Coalescence Dynamics of Acoustically Levitated Droplets

Abstract

The contactless coalescence of a droplet is of paramount importance for physical and industrial applications. This paper describes a coalescence method in mid-air via acoustic levitation using an ultrasonic phased array system. Acoustic levitation using ultrasonic phased arrays provides promising lab-on-a-drop applications, such as transportation, coalescence, mixing, separation, evaporation, and extraction in a continuous operation. The mechanism of droplet coalescence in mid-air may be better understood by experimentally and numerically exploring the droplet dynamics immediately before the coalescence. In this study, water droplets were experimentally levitated, transported, and coalesced by controlling acoustic fields. We observed that the edge of droplets deformed and attracted each other immediately before the coalescence. Through image processing, the radii of curvature of the droplets were quantified and the pressure difference between the inside and outside the droplet was simulated to obtain the pressure and velocity information on the droplet surface. The results revealed that the sound pressure acting on the droplet clearly decreased before the impact of the droplets. This pressure on the droplets was quantitatively analyzed from the experimental data. Our experimental and numerical results provide deeper physical insights into contactless droplet manipulation for futuristic lab-on-a-drop applications.

Evaporation and drying kinetics of water-NaCl droplets via acoustic levitation

“…Furthermore, in recent years, non-contact manipulation by using an ultrasonic phased array has also been investigated, thereby expanding the application of the ALM for non-contact manipulation. 21,22 However, the use of the ALM leads to the introduction of nonlinear dynamics on a levitated droplet, such as in the form of acoustic streaming [23][24][25][26][27][28] and dynamic behavior, 29,30 owing to the levitation of the sample by a nonlinear acoustic eld. These phenomena may affect the evaporation and precipitation of the levitated samples.…”

Diffusion enhancement in a levitated droplet via oscillatory deformation

“…Then, we consider the flow field in the order of ε 2 in the case that the flow field in the order of ε is expressed in Eqs. (19) and (20). From the equations for the order of ε 2 in Eqs.…”

“…Recently, a contactless mixing technique in an acoustic levitation system is reported [18,19]. By applying the frequency modulation to the sound pressure field, the levitated droplet can exhibit a periodical deformation.…”

Acoustic manipulation of multi-body structures and dynamics

“…Positive acoustic contrast, in which case all particles will collect in a nodal plane, requires that Φ = f 0 + 3 2 f 1 > 0. In practice, this is the situation not only for solid particles levitating in air, for which the set of scattering coefficients (f 0 , f 1 ) can be well approximated by (1,1), but also for liquid droplets in air, so long as their shape remains approximately constant (large shape oscillations, breakup and coalescence introduce considerable complications [41,42]). Φ is also positive for solid spheres in water, such as polystyrene particles with (f 0 , f 1 ) = (0.47, 0.038), and for many types of live cells and liquid droplets in water, e.g.…”