Enhanced Adhesion of Dry Adhesives through Interface Stress Distribution Control

In contrast to conventional chemical adhesives, dry adhesive materials acquire adhesion by surface interactions such as van der Waals forces rather than chemical bonding. Dry adhesive materials demonstrate a wide range of applications across multiple areas owing to their simplicity in preparation and recyclability. For instance, biomimetic robots inspired by creatures like geckos exploit dry adhesive materials for space or outdoor scenarios. These materials are used in medical equipment, wearable materials, and transfer printing of micro/nano devices. However, when dry adhesive materials are subjected to normal tensile force, they may experience stress concentration at the edge of their contact interface with the substrate, which results in an adhesive strength being less than the theoretical value. To address this issue, this study designed micrometer-scale carbonyl iron powder particles/polydimethylsiloxane magnetorheological elastomer-based dry adhesive materials. By adjusting the mass fraction of carbonyl iron powder and applying an external magnetic field, disk-like composites in centimeter scale with variable Young's modulus along the radial direction in stages or continuously were prepared. The adhesive strength of the two test samples reached 8.0 N/cm² and 11.4 N/cm², respectively, which was 35% and 77% higher than that of control samples with the same carbonyl iron powder mass fraction (60%). Based on finite element simulation, the radial distribution of Young's modulus effectively reduces stress concentration along the edge region, which in turn homogenizes the normal stress distribution at interfaces, thus enhancing normal adhesion performances. Comparing to homogeneous and multi-stage composites, the continuously varying modulus samples exhibited a more uniform normal stress distribution at the interface, which further improved adhesion. Our design of radial distribution of modulus over dry adhesive materials and the achievement of a continuously change of modulus through spatial distribution of magnetic fields provides valuable insights into the optimization of dry adhesive materials for various engineering applications.