Collisions and rebounds of chemically active droplets

Kevin Lippera (1) , Matvey Morozov (1) , Michael Benzaquen (1) , Sébastien Michelin (1)

1 LadHyX - Laboratoire d'hydrodynamique

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.

Domaines: Mécanique des fluides [physics.class-ph]

Kevin Lippera, Matvey Morozov, Michael Benzaquen, Sébastien Michelin. Collisions and rebounds of chemically active droplets. Journal of Fluid Mechanics, 2020, 886, ⟨10.1017/jfm.2019.1055⟩. ⟨hal-02536797⟩