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.

Acoustic levitation method (ALM) is a container-less processing method with applications in various fields, including material processing, biology, and analytical chemistry. Because it is a container-less processing technique, ALM could prevent nucleation and contamination of materials being processed via contact with a container wall. It is well-known that evaporation of a sample is an important process in container-less processing of materials; however, the mechanism of evaporation in multicomponent droplets in a single acoustic levitator is still unclear. Thus, we evaluate and understand the evaporation of an acoustically levitated multicomponent droplet and manipulate the evaporation process of the sample in this study. Specifically, we investigate the evaporation process of pure and multicomponent droplets using container-less processing experimentally. The evaporation processes and temporal evolution of the surface temperature of a multicomponent droplet were evaluated using a high-speed camera and radiation thermometer, respectively. We used water, ethanol, methanol, hexane, acetone, pentane, and binary solutions (solution of 25 wt%, 50 wt%, and 75 wt% ethanol, methanol, and acetone, respectively) as test samples to study the effect of saturated vapor pressure on evaporation. Ethanol, methanol, and acetone droplets evaporate in two different stages. It was observed that the water vapor in the air condensed during the evaporation process of these water-soluble droplets; hence, our experimental data did not agree with the theoretical prediction in accordance with the d² law. Nevertheless, the evaporation behavior in the first stage of evaporation was consistent with the theoretical prediction. Furthermore, for binary droplets, as the concentration of the resultant solution increased owing to evaporation, the transition time from the first to the second stage of evaporation also increased. Based on these observations, estimation equations for binary droplets were developed to ensure that the experimental and theoretical values were in good agreement.

The acoustic levitation method (ALM) is expected to be applied as a container-less processing technology in the material science, analytical chemistry, biomedical technology, and food science domains because this method can be used to levitate any sample in mid-air and prevent nucleation and contamination due to the container wall. However, this approach can lead to nonlinear behavior, such as acoustic streaming, which promotes the evaporation of a levitated droplet. This study aims to understand the evaporation and precipitation kinetics of an acoustically levitated multicomponent droplet. An experimental investigation of the evaporation process of a salt solution droplet was performed, and the experimental results were compared with those of the d²-law. The droplet was noted to evaporate in two stages owing to the precipitation of the salt. Because of the vapor pressure depression, the experimental data did not agree with the classical prediction obtained using the d²-law. However, the experimental results were in partial agreement with those of the d²-law when the vapor pressure depression was considered by using the concentration estimate at each time, as obtained from the experimental results. In addition, it was observed that the time when the salt completely precipitated could be estimated by using the extended theory. These findings provide physical and practical insights into the droplet evaporation mid-air for potential lab-in-a-drop applications.

We experimentally investigate the flow structure and evaporation behavior of a droplet in an ∼19 kHz single-axis acoustic levitator. Decane, nonane, octane, heptane, hexane, and pentane are used as test fluids to investigate the effect of saturated vapor pressure on the internal and external flow fields. Under low saturated vapor pressure (decane and nonane), the direction of the external flow is away from the surface of the droplet. However, at a relatively higher saturated vapor pressure (octane, heptane, hexane, and pentane), the direction of the external flow is toward the surface of the droplet, with vortices forming near the droplet surface. For droplets with a low saturated vapor pressure (decane, nonane, octane, and heptane), the internal flow is similar to that in the case of rigid body rotation. Finally, under high saturated vapor pressure (hexane and pentane), the internal flow is an unsteady 3D complex flow. The experimental results indicate that the vapor concentration distribution around a levitated droplet surface correlates closely with changes in the external and internal flows.

To help realize stable droplet manipulation, we present a study on the physical mechanism of interfacial instability and droplet atomization dynamics in acoustic levitation with ultrasonic phased arrays. Acoustic levitation is highly applicable in the fields of analytical chemistry and biology because container-less processing can prevent undesirable wall effects, such as nucleation and contamination resulting from the container walls. Although many studies demonstrated the atomization behavior in single-axis acoustic levitation, the breakup characteristics of levitated droplets in ultrasonic phased array levitation have been less studied. Here, we visualized the atomization behavior of ethanol, ethanol solution, and silicone oil droplets using a high-speed camera. The time evolution of the interfacial velocity of the droplets indicates a threshold for the droplet atomization. To elucidate atomization dynamics, the critical interfacial velocities of the levitated droplet immediately before its atomization are discussed and compared to the theoretical prediction based on the Kelvin–Helmholtz instability. Our experimental findings provide deeper physical insights into the levitation stability of droplets in ultrasonic phased array levitation for futuristic lab-in-a-drop applications.