Seminar: Multiphysics of Acoustically Levitated Droplets

Thursday, November 17, 2022

Mining Building

Speaker: Prof. Kai-Tak Wan (Mechanical Engineering, Northeastern University, USA)

Host: Xinyu Liu (MIE);


Two aligned acoustic transducers set up a standing wave at 40kHz.  A glycerol droplet placed at the low-pressure node is supported by the high-pressure anti-node underneath and is squashed into an oblate disk by the sandwiching anti-nodes.  When the now levitated droplet is subject to a modulated field at frequency on the order of 150 Hz, it behaves as a solid plate and exhibits flexural bending resonance rather than the conventional equatorial star-shape oscillations.  Three new oscillation modes are observed: seesaw, saddleback, and monkey saddle with increasing energy levels.  Finite element analysis generates droplet shapes consistent with the experimental observation and yields apparent plate flexural rigidity in terms of surface tension and aspect ratio of plate thickness to diameter.  High viscosity leads to plate-like behavior of the droplet as the Deborah number exceeds 2.0.  A large air bubble is introduced into an levitated droplet which is then set into an out-of-phase azimuthal sloshing resonance by a modulated frequency with modes n = 4 to 9.   Waveforms of the inner and outer liquid-air interfaces conform to the classical Saffren model.  Resonance peaks and their harmonics in the frequency spectrum are found to be a function of drop dimension and resonance modes.  Drops with multiple small air bubbles do not resonate in synch because of asymmetry.  We further investigate the evaporation of a levitated Ouzo droplet.  Temperature, droplet dimension and optical transparency are monitored simultaneously.  Since the levitated droplet is not in intimate contact with any substrate, heat transfer process is significantly different from the classical evaporation of Ouzo drop resting on a glass surface. The contactless measurement thus represents a unique way to characterize intrinsic properties of the liquid.


Dr. Kai-tak Wan is currently professor at Mechanical Engineering, Northeastern University, Boston.  He graduated with B.Sc. (1st Hon) in Physics, University of New South Wales, Australia in 1988 and Ph.D. in Chemical Physics, University of Maryland at College Park.  He worked at various institutes including NIST, Sydney University, Nanyang Technological University, and Missouri University of Science and Technology.  His main research interests lie in dynamics of non-Newtonian liquid droplets, palpation mechanics to detect breast cancer, shell mechanics and application in robotic gripper, and bacterial adhesion in water filtration.

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