Nikhil Desai and Sébastien Michelin awarded for their research on microswimmers
Many micro-organisms and living cells (bacteria, plankton, spermatozoa, etc.) are able to move in fluids despite their small size and low mass. But inert systems, sometimes as simple as a drop of oil in water, can become "microswimmers" by extracting energy from the fluid that surrounds them. In a paper published in Physical Review Fluids, Nikhil Desai and Sébastien Michelin, working at the Hydrodynamics Laboratory (LadHyx*), have focused on these active drops as they move along a wall. They have shed light on a previously unexplored area. For this work, they have just received the François Frenkiel Award for Fluid Mechanics from the American Physical Society (APS).
The general model they consider is a drop whose chemical composition differs from that of the liquid. On the surface of the drop, chemical exchanges take place (release or capture of a compound). This mechanism takes place equally in all directions. But if the drop moves slightly, the transport of the chemical compound by the flow around the drop generates a concentration difference, which in turn propels the drop. This phenomenon of propulsion was already well understood by scientists, but all theoretical models until then considered that the drop was swimming in an infinite fluid. In experiments, however, they are always close to a wall. Does the model become inoperative? The whole community was aware of the problem, but it is more difficult to solve than the case of a simple spherical drop.
The difficulty lies in the fact that one has to describe precisely what is happening at the interface between the drop and the liquid. Nikhil Desai and Sébastien Michelin have succeeded in doing this without approximation, thanks to the use of a particular 'bispherical' coordinate system, which had never been used for these systems. By combining modelling and numerical simulation, they carried out the first linear stability analysis of an active drop subjected to a small pertubation near a wall. The verdict: the drop can move. "Its propulsion is even more efficient in the vicinity of the wall" explains Sébastien Michelin. The model used by the community is therefore strengthened.
With Nikhil Desai (now a post-doctoral fellow at Cambridge University), Sébastien Michelin continues to study this system. This has led to a second article analysing how the drop behaves once it is set in motion. More broadly, this work is part of the CollectSwim project led by Sébastien Michelin and supported by a Starting Grant from the European Research Council (ERC). The aim of this project is to go from detailed individual modelling (as here) to understanding the collective dynamics of a group of microswimmers.
About François Frenkiel Award for Fluid Mechanics
The APS Division of Fluid Dynamics awards the François Frenkiel Award to young investigators in recognition of significant contributions to Fluid Mechanics that have also been published during the previous year in Physical Review Fluids. Eligible authors must have not more than 12 years of full-time employment after their most advanced academic degree was awarded, prior to the paper’s year of publication.
*LadHyX: a joint research unit CNRS, École Polytechnique - Institut Polytechnique de Paris