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Collisions and rebounds of chemically active droplets

Abstract : Active droplets swim as a result of the nonlinear advective coupling of the distribution of chemical species they consume or release with the Marangoni flows created by their non-uniform surface distribution. Most existing models focus on the self-propulsion of a single droplet in an unbounded fluid, which arises when diffusion is slow enough (i.e. beyond a critical Péclet number, Pec). Despite its experimental relevance, the coupled dynamics of multiple droplets and/or collision with a wall remains mostly unexplored. Using a novel approach based on a moving fitted bispherical grid, the fully-coupled nonlinear dynamics of the chemical solute and flow fields are solved here to charac-terise in detail the axisymmetric collision of an active droplet with a rigid wall (or with a second droplet). The dynamics is strikingly different depending on the convective-to-diffusive transport ratio , Pe: near the self-propulsion threshold (moderate Pe), the rebound dynamics are set by chemical interactions and are well captured by asymptotic analysis; in contrast, for larger Pe, a complex and nonlinear combination of hydrodynamic and chemical effects set the detailed dynamics, including a closer approach to the wall and a velocity plateau shortly after the rebound of the droplet. The rebound characteristics, i.e. minimum distance and duration, are finally fully characterised in terms of Pe.
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Submitted on : Wednesday, April 8, 2020 - 12:38:53 PM
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Kevin Lippera, Matvey Morozov, Michael Benzaquen, Sébastien Michelin. Collisions and rebounds of chemically active droplets. Journal of Fluid Mechanics, Cambridge University Press (CUP), 2020, 886, ⟨10.1017/jfm.2019.1055⟩. ⟨hal-02536797⟩

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