Confined self-propulsion of an isotropic active colloid

Abstract

To spontaneously break their intrinsic symmetry and self-propel at the micron scale, isotropic active colloidal particles and droplets exploit the non-linear convective transport of chemical solutes emitted/consumed at their surface by the surface-driven fluid flows generated by these solutes. Significant progress was recently made to understand the onset of self-propulsion and non-linear dynamics. Yet, most models ignore a fundamental experimental feature, namely the spatial confinement of the colloid, and its effect on propulsion. In this work, the self-propulsion of an isotropic colloid inside a capillary tube is investigated numerically. A flexible computational framework is proposed based on a finite-volume approach on adaptative octree-grids and embedded boundary methods. This method is able to account for complex geometric confinement, the nonlinear coupling of chemical transport and flow fields, and the precise resolution of the surface boundary conditions, that drive the system's dynamics. Somewhat counter-intuitively, spatial confinement promotes the colloid's spontaneous motion by reducing the minimum advection-to-diffusion ratio or Pćlet number, Pe, required to self-propel; furthermore, self-propulsion velocities are significantly modified as the colloid-to-capillary size ratio $\kappa$ is increased, reaching a maximum at fixed Pe for an optimal confinement $0<\kappa <1$. These properties stem from a fundamental change in the dominant chemical transport mechanism with respect to the unbounded problem : with diffusion now restricted in most directions by the confining walls, the excess solute is predominantly convected away downstream from the colloid, enhancing front-back concentration contrasts. These results are confirmed quantitatively using conservation arguments and lubrication analysis of the tightly-confined limit, $\kappa \to 1$.

arXiv comment23 pages, 13 figures, to appear in Journal of Fluid Mechanics

References in corpus (22)

• Active Particles in Complex and Crowded Environments
• The hydrodynamics of swimming microorganisms
• Self-motile colloidal particles: from directed propulsion to random walk
• Designing phoretic micro- and nano-swimmers
• Circular motion of asymmetric self-propelling particles
• Self-propulsion of pure water droplets by spontaneous Marangoni stress driven motion
• Spontaneous autophoretic motion of isotropic particles
• Low-Reynolds number swimming in a capillary tube
• Phoretic Interactions Between Active Droplets
• Autophoretic locomotion from geometric asymmetry
• Nonlinear dynamics of a chemically-active drop: from steady to chaotic self-propulsion
• Emergence of bimodal motility in active droplets
• Self-propulsion near the onset of Marangoni instability of deformable active droplets
• A regularised singularity approach to phoretic problems
• Collisions and rebounds of chemically-active droplets
• Instability and self-propulsion of active droplets along a wall
• The flow field around a confined active droplet
• Bouncing, chasing or pausing: asymmetric collisions of active droplets
• A fine balance of chemotactic and hydrodynamic torques: when microswimmers orbit a pillar just once
• Speed-Dispersion Induced Alignment : a 1D model inspired by swimming droplets experiments
• Hydrochemical interactions of phoretic particles: a regularized multipole framework
• Swimming droplet in 1D geometries, an active Bretherton problem

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