Droplet Migration in the Presence of a Reacting Surfactant at Low P\'eclet Numbers

Abstract

A surfactant-laden droplet of one fluid dispersed in another immiscible fluid serves as an artificial model system capable of mimicking microbial swimmers. Either an interfacial chemical reaction or the process of solubilization generates gradients in interfacial tension resulting in a Marangoni flow. The resulting fluid flow propels the droplet toward a region of lower interfacial tension. The advective transport of surfactants sustains the active propulsion of these droplets. In these systems, the local interfacial tension is affected by the interfacial reaction kinetics as well as convection and diffusion induced concentration gradients. The migration of such a surfactant-laden viscous droplet undergoing an interfacial reaction, suspended in a background Poiseuille flow is investigated. The focus is specifically on the role of the surface reaction that generates a non-uniform interfacial coverage of the surfactant, which in turn dictates the migration velocity of the droplet in the background flow. Assuming negligible interface deformation and fluid inertia, the Lorentz reciprocal theorem is used to analytically determine the migration velocity of the droplet using regular perturbation expansion in terms of the surface P\'eclet number. We show that the presence of interfacial reaction affects the magnitude of both stream-wise and cross-stream migration velocity of the droplet in a background Poiseuille flow. We conclude that the stream-wise migration velocity is not of sufficient strength to exhibit positive rheotaxis as observed in recent experimental observations. Additional effects such as the hydrodynamic interactions with the adjacent wall may be essential to capture the same.