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 characterise 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. * Electronic address: sebastien.michelin@ladhyx.polytechnique.fr arXiv:1912.04621v1 [physics.flu-dyn]

communication and help regulate the function on the scale of whole organs and organism. The different types of molecules are present at different concentrations in vivo and act over a range of time scales, from seconds to days. Indeed, variations in the rate of secretion of different molecules indicate the state of the cells, for example, as a response to changes in their environment. Standard methods for measuring secreted molecules in typical cell culture experiments, however, require the presence of a large number of molecules, which translates into volumes of tens to hundreds of µL and give information at the level of large populations of cells. As a result, these techniques hide the heterogeneities that may exist on the scale of individual cells. It is becoming increasingly clear, however, that these heterogeneities are present and play a determining role in many biological processes, such as the immune response to an unknown pathogen, [1] or as a prognostic marker for some cancer types. [2] 2002303 (2 of 11)