Active Droplets in Polymer Solutions | Seminar Talk | Dr. Rahul Mangal, IIT Kanpur #physics #science
Channel: IISER Pune Optics League
Date: 21/07/2023

Rahul Mangal is presently an Associate Professor in Chemical Engineering at IIT Kanpur. He completed his PhD from Cornell University where he worked on understanding fundamental properties of Polymer Nanoparticle composites. Subsequently, he did his Post Doc at University of Wisconsin Madison where he worked on Active Janus Particles in Liquid Crystals. Presently, at IIT Kanpur his group is focusing on investigating the motion of Artificial micro-swimmers, which includes self-propelled Janus colloids and self-propelled emulsion droplets, in complex surroundings. 

Abstract of the Talk:
Active Droplets in Polymer Solutions
Rahul Mangal, IIT Kanpur

Typical bodily and environmental fluids encountered by biological swimmers consist of dissolved macromolecules such as proteins and 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-induced swimming oil droplet in an aqueous medium dispersed with macromolecular solutes. 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 a 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 the suitable choice of macromolecules.
On further increasing the polymer concentration, the surrounding medium was made viscoelastic in nature. The associated Deborah number (De) is tuned by varying the surfactant (fuel) and polymer concentration. At moderate De, the droplet exhibits a steady deformed shape, markedly different from the spherical shape observed in Newtonian media. A simple theoretical analysis based on the normal stress balance at the interface is shown to accurately predict the droplet shape. With a further increase in De, time-periodic deformations accompanied by oscillatory transitions in swimming modes are observed. The study unveils the rich complexity in the motion of active droplets in viscoelastic fluids, which has been hitherto unexplored.

Talk date:
Thursday 09, 2023, 04:00 PM IST