Contactless Fluid Manipulation in Air: Droplet Coalescence and Active Mixing by Acoustic Levitation

Abstract

Acoustic manipulation by an ultrasonic phased array provides an entirely new approach to processes such as coalescence, mixing, separation, and evaporation occurring in the generation of new materials, physical property measurement, the biomedical industry, etc. However, to date, ultrasonic phased arrays have not been fully investigated for applications in fluid manipulation. This paper provides contactless coalescence and mixing techniques for droplets in air by controlling the acoustic potential by using an ultrasonic phased array. We focused on mode oscillation to propose an efficient mixing technique for liquid without contact. A comparison of mixing performance between cases with mode oscillation and without mode oscillation showed that the flow induced by mode oscillation promotes droplet mixing. Our paper demonstrates the feasibility of contactless coalescence and mixing as a first step in fluid manipulation with a phased array.

Figure 1

Acoustic levitation by an ultrasonic phased array. (a) Calculation result of acoustic field generated by focused ultrasound. (b) Snapshot of acoustically levitated droplets in localized standing wave. (c) Comparison of sound pressure between experiment and calculation (DPSM). The error bars in the experimental plot represent standard deviations.

Figure 2

Contactless transport and coalescence by controlling the acoustic potential. (a) Calculation results of an acoustic potential obtained by DPSM (a-1, L = 12 mm; a-2, L = 10 mm; a-3, L = 8 mm; a-4, L = 6 mm). (b) Calculation results of an acoustic potential at a pressure node (z = 3λ/4) (b-1, L = 12 mm; b-2, L = 10 mm; b-3, L = 8 mm; b-4, L = 6 mm). (c) Snapshots of levitation behavior when a pair of focal points is generated (c-1, L = 12 mm; c-2, L = 10 mm; c-3, L = 8 mm; c-4, L = 6 mm). (d) Snapshot of contactless coalescence of water droplets.

Figure 3

Induction of an oscillation mode by modulation of sound. (a) Typical 4^th to 7^th mode behavior of acoustically levitated droplet. (b) Condition under which oscillation mode appears. The symbols are the experimental data for the 2^nd to 8^th mode. Solid curves are the calculation results obtained using Eq. (2).

Figure 4

Active mixing of an acoustically levitated droplet by an oscillation mode. (a) Experimental procedure for observing mixing behavior. (b) Comparison of mixing performance between the case without a mode and the case with a mode. (c) Comparison of transition of mixing parameter. (d) Comparison of mixing pattern between (d-1) the case without an oscillation mode and (d-2) the case with an oscillation mode. (e) Comparison of flow structure between (e-1) the case without an oscillation mode and (e-2) the case with a 6^th-mode oscillation.

Figure 5

Schematic diagram of experimental apparatus. (a) The experimental system for observing droplet behavior. (b) The experimental PIV and LIF system. (c) Modulation of the voltage applied to the transducers. V is the voltage, Vo is the voltage amplitude, and T_m is the modulation period.