Ultrasonic waves are known induce complex and non-intuitive phenomena in fluids, like for instance the phenomenon of acoustic streaming when a beam of sound can generate an average flow due either to bulk dissipation either to creation of vorticity near solid boundaries. From acoustic levitation, to acoustic fountains or cavitation bubbles, often associated to nebulization of water into a mist of small droplets, ultrasounds also induce dynamics in liquid free-surfaces. Although the typical forcing frequency is within the MHz range, the characteristic time of free-surface dynamics is rather ruled by hydrodynamics, hence often accessible to visualisation with standard high-speed cameras. Still, a deep understanding of the underlying mechanisms often requires the additional measurements of quantities at the time-scale of excitation, like the local acoustic pressure. I will present experiments of high-speed visualisation of droplets excited by MHz surface acoustic waves, leading to internal flow, droplet oscillations and directional motion. I will also identify and present the acoustofluidics mechanisms at play.
|Title||Droplet and free-surface actuation by MHz acoustic waves|
Top: Examples of a free-surface destabilized by an immersed focalized acoustic transducer. Bottom: Free-surface of a sessile drop destabilized by a 5 MHz surface acoustic wave.
CNRS - Université Paris Diderot, Laboratoire Matière et Systèmes Complexes
|Session||15. Bubbles and droplets I|
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