Internal flow during mixing induced in acoustically levitated droplets by mode oscillations

In this paper, we describe a mixing method with mode oscillation on the internal flow field of a levitated droplet. The effect of internal flow on the mixing performance of droplets acoustically levitated via ultrasonic phased arrays remains unclear. To better understand the mixing mechanism of a levitated droplet, clarifying the effect of the internal flow field on droplet mixing from mode oscillation during acoustic levitation is necessary. We used a 50 wt. % glycerol aqueous solution with 6th mode oscillation. We applied particle image velocimetry (PIV) to study the internal flow fields under interfacial oscillation. The PIV results indicated that the visualized flow field enhanced mixing performance with increasing Reynolds number. We demonstrated the nonlinear characteristics of droplet mixing compared to potential flow. The nonlinearity of the droplet oscillation was driven by the nonlinear acoustic field exerted on the levitated droplet. Mode oscillation on the droplet surface induced a pressure gradient and caused internal flow in the droplet. The pressure gradient in the droplet from the interfacial oscillation was quantitatively analyzed. Pressure induced by the interfacial oscillation, which can be roughly ten times larger than the hydrostatic pressure in the droplet, drastically enhanced the mixing performance in the droplet. Our experimental findings provide deeper physical insights into noncontact fluid manipulation for potential lab-in-a-drop applications. Published under license by AIP Publishing.