectively. The result shows

6

that the contact angle hysteresis (difference between the advancing and receding contact angles)

7

is significantly increased by the hydrophilic tips, i.e., 51±4º for the superhydrophobic surface

8

decorated with hydrophilic tips vs 14±4º for the superhydrophobic surface with no decoration of

9

hydrophilic tips. As reported by Butt et al. [53], the “microscopic” advancing contact angle of a

10

droplet on post-structured surfaces should be 180° irrespective of the structure dimensions, and

11

the droplet pinning is solely determined by the receding boundary motion. The significant lowered

12

receding contact angle on the superhydrophobic surface with hydrophilic tips mainly results in the

13

high contact angle hysteresis and droplet retention. The significant difference in the droplet

14

mobility and retention was also shown in the measurement of sliding angles (Figure 6e and 6f).

15

The sliding angle, which was measured at the onset of the sliding motion, on the superhydrophobic

16

surface with no decoration of hydrophilic tips was 6±1º (Figure 6e). However, for the

17

superhydrophobic surface decorated with hydrophilic tips, the water droplet remained pinned on

18

the surface even when the surface was inclined to 90º. Even when the surface was further tiled to

19

be upside down, the droplet still clung to the surface with complete pinning (Figure 6f). This

20

demonstrate that, compared with the homogeneous hydrophobic tips, the hydrophilic nature of the

21

metal tips contributes to the strong pinning effect on the droplet.


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1

Without hydrophilic tips

With hydrophilic tips

(a)

(b)

(c)

(d)

(e)

(f)

2 3

Figure 6. Profiles of water droplets on (a, c, e) the superhydrophobic surface with no decoration

4

of hydrophilic tips and (b, d, f) the superhydrophobic surface with decoration of hydrophilic tips.

5

(a, b) Measurement of apparent static contact angles. (c, d) Measurement of apparent advancing

6

and receding contact angles by dragging water droplets on the surfaces. (e, f) Tilting test for the

7

measurement of sliding angles.

8


20

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Table 1. Comparison of wetting properties of the superhydrophobic surfaces without and with

2

hydrophilic tips. Surface type Without hydrophilic tips With hydrophilic tips

Static angle

Advancing angle

Receding angle

Contact angle hysteresis

Sliding angle

Depinning force (mN/m)

146±2º

147±2°

133±2°

14±4 °

6±1°

10±3

148±2º

154±2°

103±2°

51±4°

No sliding

45±4

3 4

The depinning force per the unit length of the apparent droplet boundary (Fd), which

5

quantifies the droplet retention, was also estimated according to the initial static contact angle (θe)

6

and the receding contact angle (θr), expressed as [30],

7

σ ⋅ (cos θ r − cos θ e ) Fd =

8

where σ represents the liquid-gas interfacial tension (e.g., 72 mN/m in room condition). The

9

estimated depinning force of the droplet on the micropost superhydrophobic surface with

10

hydrophilic tips (45±4 mN/m) is three times greater than that (10±3 mN/m) of the droplet on the

11

superhydrophobic surface without hydrophilic tips. The result further shows that the hydrophilic

12

tips significantly promote the droplet pinning and retention. The shape and size (the re-entrant

13

shape and the increase of a solid fraction) of the fabricated tips can also modify the surface

14

hydrophobicity and droplet retention, whereas the main effect of a re-entrant shape of the tip is to

15

enhance the robustness of the suspended liquid-gas interface [54, 55]. Regarding the increase of a

16

solid fraction, however, its impact should be less significant than the effect of surface energy. The


21

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previous study suggested that the increase of a solid fraction only (i.e., with no change in the

2

surface energy) should increase a droplet pinning force minutely, e.g., the increase of a solid

3

fraction by 162.5% induced the increase of a depinning force no more than 87.5% [54]. However,

4

in this study, the enhancement of a depinning force as high as 350% has been achieved by the

5

increase in the solid fraction no more than 70%.

6

Droplet evaporation. The effect of the hydrophilic tips of the superhydrophobic surfaces on

7

the evaporative behaviors of the sessile water droplets was further investigated (see Movie 3 and

8

4 in the Supporting Information). The hydrophobicity of the tips affects the pinning of a contact

9

line of an evaporating droplet and hence the evolution and kinetics of the evaporating droplet [56].

10

Thus, the distinct behaviors of the droplet profile during the evaporation serve as another evidence

11

of the different hydrophobicity achieved by the proposed fabrication method and their unique

12

effects in the thermal transport phenomena. Figure 7a and 7b show the images and contours of an

13

evaporating water droplet over time on the superhydrophobic surfaces without and with

14

hydrophilic tips, respectively. The drop volume, contact angle, and contact diameter were

15

measured from the captured images, as presented in Figure 7c, 7d, and 7e. There is no significant

16

difference in the volume evolution of the droplet from the two different surfaces (Figure 7a).

17

However, with regard to the evolution of droplet contact angles and contact diameters, the two

18

surfaces demonstrate distinctive behaviors despite the almost same initial contact angles. The

19

droplet contact angle on the surface without hydrophilic tips remained roughly constant (130-135°)

20

for the first ~18 min (Figure 7d), while the contact diameter continuously decreased with time

21

(Figure 7e). That corresponds to a constant contact angle (CCA) mode for an evaporating droplet,


22

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indicating that the droplet boundary depinned from the very beginning of evaporation. In contrast,

2

the contact diameter of the droplet on the surface with hydrophilic tips remained roughly constant

3

(1.2 - 1.3 mm) for the first ~14 min (Figure 7e), while the contact angle continuously decreased

4

with time (Figure 7d). That corresponds to a constant contact diameter (CCD) mode, indicating

5

that the droplet boundary was highly anchored on the hydrophilic tips. Based on the onset of the

6

boundary detachment (the apparent decrease of the contact width), the apparent receding angle of

7

droplets on the surfaces without and with hydrophilic tips estimated from Figure 7e are ~135° and

8

~107°, respectively, agreeing with the results measured by laterally dragging the droplets. The

9

much smaller receding angle in the case with hydrophilic tips also indicates the greatly prolonged

10

pinned boundary during the CCD mode. At the later evaporation stage, both droplets on the

11

surfaces without and with hydrophilic tips translated to a mixed mode, where both the contact

12

angle and the contact width decreased with time.


23

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4 min

0 min

Volume (μL)

(c)

8 min

Time (min)

24 min

20 min

16 min

12 min

Contact angle (degree)

(d)

Time (min)

(a)

(b)

Contact diameter (mm)

28 min

(e)

Contour

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Page 24 of 36

Time (min)

1 2

Figure 7. Water droplet evaporation test. (a, b) Images and contour of the droplets on the

3

superhydrophobic surfaces without and with the decoration of hydrophilic tips, respectively. Scale


24

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bar in each image is 1 mm. (c) Change in droplet volumes during evaporation. (d) Change in

2

droplet contact angles during evaporation. (e) Change in droplet contact diameters during

3

evaporation.

4 5

CONCLUSIONS

6

In this work, we have developed a heterogeneous superhydrophobic surface decorated with

7

hydrophilic tips, by exploiting a novel meniscus-confined electrodeposition method. Owning to

8

the inherent Cassie-Baxter superhydrophobicity (i.e., capability of retaining air in cavities between

9

hydrophobic microstructures) of the original superhydrophobic surface, the selective deposition of

10

hydrophilic materials (nickel) only on the structural tips could be achieved. Such heterogeneous

11

hydrophilic tips allow the superhydrophobic surface to have unique properties compared to

12

conventionally (either slippery or sticky) superhydrophobic with homogenous chemical coating,

13

especially dynamic properties such as significantly enhanced droplet retention (i.e., higher contact

14

angle hysteresis and depinning force) in the droplet movement, while the Cassie-Baxter wetting

15

state is still sustained. The meniscus-confined electrodeposition method that we have developed is

16

a convenient process that can be easily scaled up, since it is based on the electroplating technique

17

that has been extensively used for surface coating and treatment for large-area substrates in

18

manufacturing industries. The superhydrophobic surfaces with regulated droplet retention would

19

be of great significance in many droplet-based applications such as bio-detection [57], inkjet

20

printing [58], evaporative cooling [56], and water harvesting [59], and droplet transportation [60].


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Page 26 of 36

1

For example, a sticky superhydrophobic surface is desirable in the application of protein chips,

2

where high contact angle (i.e., small footprint) as well as good adhesiveness of the droplet of

3

analyte are both necessary to facilitate the detection [57]. Besides, the unique combination of

4

hydrophilic pins on a superhydrophobic surface provides a promising approach of realizing long-

5

term gas-retaining for underwater applications, such as drag reduction in turbulent flow [33], anti-

6

corrosion [10] and anti-fouling surfaces [9].

7 8

ASSOCIATED CONTENT

9

Supporting Information.

10

The initiation of electrodeposition around the sharp edges of microposts’ top surfaces; The

11

scanning electron microscope (SEM) image (inclined views) of the microposts; Measurement of

12

contact angles of the electrolyte droplet on the surfaces; Scanning electron microscope (SEM)

13

image (side view) of the electrodeposition result due to the failure of Cassie-Baxter wetting state;

14

Table of the DRIE parameters; Videos of the underwater immersion test; Videos of the droplet

15

evaporation test.

16

AUTHOR INFORMATION

17

Corresponding Author

18

*E-mail:

19

ACKNOWLEDEMENTS

[email protected]


26

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1

This work has been supported by the National Natural Science Foundation of China (award

2

numbers 51405245 and 61327802), the Key Program of Science Foundation Tianjin (award

3

number 16JCZDJC30200), and the U.S. National Science Foundation (award numbers 1462499

4

and 1537474).

5


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Si

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