Mode switching of active droplets in macromolecular solutions

Abstract

Typical bodily and environmental fluids encountered by biological swimmers consist of dissolved macromolecules such as proteins or polymers, rendering them even non-Newtonian at times. Active droplets mimic the essential propulsive characteristics of several biological swimmers, and serve as ideal model systems to widen our understanding of their locomotive strategies. Here, we investigate the motion of a micellar solubilization driven active oil droplet in an aqueous medium consisting of polymers as macromolecular solutes. Experiments reveal extreme sensitivity of the droplet motion to the presence of macromolecules in its ambient medium. Through in situ visualization of the self-generated chemical field around the droplet, we notice unexpectedly high diffusivity of the filled micelles in the presence of high molecular weight polymeric solutes. This highlights the breakdown of continuum approximation due to a significant size difference between the macromolecular solutes and the micelles. It is shown that the Péclet number, defined based on the experimentally determined filled micelle diffusivity (taking into account the local solvent viscosity) successfully captures the transition from smooth to jittery propulsion mode for both molecular and macromolecular solutes. With an increase in macromolecular solute concentration, particle image velocimetry reveals another mode switching from the conventional pusher mode to puller mode of propulsion, characterized by a more persistent droplet motion. By doping the ambient medium with suitable choice of macromolecules, our experiments unveil a novel route to orchestrate complex transitions in active droplet propulsion.