Highly prized in the biomedical field, microfluidics involves manipulating very small volumes of liquid. While mixing remains a complex operation on these scales, it can be achieved using surface acoustic waves. These waves, with a frequency in the megahertz range, gradually transfer their momentum to the fluid, which then becomes agitated, forming micro-vortices. Researchers at the Institute of Electronics, Microelectronics and Nanotechnology (IEMNCNRS/Université Lille 1/ISEN Lille/Université Valenciennes/UVHC/École Centrale Lille), the Matière et systèmes complexes (MSCCNRS/Université Paris Diderot) and the Institut des nanosciences de Paris (INSP(CNRS/UPMC) have highlighted the particular topologies of the vortices in a phenomenon similar to a whispering gallery. In this architectural curiosity, sound can be perceived over long distances because it is focused along the vaults. In this way, two people can converse in hushed tones on either side of certain domes.

Here, numerical calculations have revealed the focusing of a group of waves that propagate in periodic orbits along the surface of the drop. They are completely reflected when they reach the inner edge of the drop, the interface between the air and the liquid, and are guided in elliptical loops. The spherical cap shape of the drop, shaped by its surface tension, is responsible for this concentration of waves on three "caustic" points, which are a kind of focal line. This highly heterogeneous arrangement of the acoustic field forces the internal flow to adopt a particular structure, made up of one or two pairs of vortices. It is these vortices that enable efficient mixing within the drop.

Murmur gallery in a droplet. (A-B) trajectory of acoustic rays guided by the droplet surface. (C) Simulation of acoustic currents generated by murmur galleries. (© IEMN - MSC - INSP)