Ultrasonic atomization of liquids in drop-chain acoustic fountains

Abstract

When focused ultrasound waves of moderate intensity in liquid encounter an air interface, a chain of drops emerges from the liquid surface to form what is known as a drop-chain fountain. Atomization, or the emission of micro-droplets, occurs when the acoustic intensity exceeds a liquid-dependent threshold. While the cavitation-wave hypothesis, which states that atomization arises from a combination of capillary-wave instabilities and cavitation bubble oscillations, is currently the most accepted theory of atomization, more data on the roles of cavitation, capillary waves, and even heat deposition or boiling would be valuable. In this paper, we experimentally test whether bubbles are a significant mechanism of atomization in drop-chain fountains. High-speed photography was used to observe the formation and atomization of drop-chain fountains composed of water and other liquids. For a range of ultrasonic frequencies and liquid sound speeds, it was found that the drop diameters approximately equalled the ultrasonic wavelengths. When water was exchanged for other liquids, it was observed that the atomization threshold increased with shear viscosity. Upon heating water, it was found that the time to commence atomization decreased with increasing temperature. Finally, water was atomized in an overpressure chamber where it was found that atomization was significantly diminished when the static pressure was increased. These results indicate that bubbles, generated by either acoustic cavitation or boiling, contribute significantly to atomization in the drop-chain fountain.

Ultrasonic Nebulization for the Elemental Analysis of Microgram-Level Samples with Offline Aerosol Mass Spectrometry

“…appears to be a necessary part of the nebulization mechanism for water samples as smaller volumes (< µL) of water do not generate such droplets and also do not appear to form aerosol. This observed mechanism is in agreement with previous studies of aerosol generation for ultrasonic nebulization, in which cavitation within the droplet (Lang, 1962) and boiling and/or jetting from a droplet chain (Simon et al, 2015) have been observed.…”

“…The size distribution and number of aerosol particles from ultrasonic nebulization have been shown to be affected by the frequency of the ultrasonic vibration, the properties of the liquid including surface tension, density, and viscosity, and the concentration of the solution (Donnelly et al, 2005;Lang, 1962;Simon et al, 2015). The present application involves relatively dilute solution, so the only parameter that could be varied was the surface tension, by use of different solvents.…”

Acoustic micro-tapping for non-contact 4D imaging of tissue elasticity

“…If the pump beam is directed from tissue to the medium/air interface, the tissue volume is partially released into air. For liquids, this effect can create an acoustic fountain495051. For the opposite case where the pump acoustic beam is directed from the air to the air/medium interface, the reflection coefficient is nearly one ( R  ≅ 1) and, therefore, nearly all acoustic intensity is converted into radiation pressure.…”

Ultrasonic atomizing-assisted spray drying: Effect on the quality of skimmed milk powders

“…and is easy for absorption in human digestive system. Unlike the traditional atomizing methods that rely on high pressure or high-speed motion to smash liquid samples, ultrasonic atomizing utilized the low ultrasonic vibration energy as the driven force [4] . Besides, by adjustment of the amplitude of the vibration, the atomizing speed can be precisely controlled.…”