Chemically-active drops swimming near a wall

Active drops are synthetic, isotropic, micron-sized “swimmers” that emit or absorb chemical solutes. Solutal gradients drive interfacial flows and the solute’s own convective transport. If solute diffusion is small, the nonlinear coupling of the fluid flow and solute transport around the drop can cause a spontaneous symmetry-breaking, leading to sustained interfacial flows and “swimming” of the drop.

Active drops are typically not neutrally-buoyant and evolve at small finite distances from rigid boundaries. Yet, existing theoretical models ignore this fundamental feature and systematically focus on unbounded flows. We bridge here this gap in understanding, to obtain a critical physical insight on the emergence of self-propulsion of active drops along a rigid wall. Specifically, by analyzing the linear stability of the full hydro-chemical problem, we show that, and explain why, a reduction in the drop-wall separation actually promotes the drop’s self-propulsion.