The 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.

arXiv comment14 pages, 6 figures, 3 appendices

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Cited by in corpus (14)

• Self-propulsion of chemically-active droplets
• Time-(ir)reversibility in active matter: from micro to macro
• Oscillatory rheotaxis of active droplets in microchannels
• Collective entrainment and confinement amplify transport by schooling micro-swimmers
• Instability and self-propulsion of active droplets along a wall
• Confined self-propulsion of an isotropic active colloid
• Swimming droplet in 1D geometries, an active Bretherton problem
• Deforming Active Droplets in Viscoelastic Media
• Gravity induced formation of spinners and polar order of spherical microswimmers on a surface
• Interfacial activity dynamics of confined active droplets
• Mode-Switching of Active Droplets in Macromolecular Solutions
• Confinement induced three-dimensional trajectories of microswimmers in rectangular channels
• Shapes and dynamic regimes of a polar active fluid droplet under confinement
• Steady state propulsion of isotropic active colloids along a wall