Article pubs.acs.org/Langmuir
Wet Self-Cleaning of Superhydrophobic Microfiber Adhesives Formed from High Density Polyethylene Jongho Lee*,† and Ronald S. Fearing‡ †
Mechatronics, Gwangju Institute of Science and Technology, 123 Cheomdan-gwagiro, Buk-gu, Gwangju 500-712, S. Korea Electrical Engineering and Computer Sciences, MC 1770, University of California at Berkeley, California 94720-1770, United States
‡
S Supporting Information *
ABSTRACT: Biologically inspired adhesives developed for switchable and controllable adhesion often require repetitive uses in general, dirty, environments. Superhydrophobic microstructures on the lotus leaf lead to exceptional self-cleaning of dirt particles on nonadhesive surfaces with water droplets. This paper describes the self-cleaning properties of a hard-polymer-based adhesive formed with high-aspect-ratio microfibers from highdensity polyethylene (HDPE). The microfiber adhesive shows almost complete wet self-cleaning of dirt particles with water droplets, recovering 98% of the adhesion of the pristine microfiber adhesives. The low contact angle hysteresis indicates that the surface of microfiber adhesives is superhydrophobic. Theoretical and experimental studies reveal a design parameter, length, which can control the adhesion without affecting the superhydrophobicity. The results suggest some properties of biologically inspired adhesives can be controlled independently by adjusting design parameters.
R
contact self-cleaning does not require special manipulation other than stepping or other substances such as water, providing a convenient means to keep the adhesives clean for applications in general environments, such as climbing robots. However, if water droplets are available, wet self-cleaning of gecko-inspired adhesives is more effective, possibly enabling recovery of adhesion up to 100%, by using lotus effects to remove dirt particles.37,38 Here, we report wet self-cleaning properties of microfiber adhesives with high-aspect-ratio structures which provide compliant contact to the surfaces. We describe, using experiments and theoretical studies, the performance of wet self-cleaning along with the surface properties such as superhydrophobicity with low contactangle hysteresis, and then describe the effects of one design parameter, length of the microfibers, on the performance of adhesion and water contact angle hysteresis. Extremely low contact angle hysteresis, enabled by hard polymer based microfiber adhesives, improves the chance to remove dirt particles with minimal efforts, compared to the previously reported gecko-inspired adhesives.33,34 In addition, we report results of repetitive wetting, dirtying, and wet self-cleaning to observe adverse affects on the microfiber adhesives. The fabrication process of the superhydrophobic microfiber adhesives appears in Figure 1a, in which two rollers apply heat and pressure to the thermoplastic, in this case, high density
evealed properties of natural gecko adhesives have supported the possibility of designing and fabricating synthetic gecko-inspired adhesives with various materials to capture remarkable properties, such as rapidly climbing walls (up to ∼1 m/s) and walking upside down on ceilings, enabled by switchable anisotropic adhesion on the gecko’s toes, combined with coordination of legs and whole bodies.1−3 These unique properties have motivated engineers to develop gecko-inspired adhesives4−11 for diverse applications including climbing robots,12 biomedical systems,13 microassembly,14 and others. Some of the designs include arrays of vertically aligned rods,15−17 angled rods,18−21 and hierachical structures,22−27 to provide compliance or anisotropy to the surfaces via bending, compressing, or stretching. Carbon nanotube arrays also provide very high adhesion.28−30 The uniqueness of the gecko adhesives is not only from their controllable adhesion, but also from self-cleaning on dry surfaces, where unbalanced forces act to shed dirt particles from adhesives to the surfaces,31 enabling geckos to keep their adhesives clean enough to hold their own body weight between molting. Some microfiber adhesives based on hard polymer have the properties of contact self-cleaning on dry surfaces without any aid from liquids.32 These microfiber adhesives recover up to 33% of shear adhesion from contact self-cleaning. Some other gecko inspired adhesives from carbon nanotubes33 and soft polymer rods,34 respectively, recover 60% and 100% of the adhesion of their pristine samples from wet self-cleaning with water droplets. Measurements of low contact-angle hysteresis predict the wet self-cleaning of surfaces with nano or microstructures.35,36 Dry © 2012 American Chemical Society
Received: July 26, 2012 Revised: September 16, 2012 Published: October 16, 2012 15372
dx.doi.org/10.1021/la303017a | Langmuir 2012, 28, 15372−15377
Langmuir
Article
microfibers make the HDPE film adhesive as reported previously. Figure 1d shows the microfiber array sustaining a weight of 200 g on a transparent glass slide in the shear direction. In addition to the adhesion, the high-aspect-ratio microfiber array makes the surface strongly water repellent as shown in Figure 1e. Water droplets, free-falling from about 7 cm high, bounce on the microfiber adhesive surface without wetting. The high-aspect-ratio microfiber array designed to be adhesive has similar water repelling properties as lotus leaves, which are known to self-clean dirty particles with water droplets, called the lotus effect.37 In the following, we present more detailed examination results of the self-cleaning properties of the microfiber adhesives with water droplets. Figure 2a shows a rolling water droplet (∼9 μL) on a dirty microfiber adhesive, prepared with dropping ceramic microspheres as dirt particles (∼1 μm, 10th percentile; 4 μm, 50th percentile; 15 μm, 90th percentile; 24 μm, 95th percentile; W410, 3M) freely from 2 cm high and shaking gently to remove the excessive microspheres. The water droplets do not wet on the surface. As the water droplet rolls on the microfiber adhesive, covered with the dirty particles, the droplet removes the particles along the path of rolling, making the path visually observable without magnification as in Figure 2a. Figure 2b shows an SEM image in the square region of the microfiber adhesive. The upper half of the image clearly indicates that the water droplets remove the dirt particles almost completely as confirmed with magnified images in Figure 2c and d. For comparison, we prepared pressure sensitive adhesives (PSA, Scotch Magic Tape, 3M) with the same microsphere particles as shown in Figure 2e. Figure 2f shows that microspheres remain after cleaning the PSA with water droplets. In contrast to the microfiber adhesives, the water droplets do not roll off but wet on the dirty PSAs. As a demonstration for self-cleaning adhesives, we quantified shear adhesion of pristine, dirty, and wet self-cleaned microfiber adhesives as well as PSAs. We measured shear adhesive force of the samples (∼1 cm2), preloaded with normal force (∼1 N/ cm2) on a transparent vertical glass slide in a configuration similar to that in Figure 1d, using a suspended cup instead of a metal weight. See details in the Experimental Section. Figure 2g summarizes the experimental results. The pristine microfiber adhesives have about 4.7 N/cm2 shear adhesion with standard deviation of 0.8 N/cm2 (3 samples, 12 measurements). As expected, dirty microfiber adhesives do not have measurable shear adhesion (10 N/cm2). The dirty PSAs show no measurable adhesion, and recovers about 0.5 N/cm2 (standard deviation ∼ 0.1 N/cm2, 4 samples, 12 measurements) with wet cleaning. These SEM images and measurement data clearly indicate that the high-aspect-ratio microfiber adhesives recover adhesive force with wet self-cleaning, as is known with the lotus effect of nonadhesive surfaces.37 The repetitive cycles of dirtying and self-cleaning do not diminish shear adhesion. The microfiber adhesives underwent 50 cycles of dirtying and wet self-cleaning do not show much degradation in shear adhesion (∼4.5 N/cm2, 3 samples, 12 measurements) as in Figure 2g. The examination of the SEM images taken every 10 cycles of dirtying and wet self-cleaning reveals no noticeable damages on the microfiber adhesives. See the Supporting Information.
Figure 1. Schematic illustrations of the fabrication process and images of biologically inspired superhydrophobic adhesive surfaces covered with an array of microfibers, holding a weight and bouncing water droplets. The surface with an array of microfibers forms adhesive and superhydrophobic surfaces. (a) Schematic illustration of the fabrication process. Two heated rollers (∼145 °C) melt a film of HDPE into the holes in the PC template. The polyimide film (PI) prevents the HDPE film from sticking to the roller. (b) Wet chemical etching of the laminated stack removes PC template selectively. (c) SEM images of microfiber array. Radius and length of the microfiber are 0.3 and 18 μm, respectively. Average center to center distance of the microfiber in the array is about 1.5 μm. The thickness of the backing layer is about 45 μm. (d) Optical image of the array of the microfibers sustaining a weight of 200 g. The microfibers are compliant enough to make contact onto the optically transparent glass substrate, maximizing shear adhesive force. (e) Image of dropping and bouncing water droplets on the array of the microfibers. The surface of the microfibers has very low wettability, thus water droplets bounce off.
polyethylene (HDPE, elastic modulus ∼0.9 GPa). The process involves preheating and stabilizing the rollers at 145 °C, followed by processing the stack of the HDPE film and polycarbonate (PC) template on a rigid glass slide. The heated rollers melt the HDPE film, whose melting temperature is about 130 °C, into the holes in the PC template that has higher melting point (∼ 265 °C). See the Experimental Section for further details. The rolling process provided high yield of the microfiber arrays, close to 100%, by reducing wrinkling and shrinking of the HDPE films during the heating, pressing, and cooling process, compared to the process reported previously.17 Selective chemical etching of the PC template with methylene chloride exposes microfibers on the HDPE backing as shown in Figure 1b and c. The radius and length of the individual highaspect-ratio microfiber (∼30) are 0.3 and 18 μm, respectively. The average spacing of the microfibers is about 1.5 μm. The dimensions (length and radius) and period are from the scanning electron microscopy (SEM) images of microfibers recorded with tilt angle 45°. Image processing of the microscope images of unfilled PC templates estimates the period with assumption of square-packed array. The compliant 15373
dx.doi.org/10.1021/la303017a | Langmuir 2012, 28, 15372−15377
Langmuir
Article
Figure 2. continued microspheres on the PSAs as some of microspheres embed on the soft PSAs. (g) Shear adhesive forces of pristine, dirty and self-cleaned microfiber adhesives and PSAs. After attaching the adhesives (∼1 cm ×1 cm) with a preload of ∼1 N/cm2 onto an optically transparent glass substrate, continuously increasing tensile shear force is applied to the adhesives until the adhesives lose contact with the glass substrate. The mean shear adhesion of the pristine microfiber adhesives is 4.7 N/ cm2 with standard deviation of 0.8 N/cm2. The dirty microfiber adhesives show no measurable macroscale adhesion (
