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Surface Instabilities for Adhesion Control

Controlling the specific adhesive properties of surfaces is a technologically complex challenge that has piqued the interest of many research groups around the world. While many scientists have used complex topographic and chemically altered surfaces to tune adhesion, others have shown that naturally occurring phenomena, such as elastic instabilities, can impact adhesion. We provide a thorough investigation into the effects of periodic surface buckling instabilities, or wrinkles, on adhesion. Wrinkles are an attractive surface patterning alternative as they form spontaneously over large areas and their dimensions, namely wavelength and amplitude, can be controlled on length scales relevant for adhesion control. We focus on the development of fundamental relationships that relate wrinkle adhesion to materials properties and topographic feature geometry. To accomplish this goal, we first investigate the separation of a flat rigid punch from a single elastic cylinder, which models the separation of a single wrinkle. The knowledge gained from this study is then utilized to develop a scaling expression relating adherence force to wrinkle geometry, materials properties, and testing geometry. This scaling theory is validated by varying these parameters systematically in a series of model wrinkle adhesion experiments. Added complexity in the form of varied crosslinker density, which alters the ratio of storage to loss moduli, and geometric confinement effects on wrinkle adhesion are then studied. Finally, a novel technique for fabricating biaxial wrinkles with two independently-adjusted wavelengths is developed, adding an additional parameter which can be tuned to further control adhesion. A single elastic cylinder was probed with a finite rigid flat probe, allowing the separation mechanism of a single "macro" scale wrinkle to be determined. Rather than a long cylinder contact mechanism, which has been utilized in describing wrinkle adhesion mechanisms in the past, an elliptical contact area approximation was found to more appropriately describe the single cylinder adhesion data. To consider the impact of an array of cylinders on adhesion, a model wrinkle system comprised of an elastomeric foundation and chemically-simple polymer film was developed. The wrinkle wavelength, amplitude, substrate modulus, and probe radius were varied, and the normal adhesive response of each aligned wrinkled surface was determined. Overall, wrinkles were found to decrease the separation force relative to a smooth surface and the separation force varied inversely with the square root of a wrinkle dimension, either wavelength or amplitude. The effects of viscoelasticity on the adhesion of a wrinkled substrate that is geometrically confined was studied. Wrinkled surface features were molded onto the surface of a rigid cylindrical probe, and the normal adhesion of these probes contacting thin elastomeric films fabricated with varying crosslinker concentrations was measured. The materials-defined length scale relating adhesion energy and modulus controlled the wrinkle feature sizes that impacted the adhesive response of each smooth film. In the most highly crosslinked films, small wrinkles increased both the separation force and adhesion energy of the interface two-fold, while large wrinkles reduced adhesion to almost nothing. Capitalizing on knowledge gained in the fabrication of many wrinkled surfaces, a novel technique for fabricating biaxial wrinkles was developed. Aligned wrinkles were formed on a partially crosslinked substrate, the modulus of the substrate was increased by allowing the material to crosslink completely, and a mechanical compressive strain was then imposed orthogonal to the primary wrinkle direction. This process resulted in the formation of biaxial wrinkled surfaces with two distinct, independently controlled lateral dimensions or wavelengths.
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