Dynamic Frictional Behavior of Microdroplets on PDMS Soft Substrate

The dynamic frictional behaviors of microdroplets on PDMS surface were studied by a self-made frictional force testing device. The effect of droplet volume, slip velocity, and mechanical properties of the PDMS substrate on the dynamic frictional behaviors were investigated. Results showed that the solid/liquid interfacial friction force consisted of maximum static friction force and dynamic friction force. The maximum static friction force was proportional to the viscosity and velocity gradient. The dynamic friction force was related to the droplet volume, sliding velocity and mechanical properties of the substrate. The dynamic friction force increased with droplet volume due to the increasing of the length of contact line. Moreover, the dynamic friction force increased with sliding velocity because the length of contact line and contact angle hysteresis increased with sliding velocity. Furthermore, solid/liquid interface adhesion force increased with decreasing of elastic modulus. As a result, the energy dissipation and dynamic friction force increased. The results would be helpful to provide theoretical guidance for the precise driving of microdroplets and optimize the motion parameters of droplet on PDMS surface, and then further to enrich the solid-liquid interface friction theory.