Droplet motion and deformation induced by acoustic streaming and radiation pressure
Résumé
Rayleigh Acoustic waves generated at the surface of a solid substrate can induce deformation, motion and even atomization of partially wetting droplets. The characteristic time scales associated with the droplets response strongly differ
from the acoustic period, suggesting the existence of nonlinear coupling between acoustic waves and droplets dynamics.
If different behaviors have been observed in different experimental conditions (droplet size, acoustic wave frequency,
wetting properties of the liquid), the underlying physics remains unclear. To understand it, a parametric experimental
study [P. Brunet et al., Phys. Rev. E., 81, 036315 (2010)] has been performed at a fixed frequency of 20 MHz, by varying
the droplet size, the liquid viscosity and the acoustic wave intensity. In these experiments, the free surface of the droplet
is modified in three different way: first a breaking of its symmetry, second global oscillations of the droplet and finally
small amplitude and higher frequency "trembling modes". To explain all these deformations, two classical nonlinear
acoustic driving can ve invoked: first the radiation pressure and second the acoustic streaming. The relative importance
of these nonlinear phenomena strongly depends on the frequency considered. At 20 MHz, the acoustic wave is multiply
reflected into the droplet and therefore the acoustic radiation pressure plays an important role. At higher frequencies,
the acoustic wave hardly reaches the surface and the radiation pressure plays no role. With our experiments, we show
that while both acoustic streaming and radiation pressure can induce the asymmetry of the droplet, the global oscillation
mode only appears when acoustic radiation is significant. We therefore exhibit for the first time the role played by the
acoustic radiation pressure on droplets dynamics in a certain frequency range. The comprehension of these phenomena
is fundamental to minimize the energy required to handle droplet in view of harmless manipulation of biofluids.