Tree-frog inspired friction pads provide positive curved contact performance in wet environment

There are numerous bionic gripping ends adapted to different environments, but stable gripping for curved objects in wet environments is still an urgent engineering challenge. In this study, inspired by the biological properties of tree frogs, a bionic friction pad with an inner concave micropillar surface (CMF-Pad) is proposed. Through the analysis of Hertzian contact theory, the optimization mechanism of the concave surface on the contact area is revealed, and the critical curvature range of the micropillar array to guarantee the drainage performance is deduced. Experiments have confirmed that the CMF-Pad exhibits significant advantages in wet environments (≥ 10 mg/cm² liquid film environment), its microchannels can actively drain interfacial fluids to enhance friction, and its transverse friction is increased by approximately 40% compared with that of flat/smooth friction pads. Simultaneously, it possesses excellent curved surface adaptation and torsion resistance. In the smooth surface contact life test, the CMF-Pad maintained more than 90% of its initial friction after 10,000 cycles of contact testing. Concurrently, practical applications have verified the stability of the friction pads when gripping, handling, and twisting curved objects such as medical reagent bottles by robotic arms, enabling reliable manipulation at liquid-mediated interfaces through dual mechanisms: active drainage and curvature-conformal contact, providing a different solution for operation in humid environments in semiconductor processing, medical equipment handling, and other fields.