Adhesive Force Gradients on a Biomimetic Superhydrophobic Surface

Article pubs.acs.org/cm

Invisible Gates for Moving Water Droplets: Adhesive Force Gradients on a Biomimetic Superhydrophobic Surface Daisuke Ishii*,†,‡,§ and Masatsugu Shimomura†,‡ †

World Premier International−Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ‡ CREST, Japan Science and Technology Agency, 4-1-8 Hon-cho, Kawaguchi 332-0012, Japan S Supporting Information *

ABSTRACT: On a hybrid surface, a gradient in the areal number density of metal domes generates an adhesive force gradient (AFG) that divides the surface into areas of water droplet sliding and adhesion. We demonstrate droplet-mass-dependent pinning of sliding water microdroplets on a tilted AFG hybrid surface. The pinning location acts as an invisible gate for the sliding water microdroplets.

KEYWORDS: biomimetics, self-organization, hybrid materials, gradient materials, superhydrophobicity

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INTRODUCTION In digital microfluidic devices and analytical systems, precise manipulations of microdroplets are desirable, such as transportation, separation, stabilization, selection, collection, sorting, and pinning. External energy sources, such as electric energy, drive such manipulations of microdroplets in artificial systems. In nature, however, plants, insects, and animals handle water microdroplets without using an external energy source. Water droplets on lotus leaves move quickly in order to keep the surfaces clean.1 A desert beetle collects water droplets from morning fog on its back, which is covered with an alternating pattern of hydrophilic and hydrophobic microstructures.2 The Australian lizard transports water to its mouth from the wet ground via a capillary system of grooved skin covering its entire body.3 The wharf roach, which is an arthropod that lives on seashores, transports water from a wet surface to its gills through open capillaries consisting of microprotrusions.4 In nature, examples abound of surfaces on which water droplets are manipulated by surface chemistry and microstructures. Mimicking natural surfaces, various structured surfaces have been reported for manipulating water droplets, including droplet repulsion from superhydrophobic structured surfaces,5 water collection on hydrophilic−hydrophobic hybrid surfaces,6 wettability regulation on surface gradient materials,7 and droplet pinning on microstructured and nanostructured surfaces.8 We have reported droplet pinning on superhydrophobic surfaces with hybrid structures composed of metal microdomes and polymer nanopillars.9 Recently, control of biomimetic water droplet manipulations was achieved by combining a structured surface and external energy source10 © 2013 American Chemical Society

(for example, on/off switching of droplet pinning by the thermal phase transition of liquid crystals,11 and on/off switching of magnetic field on a superhydrophobic iron surface12). A remaining challenge is the control of water droplet manipulations by using only surface structures. Herein, we report the control of water droplet pinning on a novel superhydrophobic hybrid surface that has a gradient structure13 of metal domes in polymer pillar arrays. We prepared the hybrid surface by electroless plating and simple peeling of a self-organized honeycomb-patterned polymer film.9 The areal number density of metal domes controlled water droplet adhesion on the superhydrophobic hybrid surface. The gradient in the number density of metal domes on the hybrid surface was formed by continuously changing the temperature of the solution during the plating process. In addition, we investigated water droplet pinning and sliding on the hybrid surface when it was tilted.

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EXPERIMENTAL SECTION

Preparation of the AFG Hybrid Surface. A polystyrene-based self-organized honeycomb-patterned porous films was prepared on a glass substrate by a previously reported method.14 A 10 mm × 30 mm piece of the honeycomb film was set vertically in a 50-mL plastic vial with a height of 50 mm. Initially, 10 mL of aqueous catalyst solution at 50 °C (0.010 mol dm−3 PdCl2 and 0.010 mol dm−3 poly(allylamine hydrochloride) was put in the vial at room temperature. Starting immediately thereafter, 40 mL of catalyst solution at 25 °C was Received: December 3, 2012 Revised: January 9, 2013 Published: January 14, 2013 509

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Figure 1. Preparation method of the superhydrophobic metal−polymer hybrid surface having a gradient in the number density of metal domes: (a) scanning electron microscopy (SEM) micrograph of the top view of a honeycomb film (inset shows a cross-sectional view); (b) schematic illustrations of a metallization process involving a gradual temperature change of the aqueous catalyst solution; (c) schematic models of the formation of a metal−polymer hybrid surfaces prepared from self-organized honeycomb-patterned polymer film; and (d) SEM micrograph of tilt view of the metal−polymer hybrid surface. gradually added to the vial at a rate of 2.5 mL s−1 (i.e., over a period of 16 s). The solution temperature in the vial was measured with an infrared thermometer (Model PT-U80, OPTEX, Japan) during the gradual addition of the catalyst solution at 25 °C. Then, the honeycomb film was transferred to a nickel electroless plating bath at 25 °C (0.050 mol dm−3 Ni(H2PO2)2·6H2O, 0.19 mol dm−3 H3BO3, 0.030 mol dm−3 CH3COONa, and 9.8 × 10−3 mol dm−3 (NH4)2SO4) for 2 h.15 After rinsing with deionized water, a nickel-coated honeycomb film was obtained. The upper face of the nickel-coated honeycomb film was bonded to a plastic plate by using an epoxy adhesive (Araldite Rapid, Huntsman Advanced Materials, Japan). A metal−polymer hybrid surface having a gradient in the areal number density of metal domes was formed on the plastic plate by cleaving the metal-coated honeycomb film sandwiched between the plastic plate and glass substrate.16 Surface structures were observed with a digital microscope (Model VHX-900, Keyence, Japan) and via a scanning electron microscopy (SEM) system (Model S-3000N, Hitachi, Japan). Surface wettability was evaluated by the contact angle and sliding angle of water droplets of various masses (1.5−20 mg), as measured with a contact angle meter (Model DM-500I, Kyowa Interface Science, Japan). Water Droplet Pinning and Sliding on the Hybrid Surface. The plastic substrate with the metal−polymer hybrid surface was fixed at a slope of 20°. A water droplet of 1.5−20 mg was placed onto the tilted surface. The sliding behavior of the droplet was recorded with a digital still camera operated in high-speed video mode (Casio EX-F1, Japan). The pinning location was measured using images captured from the video data.

metal deposited in the honeycomb holes constitutes the metal domes within the polymer nanopillar array (Figure 1d). Our previous work showed that the areal number density of metal domes on the hybrid surface changes in accordance with the temperature of the catalyst solution.9 The temperature of the catalyst solution was gradually decreased from 50 °C to 30 °C by adding catalyst solution at 25 °C (Figure 2a). The honeycomb films were highly wettable by the high-temperature solution, because of its low surface tension. Once the honeycomb holes were wetted by the solution, the solution could not escape from the holes, because of the high interfacial energy between the solution and the surface of the hole. A composite micrograph of the hybrid surface shows the gradient in the number density of metal domes (Figure 2b). Through the plating process, the substrate was modified such that the areal number density of metal domes (N) was low at the top (d = 0 mm) and high at the bottom (d = 30 mm). The relationships between contact angle and N at different positions (d) along the hybrid surface were investigated (Figure 2c). As d increased from the top of the substrate (d = 0 mm), N substantially increased, from ∼100 mm−2 to 3000 mm−2. The contact angle of the hybrid surface slightly decreased, from 162° to 141°, with increasing d. The contact angle at d = 30 mm had a large margin of error, because the large N area was strongly affected by the difference in water droplet mass. These results show that a gradient in the number density of metal domes was obtained by continuously changing the catalyst solution temperature, and the resulting hybrid surface remained highly hydrophobic, even where N was large. The metal domes have good affinity for water and, therefore, affect the adhesion properties of water microdroplets on the hybrid surface.17 The sliding angle, which is a measure of the adhesion strength of a droplet, was investigated to elucidate the dependence of adhesion strength on position along the substrate. In Figure 3a, the sliding angles of water droplets of various masses are plotted versus position d along the hybrid

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RESULTS AND DISCUSSION The polystyrene-based honeycomb-patterned porous film (Figure 1a) was set vertically in a vial containing 10 mL of an aqueous catalyst solution at 50 °C. Then, 40 mL of the catalyst solution at 25 °C was gradually added to the vial at 2.5 mL s−1. Lastly, the honeycomb film was transferred from the catalyst solution to a plating bath (Figure 1b). The metalcoated honeycomb film was cleaved to reveal metal−polymer hybrid structures and a polymer pillar array (Figure 1c). The 510

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Figure 3. Adhesive force gradient (AFG) of the superhydrophobic metal−polymer hybrid surface: (a) sliding angles of water droplets of various masses on the hybrid surface plotted versus position d (“adhesion” on the vertical axis indicates that the water droplet does not slide off the surface at a tilt angle greater than 90°); (b) the adhesive force of a single metal dome 9.0 μm in diameter (Fdome), plotted against water droplet mass.

Figure 2. Gradient in the areal number density of metal domes (N) of the superhydrophobic metal−polymer hybrid surface: (a) temperature change of the catalyst solution in the vial with vertical-fixed honeycomb film by gradual addition of 40 mL of catalyst solution at 25 °C at a rate of 2.5 mL s−1 into the initial 10 mL of catalyst solution at 50 °C (the position at d = 0 mm is the top of honeycomb-patterned porous film); (b) a composite micrograph of the hybrid surface (black dots indicate the metal domes); and (c) areal number density of metal domes and contact angles of water droplets of various masses on the hybrid surface plotted versus position d.

where F is the adhesive force, θSA the sliding angle, m the mass of the water droplet, and g the acceleration due to Earth’s gravity. From the value of F, the adhesive force of a single metal dome 9.0 μm in diameter (Fdome) was calculated as Fdome =

surface having a gradient in the number density of metal domes. The sliding angle increased with increasing d for a given water droplet mass, but decreased with increasing water droplet mass for a given d. Notably, a small water droplet placed or moved to a position with high N adhered to the surface when the substrate was turned upside down; in other words, the adhesive force was greater than the gravitational force acting on the droplet. The pinning location of the water droplets moved further down the substrate (increasing d) as the water droplet mass was increased. The adhesive force at each position along the hybrid surface was calculated as follows:

F S ( m ) N (d )

where S(m) is the apparent contact area with a water droplet of a given mass m, and N(d) is the areal number density of metal domes at position d. S(m) was measured from the projection of the water droplet onto the hybrid surface. Fdome was nearly constant at 7.3 ± 0.9 nN, regardless of position d and water droplet mass m (Figure 3b). These results indicate that the areal number density of metal domes (N) dominates the adhesive force. In other words, a hybrid surface having a gradient in the number density of metal domes has an adhesive force gradient (AFG) acting on the water droplet. We next investigated the location-selective pinning of a water droplet on the AFG hybrid surface. The relationship between the pinning location and water droplet mass was examined through the sliding behavior of water droplets of various

̇ F = mg sin θSA 511

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Figure 4. Sliding and pinning behavior of the (a) 6.0 mg, (b) 8.0 mg, (c) 15 mg, and (d) 20 mg water droplet on a tilted AFG hybrid surface with slope of 20°. Arrows indicate the pinning locations.

masses, which were dropped onto the AFG hybrid surface (d = 0 mm) with a slope of 20° see Figure 4, as well as Movies S1− S3 in the Supporting Information). The 20-mg water droplet passed over the entire length of the AFG hybrid surface (Movie S1). In contrast, droplets of 15 mg or less slid and were then pinned on the AFG hybrid surface (Movies S2 and S3). The pinning location became closer to the starting position (d = 0 mm) as the droplet mass was decreased (Figure 5a and Movie S4 in the Supporting Information). The pinning location on the tilted AFG hybrid surface acts as an invisible gate for the sliding water droplets, dividing the surface into a sliding area and an adhesion area. We evaluated the sliding behavior of water droplets in this experiment in terms of kinetic energy, gravitational potential energy, and frictional energy as follows: mv 2 = mg Δd sin 20° − F Δd 2 = mg Δd sin 20° − FdomeS(m)N (d)Δd

where v is the instantaneous sliding velocity after sliding for 1 mm (sliding distance; Δd = 1 mm) and Fdome is the adhesive force of a single metal dome (7.3 nN calculated from the sliding angle measurements). Figures 5b and 5c respectively show plots of calculated and measured velocities of water droplets (6.0, 8.0, 15, and 20 mg) versus position d. All the measurement results agreed well with the calculation results. Thus, the gate location could be accurately estimated from the energy conservation law of the moving water droplet. When the droplet was placed on a substrate at a higher tilt angle, the gate location was farther from the starting position, because of the larger gravitational potential energy of the water droplet. Droplet mass, tilt angle of substrate, and also gradient of the areal number density of the metal dome, which is controlled by preparation conditions, and structural parameters of the template honeycomb film govern the gate location on the AFG hybrid surface. Moreover, further tilting the substrate (Movie S5 in the Supporting Information) or adding water to a droplet (Movie S6 in the Supporting Information) induces a pinned water droplet to start sliding again. This study provides the first demonstration of location-selective pinning of a sliding water droplet on a metal−polymer hybrid surface having a gradient in the adhesive force that acts on the water droplet.

Figure 5. Invisible gates for the sliding water droplets on the tilted AFG hybrid surface with slope of 20°: (a) sliding and pinning behavior of water droplets of various masses (the pinning location acts as an invisible gate for the sliding water microdroplet); (b) calculated and (c) measured instantaneous sliding velocities of water droplets ((■) 6.0, (○) 8.0, (△) 15, and (▲) 20 mg) plotted against position d. Arrows indicate the pinning locations where the droplet velocity goes to zero.

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CONCLUSION In conclusion, we prepared a superhydrophobic metal−polymer hybrid surface having a gradient in the number density of metal domes by a simple method of wetting porous cavities of a honeycomb-patterned polymer film. The gradient in the number density of metal domes enabled location-selective and mass-dependent pinning of water microdroplets, and it

acted as an invisible gate for sliding water microdroplets. By setting the sliding conditions, we precisely controlled the gate 512

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(16) Yabu, H.; Takebayashi, M.; Tanaka, M.; Shimomura, M. Langmuir 2005, 21, 3235. (17) Ishii, D.; Yabu, H.; Shimomura, M. Biomedical Engineering Systems and Technologies in Communications in Computer and Information Science, Vol. 52; Fred, A., Filipe, J., Gamboa, H., Eds.; Springer: Berlin, 2010; pp 136.

location, which was easily estimated from the energy conservation law of the moving water droplet. We anticipate that this water droplet manipulation on a superhydrophobic surface by an adhesive force gradient (AFG) will provide not only a deeper understanding of functional superhydrophobic surfaces but also practical applications, for example, in digital microfluidic devices.

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ASSOCIATED CONTENT

S Supporting Information *

Movies of water droplet pinning and sliding on the hybrid surface. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

Center for Fostering Young and Innovative Researchers, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466−8555, Japan. Author Contributions

All authors have given approval to the final version of the manuscript. Funding

This research was partly supported by a Grant-in-Aid for Young Scientists (A), Ministry of Science, Education, Culture, Sports, Science, and Technology, Japan (No. 21686065). Notes

The authors declare no competing financial interest.

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