Mechanics of a Novel Shear-activated Microfiber Array Adhesive
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Mechanics of a Novel Shear-activated Microfiber Array Adhesive Carmel Majidi1,2, and Ronald S Fearing1 1 Electrical Engineering and Computer Sciences, University of California, Berkeley, CA, 94720 2 Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, NJ, 08544 ABSTRACT Elastic rod theory and principles of contact mechanics motivate the development of a novel, shear-activated, microfiber array adhesive. Unlike with conventional Pressure Sensitive Adhesives (PSAs), the microfiber array and backing are composed entirely of a stiff, glassy polymer (polypropylene, elastic modulus E = 1 GPa) and an externally applied shear load is required to achieve contact with a substrate. Previously, results from a Shear Power Test on glass indicated a maximum interfacial shear strength of 10 kPa over 4 cm2, a factor of 1000 greater than with a smooth polypropylene sheet of similar thickness. Here we present a theoretical model that describes the mechanism for shear-activated adhesion and predicts a shear strength of 27 kPa, on the order of the experimental measurement. INTRODUCTION Motivated by principles of rod theory and contact mechanics, scientists and engineers are developing a new class of microfiber array adhesives [1]. Still in its early stages, this emerging field aims to introduce adhesives that are directional, reusable, biocompatible, temperature resistance, self-cleaning, and have a non-sticky default state. An example of adhesion with a microfiber array is presented in Figure 1. Unlike a conventional Pressure Sensitive Adhesive (PSA), this adhesive is composed entirely of a stiff, glassy polymer (polypropylene, elastic modulus E = 1 GPa). High elastic modulus correlates with high wear resistance and low tack and so may be essential for reusability and self-cleaning. The polypropylene fibers shown in Figure 1 have a radius R = 0.3 µm, length L = 20 µm, and density D = 42x106 fibers/cm2 and are on a 35 µm thick polypropylene backing [2]. Unlike the ultrahigh friction arrays presented in [3], these samples have a naturally planar backing. Hence, intimate contact is possible without needing to press the sample into a glass substrate [4]. As seen in the figure, the array permanently holds a 400 gram weight and there is little evidence of creep over time [2]. However, such contact does require an applied shear load. Once this shear load is removed, the sample spontaneously delaminates from the substrate and can be easily removed. This unique shear-activated adhesion property follows from elastic rod theory and contact mechanics. Here, a model is derived that predicts shear strength based on the mechanics and geometry of the microfibers and the interfacial properties between the microfiber tip and substrate. Design criteria are presented to determine the appropriate microfiber geometry for shear-activated adhesion with any selected material.
MODEL The remarkable adhesion demonstrated in Figure 1 is explained by a shearactivated adhesion model similar to that used by
