Shapes and dynamic regimes of a polar active fluid droplet under confinement

TL;DR

This study explores how geometrical confinement influences the shapes and movement patterns of active polar fluid droplets, revealing diverse dynamic regimes and potential control strategies for their propulsion.

Contribution

It uncovers the complex interplay between activity, elasticity, and confinement in determining droplet morphology and motility, advancing understanding of active matter under geometric constraints.

Findings

Diverse shapes including worm-like, cell-like, and bullet-shaped droplets.

Dynamic regimes range from oscillating to rectilinear motion.

Confinement can be used to control droplet propulsion direction.

Abstract

Active droplets are artificial microswimmers built from a liquid dispersion by microfluidic tools and showing self-propelled motion. These systems hold particular interest for mimicking biological phenomena, such as some aspects of cell locomotion and collective behaviors of bacterial colonies, as well as for the design of droplet-based biologically inspired materials, such as engineered tissues. Growing evidence suggests that geometrical confinement crucially affects their morphology and motility, but the driving physical mechanisms are still poorly understood. Here we study the effect of activity on a droplet containing a contractile polar fluid confined within microfluidic channels of various sizes. We find a surprising wealth of shapes and dynamic regimes, whose mechanics is regulated by a subtle interplay between contractile stress, droplet elasticity and microchannel width. They range from worm-like and cell-like shaped droplets displaying an oscillating behavior within wider channels to bullet-shaped droplets exhibiting rectilinear motion in narrower slits. Our findings support the view that geometrical confinement can provide a viable strategy to control and predict the propulsion direction of active droplets. It would be of interest to look for analogues of these motility modes in biological cells or in synthetic active matter.