Jet-driven viscous locomotion of confined thermoresponsive microgels

TL;DR

This paper demonstrates how thermoresponsive microgels can achieve net locomotion in confined spaces through jet-driven propulsion caused by cyclic volume changes, with a theoretical model matching experimental results.

Contribution

It introduces a novel jet-driven propulsion mechanism for microgels, expanding understanding of non-reciprocal swimming in confined environments.

Findings

Microgels exhibit net movement due to water flux during swelling and shrinking.

Theoretical model accurately predicts experimental propulsion speeds.

Jet-driven flow is key to the microgel's locomotion mechanism.

Abstract

We consider the dynamics of micro-sized, asymmetrically-coated thermoresponsive hydrogel ribbons (microgels) under periodic heating and cooling in the confined space between two planar surfaces. As the result of the temperature changes, the volume and thus the shape of the slender microgel change, which lead to repeated cycles of bending and elastic relaxation, and to net locomotion. Small devices designed for biomimetic locomotion need to exploit flows that are not symmetric in time (non-reciprocal) to escape the constraints of the scallop theorem and undergo net motion. Unlike other biological slender swimmers, the non-reciprocal bending of the gel centreline is not sufficient here to explain for the overall swimming motion. We show instead that the swimming of the gel results from the flux of water periodically emanating from (or entering) the gel itself due to its shrinking (or swelling). The associated flows induce viscous stresses that lead to a net propulsive force on the gel. We derive a theoretical model for this hypothesis of jet-driven propulsion, which leads to excellent agreement with our experiments.