Flow field around a confined active droplet

Abstract

We present here first-of-a-kind experimental measurements of the flow field around a swimming water droplet, using confocal PIV in three dimensions. The droplet is denser than the continuous oil phase, and swims close and parallel to the bottom wall. The measured flow field is first quantitatively characterized and compared to the flow field obtained for an axisymmetric swimmer in unbounded flow. Important qualitative differences are observed, in particular the emergence of a strong isotropic radial flow field in the planes parallel to the wall. These fundamental differences stress the critical impact of confinement on the flow field around the swimming droplet. We then propose an analytical formulation, based on a reduced order description of the swimming droplet in terms of fundamental hydrodynamic singularities as well as the classical method of images. This model is able to account exactly for the effect of the wall and provides a simplified description of the flow field as the superposition of axisymmetric dipole and quadrupole singularities. We demonstrate that this simplified description quantitatively captures the effect of the wall on the dipolar and quadrupolar components of the flow field. Further including the confinement-induced asymmetry of the concentration field responsible for the droplet's Marangoni propulsion, our model is also able to account for the monopolar contribution to the experimental flow field which drives the dominant far-field signature of the droplet.

Confined self-propulsion of an isotropic active colloid

“…Given the characteristic size and velocities of confined phoretic microswimmers (de Blois et al. 2019; Lippera et al. 2020 b ; Hokmabad et al.…”

“…Recent experimental measurements have shown significant modifications of the flow field around the droplet when placed close to or between rigid walls (de Blois et al. 2019), and theoretical modelling unveiled the non-trivial alterations of the hydrochemical coupling induced by confinement (Lippera et al. 2020 b ).…”

Collective Entrainment and Confinement Amplify Transport by Schooling Microswimmers

“…well-developed toolbox for tuning their motility, flow generation, and collective dynamics, and solving microfluidic mazes [57][58][59][60][61][62][63][64][65][66][67][68].…”

Confined self-propulsion of an isotropic active colloid

“…The activity A and mobility M coefficients characterise the physico-chemical properties of the particle surface and can be positive or negative; from these, a characteristic phoretic velocity scale can be defined as V = |AM|/D. Given the characteristic size and velocities of confined phoretic microswimmers [28,37,38], the fluid and colloid inertia can be neglected, i.e. the Reynolds number Re = ρVa/η is negligible, so that the motion of the particle can be described using the steady Stokes equations.…”

“…confocal microscopy 28], theoretical models most often ignore the presence of confining boundaries and focus on droplets in unbounded fluid domains, leaving unexplored their role on the emergence and persistence of self-propulsion. Recent experimental measurements have shown significant modifications of the flow field around the droplet when placed close to or between rigid walls [37], and theoretical modelling unveiled the non-trivial alterations of the hydro-chemical coupling induced by confinement [38]. Beyond the influence of a single flat wall, recent experiments have also shown that self-sustained motion can also occur in strongly-confined settings, such as small capillary tubes [34,39].…”