Visualization of Internal Flow of Acoustically Levitated Droplet with Interfacial Oscillation

Abstract

Acoustic levitation is a technique for the contactless manipulation of objects. In previous study, the contactless manipulation and coalescence of droplets were succeeded by using ultrasonic phased array. Additionally, a mixing of the droplet was enhanced by introducing oscillation on the interface of droplet. However, the mechanisms of the enhancement and the optimal conditions for mixing of the droplet are not yet understood. Therefore, we focused on the stretching of fluid particles that directly contributes to mixing and measured it experimentally. First, we performed the LIF method. It was confirmed that the mixing of droplet was enhanced by inducing interfacial oscillation. Moreover, the distribution of the fluorescent dye changed into a complex stripe pattern. Secondly, we visualized the internal flow of droplet by using fluorescent particles. As a result, the trajectories of the particles were sufficiently mixed. For quantitative evaluation of the mixing characteristics, we calculated the increase rate of the distance between nearest neighbor particles to reveal the agitation. The time averaged distribution demonstrated the agitation was changed by the interfacial oscillation. Finally, finite time Lyapunov exponent (FTLE) was introduced to characterize the mixing behavior of the droplet. FTLE was proportional to the amplitude of oscillation. From these results, it is considered that the mixing of droplet was enhanced by the chaotic advection occurred due to the interfacial oscillation of droplets. Furthermore, the stretching effect of the fluid particles inside the droplet increases in proportion to the amplitude of the interfacial oscillation relative to the droplet diameter.