Self-Assembled Asperities for Pressure Tunable Adhesion

Control of adhesion is important in a host of applications including soft robotics, pick-and-place manufacturing, wearable devices, and transfer printing. Many studies have investigated materials able to achieve a singular but tunable adhesion strength or switchable adhesion. However, there have not been extensive studies in developing a single material with multiple, tunable adhesion strengths. In this work, we present a versatile and scalable pressure tunable adhesive (PTA) that is based on the self-assembly of stiff microscale asperities on an elastomeric substrate. The adhesive strength of the PTA is tuned by controlling the amount of pressure applied to the patterned material. To pattern the elastomer, a glassy polymer thin film (polystyrene) is dewetted from a soft elastomer (polydimethylsiloxane) by thermally annealing the bilayer system. The PS droplets arrange in a Voronoi pattern that solidify upon quenching, leaving stiff cap-shaped asperities on the elastomer surface. The patterning parameters, specifically size and spacing of the asperities, are altered by changing the pre-annealing film thickness. Flat punch indentation testing is used to characterize the adhesion strength of PTAs ranging in pattern parameter size. During testing, the PTA is compressed by the punch to a pre-defined maximum compressive load, P_m, and retracted until reaching the critical pull-off load, P_c, where separation occurs. We demonstrate that the adhesion strength of the PTA increases with the applied compressive preload due to the unique contact formation mechanism caused by the asperities. Finally, we demonstrate the applicability of precision control of adhesion strength by utilizing the PTA for pick-and-place material handling. We show that P_c increases with P_m for these PTA materials. A mechanics analysis is presented to describe the adhesive contact between a rigid body and the PTA. Our pressure tunable adhesive design based on self-assembly of asperities presents a scalable and versatile approach that is applicable to a variety of material systems having different mechanical or surface properties.