Article pubs.acs.org/Langmuir
Role of Counter-substrate Surface Energy in Macroscale Friction of Nanofiber Arrays Yongkwan Kim,† Francesca Limanto,† Dae Ho Lee,‡ Ronald S. Fearing,‡ and Roya Maboudian*,† †
Department of Chemical and Biomolecular Engineering, and ‡Department of Electrical Engineering and Computer Science, University of California, Berkeley, California 94720, United States ABSTRACT: The effect of counter-substrate surface energy on macroscale friction of nanofiber array is studied. Low-density polyethylene (LDPE) fibrillar array fabricated from silicon nanowire template is tested against glass substrates modified with various self-assembled monolayers, which exhibit a wide range of surface energy. A large drop in friction over a narrow range of surface energy is observed and explained in terms of drastically reduced number of fibers in actual contact, in addition to the reduced surface energy. The relationship between surface energy and fiber engagement is discussed with Johnson−Kendall−Roberts (JKR) and elastic beam models.
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INTRODUCTION Geckos have a unique climbing ability made possible by their dense array of foot-hairs.1,2 The remarkable adhesion and friction performance of their feet, which has been explained by the analyses based on the pull-off force3−5 as well as the peeling mechanism of a fibrillar array,5,6 has attracted an intense research in fabrication of gecko-like structures and adhesion/ friction testing of both natural and synthetic fibrillar surfaces.7−10 While increasingly complex structures have been fabricated, their performance has been mostly tested against smooth glass (or on oxidized silicon wafer),7−10 which is an ideal surface for high adhesion/friction due to its high surface energy. However, because it is critical for synthetic gecko-like adhesives to perform on a wide variety of surfaces as the natural gecko foot does, the role of counter-substrate surface energy in adhesion and friction remains an important issue. Only a few experimental studies have compared adhesion or friction of fibrillar surface against substrates with different surface energies. Autumn et al.2 reported similar shear stress of a gecko seta on hydrophilic oxidized Si and hydrophobic GaAs surfaces, which they indicated as evidence that capillary force does not play a significant role in fiber adhesion. Huber et al.11 reported significantly higher pull-off force for an individual gecko spatula against glass in comparison to silicon wafer treated with hydrophobic monolayer at various relative humidities, and concluded that the pull-off force is controlled by short-range forces with capillarity contribution due to adsorbed water layer. However, Puthoff et al.12 later reported that the material property change of the foot-hairs due to humidity may be responsible for the change in the observed adhesion, rather than any significant capillarity contribution. Ge et al.13 showed that for their carbon nanotube-based synthetic adhesive, shear force was similar against substrates with varying surface energies, but the peel-off strength was correlated with surface energies. While it is difficult to directly compare such © 2012 American Chemical Society
experiments, it seems that the quantitative role of countersubstrate surface energy in fibrillar friction is unclear. This study presents a systematic study on macroscopic friction of highly ordered low-density polyethylene (LDPE) nanofiber arrays against substrates with a wide range of surface energies. Glass surfaces treated by commercially available selfassembled monolayer (SAM) with various terminal groups are used as test substrates. The contact areas of the samples are simultaneously observed to provide insights on macroscopic contact fraction as well as the number of fibers in contact. The observed trend is discussed in the context of simple theoretical models relating substrate surface energy to fiber friction.
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EXPERIMENTAL DETAILS
Fabrication of LDPE Nanofiber Arrays. Ordered nanofiber arrays were fabricated by molding from silicon nanowire (SiNW) masters obtained by colloidal lithography and electroless etching as previously developed.14 Briefly, the fabrication steps are as follows. Polystyrene microspheres (1 μm, Thermo Scientific) were floated on a NaCl solution (∼0.5 wt %) by gently introducing the microsphere solution (1 wt % in water/ethanol by 1:1 volume ratio). The twodimensional array was densely packed on the water surface by a small droplet of a surfactant solution (sodium dodecylsulfate, 3 wt % in water) and transferred to ∼1 × 1 cm2 Si(100) chips (Silicon Quest International, 10−30 Ω). The spheres were reduced in size by plasma etching (Plasma-Therm PK-12 RIE), and a ∼20 nm film of gold was evaporated (Thermoionics VE-100 Vacuum Evaporator) onto the substrates. After the removal of the spheres by sonication in deionized water, the resulting substrates covered with gold mesh were exposed to etching solution composed of hydrofluoric acid (48%, EMD), hydrogen peroxide (35%, Fisher Scientific), deionized water, and acetonitrile (Fisher Scientific) in a 2/1/5/2 ratio by volume, which selectively etches the silicon region in contact with gold, producing the Received: October 18, 2011 Revised: January 6, 2012 Published: January 20, 2012 2922
dx.doi.org/10.1021/la204078z | Langmuir 2012, 28, 2922−2927
Langmuir
Article
master templates with well-ordered SiNWs. The SiNW templates were treated with a hydrocarbon monolayer by reaction with octadecyltricholorosilane (Sigma-Aldrich) in toluene (Fisher Scientific) with 1:1000 volume ratio for ∼15 h. For molding, a polycarbonate (PC) film (McMaster-Carr, ∼130 μm thick) was melted in a vacuum (∼10 Torr) onto the SiNW templates at 300 °C for 1.5 h. After cooling to room temperature, the PC film was mechanically peeled off from the SiNW templates to produce negative intermediate templates. Finally, a second melt-in process (1 h at 160 °C) of an LDPE film (McMasterCarr, ∼100 μm thick) onto the PC templates and dissolving of PC in a methylene chloride bath for 1 h produced ordered nanofiber arrays for friction testing. The fabricated samples were observed under a scanning electron miscroscope (Agilent NovelX-MySEM) at 1 kV with ∼10 nm of sputtered gold (SPI Supplies, Division of Structure Probe, Inc. Au sputtering system). Counter-substrate Preparation. Glass slides (Fisher Scientific) were successively sonicated in acetone and isopropanol for 10 min each, and then cleaned in boiling piranha solution (3:1 ratio of 97% H2SO4 and 35% H2O2) for 30 min. All SAM coating procedures were performed in a nitrogen environment with less than 5% relative humidity with a 1:1000 volume ratio of silane precursor to solvent (50 μL:50 mL). 1H,1H,2H,2H-Perfluorodecyltrichlorosilane (FDTS, Alpha Aesar) was deposited in 2,2,4-trimethylpentane (EMD) for 1 h. n-Decyltrichlorosilane (DTS, Gelest) and 11-bromoundecyltrichlorosilane (BUTS, Gelest) were deposited in toluene (Fisher Scientific) for 2 and 1.5 h, respectively. 11-Aminoundecyltrichlorosilane (AUTS, Gelest) was deposited in chloroform (Fisher Scientific) for 1 h. After the deposition step, the substrates were sonicated for 10 min in fresh baths of respective solvents, followed by another 10 min in isopropanol. Finally, they were postbaked at 130 °C in ambient pressure for 30 min. Static contact angles were measured with 3 μL drops of deionized water using a tensiometer (Ramé−Hart model 290). Surface topography measurements on the substrates were carried out over 10 × 10 μm areas with an atomic force microscope (Digital Instrument Nanoscope IIIa) in the tapping mode with a silicon tip (MikroMasch, NSC14/AIBS). Friction Measurements and Contact Area Observation. A fabricated ∼1 × 1 cm2 sample was placed on a prepared countersubstrate with no intentional preload. A rubber pad of the same size was placed on top of the sample, and a small piece of metal (∼10 g, corresponding to 0.1 N/cm2) was placed on top of the rubber pad to ensure consistent sample contact against the counter-substrate. The sample was connected to a weighing cup with a string over a pulley, which allowed friction measurement by progressively adding weight to the cup. Each sample went through multiple friction cycles, and the static friction before sliding was recorded for each cycle. Samples often exhibited increasing friction over several cycles followed by a decrease in friction due to plastic deformation, as reported in another study15 with similar thermoplastics. For consistency, the maximum friction value observed was taken as the friction value for each sample. Contact area was simultaneously observed by frustrated total internal reflection method.16 A white light directed parallel to the glass substrate created bright/dark contrast of the contacting/noncontacting regions, and this was recorded by placing a 45° mirror under the glass substrate and recording with a CCD camera. All recorded images were analyzed by software ImageJ (version 1.43u). Light intensity analysis was done by first converting the images to 256 grayscale, and finding the difference between the grayscale values of contacting region and the background dark region. All measurements, unless otherwise noted, were performed in an ambient condition (∼25 °C, ∼35% RH). Tests under low humidity (
