Open Access
Numerical and experimental investigation of the stability of a drop in a single-axis acoustic levitator
(American Institute of Physics, 2019) Brizzotti Andrade, Marco Aurélio; Marzo Pérez, Asier; Estadística, Informática y Matemáticas; Estatistika, Informatika eta Matematika
Acoustic levitation can be employed to hold liquid drops in midair, enabling novel applications in X-ray scattering of proteins, amorphous crystallization of solutions, or contactless mixing. Multiple studies have characterized the physical behavior of a levitated drop inside an acoustic field. Here, we present a numerical and experimental study on the acoustic levitation of water drops in a single-Axis acoustic levitator consisting of an ultrasonic transducer and an opposing reflector. Instead of modeling an abstract incident acoustic field, our model considers the shape of the drop as well as the real geometry of the levitator. We also use a high-speed camera to observe the disintegration and the undesired oscillations of the drops. Our results show that the insertion of a drop in the levitator provokes a shift in its resonant frequency that depends on the shape of the drop. Second, the levitation behavior depends on whether the levitator operates slightly below or above the resonance. Third, if the levitator is driven above the resonant frequency, it is possible to levitate with more strength and avoid disintegration of the drop. This research provides an insight on how to achieve more stable experiments that avoid the bursting and undesired oscillations of the levitated sample. We hope that it will facilitate numerous experiments involving acoustically levitated liquid drops.
Open Access
Experimental investigation of the particle oscillation instability in a single-axis acoustic levitator
(AIP Publishing, 2019) Brizzotti Andrade, Marco Aurélio; Polychronopoulos, Spyros; Memoli, Gianluca; Marzo Pérez, Asier; Estadística, Informática y Matemáticas; Estatistika, Informatika eta Matematika
Single-axis acoustic levitators are employed in biomedicine, chemistry and physics experiments due to their ability to trap in mid-air objects of a wide range of materials and sizes. Although this type of levitator has been studied for decades, there are effects that are not well understood. One of these effects is the particle oscillation instability, in which the levitating particle starts to oscillate with increasing amplitude until it is ejected out of the levitator. Most of the operations performed with acoustic levitation require high accuracy regarding the positioning of the particle, thus a lack of stability severely hinders the experiments. In this paper, we present an experimental setup that consists of a single-axis levitator, a mechanized stage to control the separation between the emitter and the reflector, a scale to measure the radiation force and a high-speed camera. We experimentally investigate the effect of the distance between the emitter and the reflector on the apparatus resonant frequency and on levitation stability. In accordance with previous theoretical studies, three types of levitation behavior were experimentally identified: stable levitation, oscillation of constant amplitude and unstable oscillation. We also show that the type of levitation behavior can be controlled by changing the distance between the emitter and the reflector.
Open Access
Automatic contactless injection, transportation, merging, and ejection of droplets with a multifocal point acoustic levitator
(AIP Publishing, 2018) Brizzotti Andrade, Marco Aurélio; Camargo, Thales S. A.; Marzo Pérez, Asier; Estadística, Informática y Matemáticas; Estatistika, Informatika eta Matematika
We present an acoustic levitation system that automatically injects, transports, merges and ejects liquid droplets in mid-air. The system consists of a phased array operating at 40 kHz on top of a plane reflector. The phase array generates multiple focal points at independent positions that form standing waves between the array and the reflector. In the reflector there is an inlet for a piezoelectric droplet injector which automatically inserts liquid droplets at the lower pressure nodes of the standing waves, and a hole that serves as an outlet for ejecting the processed droplets out of the system. Simulations of the acoustic radiation potential acting on the levitating droplets are in good agreement with the experiments. High-speed footage captured the functioning of the system in four fluidic operations: injection, transport, merging and ejection of liquid droplets. Having these operations integrated reliably into a single automatic system paves the way for the adoption of mid-air acoustophoretic processing in biological, chemical and pharmaceutical applications.

Marzo Pérez, Asier

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