WRINKLING, CRUMPLING, AND SNAPPING SURFACES FOR ADHESION CONTROL Alfred J. Crosby, Associate Professor, University of Massachusetts, Amherst, MA Douglas Holmes, Ph.D. Candidate, University of Massachusetts, Amherst, MA Edwin P. Chan, Postdoctoral Researcher, NIST, Gaithersburg, MD Chelsea Davis, Ph.D. Candidate, University of Massachusetts, Amherst, MA Introduction From the adhesion of geckos to the wrinkling of skin, nature displays numerous examples of materials structures that either result from or use mechanical instabilities. 1-4 Mechanical instabilities exhibit large fluctuations in deformation or geometry upon a small change in stress. We use mechanical instabilities to provide novel design, fabrication, and characterization strategies for materials surfaces. Here, we highlight the use of surface wrinkles in soft elastomers for the control of adhesion, optics, and sensors. Stable, Aligned Surface Wrinkles Surface buckling, or wrinkling, can be generated in a variety of materials systems that include: (1) thermally or mechanically stressed metallic, polymeric, and silicate thin films supported on elastomeric substrates, 5, 6 (2) dried thin films prepared by the sol-gel method, 7, 8 as well as (3) soft gels placed under geometric confinement that are swollen or dried. 9, 10 These surface relief structures are interesting for their pattern complexity and ease of well-defined feature formation. In this report, we demonstrate the spontaneous alignment of surface wrinkles through pre-defined regions of local moduli-mismatch combined with osmotic pressure. The moduli-mismatch is produced by converting selective areas of a poly(dimethyl siloxane) (PDMS) elastomer surface into a silicate thin surface layer through a UV Ozone (UVO) oxidation process. Geometric confinement of the oxidized regions coupled with an osmotic stress leads to the alignment of the surface wrinkles. The osmotic stress is induced by swelling the elastomer with a photopolymerizable n-butyl acrylate (nba) monomer, thus allowing the aligned wrinkles to be stabilized upon ultraviolet (UV) exposure. The shape of oxidized regions controls the local stress state upon swelling which in turn orients the formation of these wrinkles. 4, 11 This approach is amenable to creating relief patterns on a variety of polymer systems. A summary of our general process is provided in Figure 1. An example of a surface wrinkle-induced pattern is shown in Figure 2. 127
Figure 1. Schematic of the process for inducing surface wrinkles in soft elastomers. Swelling in confined state occurs between steps 3 and 4. Figure 2. Surface wrinkle pattern due to confined swelling stress. Width of image is 1mm. Adhesion of Surface Wrinkles Surfaces wrinkles provide the isolation of discrete wavelengths that can be used to tune properties such as adhesion and friction. Using a processing method similar to Tanaka et al, 9 we have fabricated surface wrinkles in soft, polymer films of crosslinked poly(n-butyl acrylate). 12 Using contact adhesion tests between a fused silica probe and the wrinkled elastomer films, we demonstrate that the adhesion can be 128
tuned, and more importantly enhanced, with surface wrinkles. Figure 3 demonstrates the impact of wrinkled surfaces on the adhesion of poly(n-butyl acrylate). We discuss how a simple scaling relationship, similar to arguments used to understand the adhesion of the gecko and numerous insects, can account for this control of polymer adhesion. An overview of these general concepts related to the patterned control of adhesion can be found in a recent review. 13 Figure 3: a) Schematic of contact adhesion test for characterizing adhesion of wrinkled adhesive materials. b) Representative force (P) versus time curve for wrinkled PnBA adhesive contact adhesion test with fused silica. Snapping and Crumpling of Surface-Attached Sheets Surfaces that respond to environmental stimuli or on-command are attractive for numerous applications, from smart adhesives to sensors. In nature, one of the most impressive responsive systems is the Venus Flytrap, which relies upon snap instabilities as a responsive mechanism, as detailed in a recent publication 14. Our idea was to create surfaces with patterned structures to transition through a snap instability at a prescribed stress state. Changes in stress state can be related to triggers such as chemical swelling, light-induced architecture transitions, mechanical pressure, or voltage. The primary advantages of the snap transition are that the magnitude of change, the rate of change, and the sensitivity to change can be dictated by a balance of materials properties and geometry. Initial structures that demonstrate these concepts were fabricated by placing a patterned PDMS substrate under equibiaxial strain through inflation and then bonding a capping layer to the patterned depressions. 15 Upon release of the equibiaxial strain, the capping layer buckles above each of the patterned depressions to create a surface array of microlens shells. 15 These shells can snap between concave and convex curvatures through application of various triggers. We have demonstrated the control of the microlens shell geometry and that the transition time follows 129
scaling relation-ships presented for the Venus flytrap. 14,15 Furthermore, the microlens arrays have been demonstrated as optical devices that can alter their focal plane by snapping between the two curvature states. Figure 4: (a) SEM images of snapping microlens shells in concave and convex states. (b) Measured time of snap as a function of microlens shell thickness. Solid line represents h 2 dependence as presented by Mahadevan for Venus Flytrap leaflets. Using the same process, non-axisymmetric structures can be achieved through the application of higher equibiaxial strains. 16 Higher strains leads to bifurcated shells and the generation of higher aspect ratio features that can snap from positive to negative states. These transitions have been used to change water contact angle and adhesion as a function of surface geometry. Figure 5: Non-axisymmetric snapping structures formed by higher order crumpling of equibiaxial PDMS plates bound to a surface. Top images are z-scale contour maps obtained from confocal microscopy. Bottom images show 2-d projections of structures from reflection microscopy as a function of applied compressive strain. By the inherent process for creating the crumpled surface patterns, the relationship between shell geometry and strain allows the creation of responsive surface patterns that can dynamically change their shape in response to strain changes on the surface. Osmotic swelling of the PDMS network with hexane changes the applied strain around the surface-attached sheets and leads higher degrees of crumpling. When hexane is added to the surface of initially spherical shells the shell swells initially, but is laterally confined by the edges of the unswollen hole below it, to which it is bound. This lateral confinement increases the compressive strain along the edge of the shell, which increases the height of the shell when a remains constant. Since the thickness of the film remains constant as the height of the shell increases, 130
the shell will go through a bifurcation point and form a stable, nonaxisymmetric geometry. This process is reversible, and it offers a novel approach for fabrication of the next generation of surface patterns, especially in the context of responsive materials. Dynamically changing surface patterns offer a unique approach to the development of smart adhesives and switchable wetting surfaces. Summary Figure 6: The responsiveness of convex shells to increasing/decreasing strain as applied by swelling via hexane. Using lessons from nature, we have created unique surface structures that can be used for the predictable control of adhesion, optical properties, and sensing. The processes and mechanisms presented are not limited by specific materials system; thus creating numerous opportunities for future applications. References 1. Autumn, K.; Liang, Y. A.; Hsieh, S. T.; Zesch, W.; Chan, W. P.; Kenny, T. W.; Fearing, R.; Full, R. J. Nature 2000, 405, (6787), 681-685. 2. Jagota, A.; Bennison, S. J. Integrative and Comparative Biology 2002, 42, (6), 1140-1145. 3. Crosby, A. J.; Hageman, M.; Duncan, A. Langmuir 2005, 21, (25), 11738-11743. 4. Chan, E. P.; Crosby, A. J. Soft Matter 2006, 2, (4), 324-328. 5. Bowden, N.; Huck, W. T. S.; Paul, K. E.; Whitesides, G. W. Applied Physics Letters 1999, 75, (17), 2557-2559. 6. Bowden, N.; Brittain, S.; Evans, A. G.; Hutchinson, J. W.; Whitesides, G. W. Nature 1998, 393, 146-149. 7. Chua, D. B. H.; Ng, H. T.; Li, S. F. Y. Applied Physics Letters 2000, 76, (6), 721-723. 8. Hayward, R. C.; Chmelka, B. F.; Kramer, E. J. Macromolecules 2005, 38, (18), 7768-7783. 9. Tanaka, T.; Sun, S.-T.; Hirokawa, Y.; Katayama, S.; Kucera, J.; Hirose, Y.; Amiya, T. Nature 1987, 325, 796-798. 10. Matsuo, E. S.; Tanaka, T. Nature 1992, 358, 482-484. 11. Chan, E. P.; Crosby, A. J. Advanced Materials 2006, 18, (24), 3238 12. Chan, E. P.; Smith, E.; Hayward, R.; Crosby, A. J. Advanced Materials 2008, 20, 711-716. 13. Chan, E. P.; Greiner, C.; Arzt, E.; Crosby, A. J. Mrs Bulletin 2007, 32, (6), 496-503. 14. Forterre, Y.; Skotheim, J. M.; Dumais, J.; Mahadevan, L. Nature 2005, 433, (7024), 421-425. 15. Holmes, D. P.; Crosby, A. J. Advanced Materials 2007, 19, 3589-3593. 16. Holmes, D. P.; Ursiny, M.; Crosby, A. J. Soft Matter 2008, 4, 82-85. Acknowledgments We gratefully acknowledge the financial support of NSF-DMR CAREER award, ARO YIP Award, and NSF-MRSEC at University of Massachusetts. 131
TECH 32 Technical Seminar Speaker Wrinkling, Crumpling, and Snapping Surfaces for Adhesion Control Alfred J. Crosby, Ph.D., University of Massachusetts Amherst Alfred J. Crosby, Ph.D., is an associate professor in the Polymer Science & Engineering Department at the University of Massachusetts Amherst. He received his B.S. degree in civil engineering and applied mechanics at the University of Virginia and his Ph.D. in materials science and engineering at Northwestern University. He was awarded a National Research Council Fellowship for his postdoctoral research in the Polymers Division at the National Institute of Standards and Technology. At UMass, he has received several awards including the 3M Non-tenured Faculty Award, the NSF CAREER Award, the Army Research Office Young Investigator Award, the Adhesion Society s Young Scientist Award and the Rohm and Haas New Faculty Award. His research interests include mechanics of hierarchical structures; adhesion; biomimetic materials design; responsive surfaces; deformation and fracture of thin films; polymer patterning; and nanocomposites. Crosby can be reached at crosby@mail.pse.umass.edu. 125