Flow fields around active droplets squeezing through tight confinements

TL;DR

This study investigates how active droplets deform and change their hydrodynamic flow fields as they squeeze through tight microchannels, revealing a transition from pusher to puller velocity signatures and asymmetric flows.

Contribution

It provides the first detailed analysis of the evolution of velocity fields around active droplets in extreme confinement using experiments and simulations.

Findings

Velocity field transitions from pusher to puller as shape deforms.

Hydrodynamic signatures become asymmetric in tighter confinements.

Numerical simulations explain the flow evolution based on physico-chemical interactions.

Abstract

Biological microswimmers, like euglena, deform their body shape to swim through tight confinements having length scales comparable to the microswimmer length scale. Recently, it was shown that self-propelling active droplets can also squeeze through tight microconfinements by elongating their shape. However, the evolution of the hydrodynamic signature, or the velocity field, generated by the active droplet, as it deforms its shape to swim through increasingly tight microconfinements, has remained scarcely studied. Using high-resolution fluorescence microscopy and $\mu$-Particle Image Velocimetry (PIV) analysis, we show here that as the swimming active droplet deforms from a spherical shape to a `stadium'-like shape, and eventually to an elongated `capsule'-like shape in increasingly tighter microchannels, its hydrodynamic signature changes from a `pusher'-like velocity field to a `puller'-like velocity field, and finally to an asymmetric velocity field. We characterize such alterations in the active droplet dynamics using the distributions of the local velocity magnitude, axial and transverse components of the local flow velocity, vorticity, and the filled micelle concentration. Finally, we use finite-element based numerical simulations to explain the aforementioned evolution of the velocity field stemming from the underlying physico-chemical hydrodynamics in the presence of a thin lubrication film between the swimming active droplet and the tight microconfinement walls. The present work provides a comprehensive understanding of the chemo-hydrodynamic characteristics of active droplets navigating extreme confined spaces, which can be beneficial for many autonomous cargo-delivery applications.