Beyond the Langevin horn: Transducer arrays for the acoustic levitation of liquid drops

Abstract

The acoustic levitation of liquid drops has been a key phenomenon for more than 40 years, driven partly by the ability to mimic a microgravity environment. It has seen more than 700 research articles published in this time and has seen a recent resurgence in the past 5 years thanks to low cost developments. As well as investigating the basic physics of levitated drops, acoustic levitation has been touted for container free delivery of samples to a variety of measurements systems, most notably in various spectroscopy techniques including Raman, Fourier Transform InfraRed (FTIR) in addition to numerous X-Ray techniques. For 30 years the workhorse of the acoustic levitation apparatus was a stack comprising a piezoelectric transducer coupled to a horn shaped radiative element often referred to as the Langevin horn. Decades of effort have been dedicated to such devices, paired with a matching and opposing device or a reflector, but they have a significant dependence on temperature and require precision alignment. The last decade has seen a significant shift away from these in favour of arrays of digitally driven, inexpensive transducers, giving a new dynamic to the topic which we review herein.

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

“…7(a) shows the evaporation constant up to the precipitation time (complete evaporation of water) for each sample. The horizontal axis represents time, and the vertical axis represents the evaporation constant with the different initial droplet size and concentration obtained from eqn (7). For the effect of the initial droplet diameter of the salt solution, it was conrmed that the evaporation constant decreased as the initial droplet size increased for each concentration.…”

“…The theoretical predictions were in good agreement with the experimental results. It could be concluded that the precipitation time, when the water component completely evaporates, can be predicted using eqn (7) and (8).…”

“…ALM is a noncontact technology that can be used to suspend a droplet near a sound pressure node of a standing wave through the action of acoustic radiation force. 6,7 Because this technology can avoid the adsorption, contamination, and heterogeneous nucleation caused by the vessel wall surface, it is expected to be used to realize non-contact manipulation in elds such as material creation, 8,9 chemistry, 10,11 and biomedicine. 12,13 Yu et al successfully synthesized single-atom Pt materials in solution and supported Pt nanoclusters on microporous L 2 O 3 by using a one-step ALM without any pretreatment/modication of raw oxide.…”

Coalescence Dynamics of Acoustically Levitated Droplets

“…The acoustic levitation [1,2] of droplets is a promising tool for contactless fluid manipulation for lab-on-a-drop applications [3][4][5][6][7][8] by achieving droplet levitation, transportation, coalescence, mixing, separation, evaporation, and extraction in a single operation. Through acoustic levitation, a sample can be suspended using the acoustic radiation force acting on the object [9]. This method enables contactless treatment to avoid the contamination, absorption, and heterogeneous nucleation through the wall effect [2].…”

Coalescence Dynamics of Acoustically Levitated Droplets

“…The acoustic levitation [1,2] of droplets is a promising tool for contactless fluid manipulation for lab-on-a-drop applications [3][4][5][6][7][8] by achieving droplet levitation, transportation, coalescence, mixing, separation, evaporation, and extraction in a single operation. Through acoustic levitation, a sample can be suspended using the acoustic radiation force acting on the object [9]. This method enables contact-free treatment such as contamination, absorption, and heterogeneous nucleation through the wall effect [2].…”