Spontaneous motion of a passive fluid droplet in an active microchannel

TL;DR

This study demonstrates how active gel layers in microchannels can induce spontaneous flows that move passive droplets, with flow patterns and droplet shapes controllable by boundary conditions, informing microfluidic device design.

Contribution

It introduces a numerical analysis of spontaneous flow-driven droplet motion in active gel-coated microchannels, highlighting control mechanisms via wall anchoring conditions.

Findings

Active gel layers induce spontaneous flow and droplet motion.

Flow symmetry and droplet shape depend on active layer coverage.

Control of flow patterns achieved through wall anchoring conditions.

Abstract

We numerically study the dynamics of a passive fluid droplet confined within a microchannel whose walls are covered with a thin layer of active gel. The latter represents a fluid of extensile material modelling, for example, a suspension of cytoskeletal filaments and molecular motors. Our results show that the layer is capable of producing a spontaneous flow triggering a rectilinear motion of the passive droplet. For a hybrid design (a single wall covered by the active layer), at the steady state the droplet attains an elliptical shape, resulting from an asymmetric saw-toothed structure of the velocity field. On the contrary, if the active gel covers both walls, the velocity field exhibits a fully symmetric pattern considerably mitigating morphological deformations. We further show that the structure of the spontaneous flow in the microchannel can be controlled by the anchoring conditions of the active gel at the wall. These findings are also confirmed by selected 3D simulations. Our results may stimulate further research addressed to design novel microfludic devices whose functioning relies on the collective properties of active gels.