Mode-Switching of Active Droplets in Macromolecular Solutions

TL;DR

This study investigates how the motion of active droplets in polymer-doped solutions is highly sensitive to macromolecular presence, revealing mode-switching behaviors and proposing a Peclet number framework to understand these transitions.

Contribution

We demonstrate that macromolecular solutes induce mode-switching in active droplet propulsion, providing new insights into controlling active matter in complex fluids.

Findings

Droplet motion is highly sensitive to macromolecular solutes.

Mode-switching from pusher to puller occurs with increased polymer concentration.

A Peclet number framework captures the observed propulsion transitions.

Abstract

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.