Mixing dynamics of droplets levitated using acoustic tweezers: an optical approach

Abstract

Abstract Acoustic levitation is well-suited to ‘lab-on-a-drop’ contactless chemical analysis of droplets. Rapid mixing is of fundamental importance in lab-on-a-drop platforms and many other applications involving droplet manipulation. Small droplets, however, have low Reynolds numbers; thus, mixing via turbulence is not possible. Inducing interfacial oscillation is effective in this regard, however, the relationship between internal flow and mixing dynamics of droplets remains unclear. Here, we conducted simultaneous optical measurements of the flow field and distribution of fluid components in acoustically levitated droplets. A relationship between internal flow due to interfacial oscillation and mixing of fluid components was revealed, and the effect of kinematic viscosity on the flow characteristics of droplets with interfacial oscillation is discussed. The experimental results were consistent with the theoretical findings. Thus, the internal flow induced by droplet oscillation could enhance droplet mixing. Our findings will be conducive to the application and further development of lab-on-a-drop devices.