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.

Automatic contactless injection, transportation, merging, and ejection of droplets with a multifocal point acoustic levitator

“…As described in the literature, [40][41][42] the acoustic radiation force acting on the droplets can be approximated by the restoring force of a spring and the oscillations can be described by making an analogy with a damped harmonic oscillator. The merging operation was achieved by a procedure similar to that described by Watanabe et al, 28 but instead of generating the focal points by rapidly switching the phases of all the transducers, our focusing scheme divided the transducers into two sets to obtain two simultaneous and independent focus points; i.e. Watanabe et al employed time multiplexing whereas we have used spatial multiplexing.…”

“…22 In the recent years, different acoustic approaches have been proposed to manipulate liquid droplets. 14,[23][24][25][26][27][28] Koyama and Nakamura 23 developed a levitation system formed by a plane reflector and a metal plate attached to transducers at its ends. By changing the relative phase between the a) Author to whom correspondence should be addressed: marcobrizzotti@ gmail.com transducers, they could transport ethanol droplets.…”

“…Ochiai et al 24 employed four arrays arranged in a cube to manipulate small solid objects and liquid droplets in three-dimensions. Recently, Watanabe et al 28 used a 7 × 7 square array of 40 kHz transducers and an opposing reflector to levitate and merge two liquid droplets. In their system, the horizontal positions of the droplets were controlled by rapidly multiplexing (i.e.…”

“…Despite the recent progress in manipulating liquid droplets by acoustic levitation methods, a remaining issue is how to insert and remove the samples from the levitator. In the state-of-the-art levitation systems, 11,14,22,23,28 samples are manually inserted and removed with a syringe, requiring a skilled person to avoid sample losses and preventing the automation of the processes. Therefore, executing different fluidic operations such as injection, 19,29,30 transportation, 14,23,24 merging 14,26,28 and ejection 31 in a single levitation device would open up multiple possibilities, enabling the automatic processing of droplets in mid-air.…”

Coalescence Dynamics of Acoustically Levitated Droplets

“…Additionally, Andrade et al [12] experimentally presented a manipulation method for a target sample for injection, levitation, transportation, coalescence, and ejection in midair. Watanabe et al [13] investigated the mixing of droplets with an ultrasonic phased array system. After the successful merging of droplets, a single droplet was effectively mixed by inducing the 6th mode oscillation on its surface.…”