Rim recoil of impacted drops with pinned contact lines*

When a liquid drop impacts a surface with sufficient speed, it initially spreads to form a thin lamellar disk of liquid surrounded by a relatively thick rim, quickly reaching a maximum spreading diameter before recoiling to minimize the droplet's total energy. While the spreading phase is dominated by inertia, the recoil dynamics depend intimately on the receding contact angle and the fluid rheology. The present work investigates the recoil of rims formed in drop impact where the contact line is 'pinned'. Experiments are performed using drops of water, glycerol mixtures, and a shear-thinning yield stress fluid impacting on clean glass slides at varying speeds, resulting in a pinned contact line for varying degrees of maximum spreading diameter and fluid viscosity. The experiments show that while the recoil rate of rim is initially fast, it quickly slows as the recession progresses. Incorporating a rate-dependent viscosity further slows the recoil. An energy model is constructed to investigate the roles of the maximum spreading extent and the lamella thickness on the recession rate. Increasing either the maximum spreading extent or lamella thickness leads to a slower recoil for a fixed total volume due to a lower rate of energy minimization. In the limiting case of no lamella, an energy minimum appears early in the recession and the rim will remain stable as a toroidal cap.