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Mode-Switching of Active Droplets in Macromolecular Solutions

Typical bodily and environmental ﬂuids encountered by biological swimmers consist of dissolved macromolecules such as proteins and polymers, often rendering them non-Newtonian. To mimic such scenarios, we investigate the motion of swimming droplets in an ambient medium doped with polymers as macromolecular solutes. 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. Our experiments reveal extreme sensitivity of droplet motion to the presence of macromolecular solutes in the ambient medium. Through in-situ visualization of the self-generated chemical ﬁeld around the droplet, we report unexpectedly high diﬀusivity of ﬁlled micelles in the presence of high molecular weight polymer solutes or macromolecules. This is attributed to the limitation of Stokes-Einstein relationship in accurately predicting micelle diﬀusivity due to signiﬁcant size disparity between micelles and the macromolecular solute. With an increase in polymer concentration, particle image velocimetry reveals a mode-switching, from the conventional pusher mode to a puller mode of propulsion, characterized by a more persistent droplet motion. With a further increase in concentration, a secondary transition from smooth to a jittery mode of propulsion occurs. A robust Peclet number framework is proposed that successfully captures the observed mode-switching of active droplets. Our experiments unveil a novel route to orchestrate complex transitions in active droplet propulsion by doping the ambient medium with suitable choice of macromolecules.