Simon Julianna C, Sapozhnikov Oleg A, Khokhlova Vera A, Crum Lawrence A, Bailey Michael R
Center for Industrial and Medical Ultrasound, Applied Physics Laboratory, University of Washington, Seattle, WA 98105, USA.
Center for Industrial and Medical Ultrasound, Applied Physics Laboratory, University of Washington, Seattle, WA 98105, USA ; Department of Acoustics, Physics Faculty, Moscow State University, Moscow, 119991, Russian Federation.
J Fluid Mech. 2015 Mar;766:129-146. doi: 10.1017/jfm.2015.11.
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.
当液体中强度适中的聚焦超声波遇到空气界面时，一串液滴会从液体表面喷出，形成所谓的液滴链喷泉。当声强超过与液体相关的阈值时，就会发生雾化，即微滴的发射。虽然空化波假说（该假说认为雾化是由毛细波不稳定性和空化气泡振荡共同作用产生的）目前是最被广泛接受的雾化理论，但关于空化、毛细波，甚至热沉积或沸腾作用的更多数据将很有价值。在本文中，我们通过实验测试气泡是否是液滴链喷泉中雾化的一个重要机制。我们使用高速摄影来观察由水和其他液体组成的液滴链喷泉的形成和雾化过程。对于一系列超声频率和液体声速，我们发现液滴直径大致等于超声波长。当用水替换其他液体时，我们观察到雾化阈值随剪切粘度增加而升高。对水进行加热时，我们发现开始雾化的时间随温度升高而减少。最后，我们在超压室中对水进行雾化，结果发现当静压增加时，雾化明显减弱。这些结果表明，由声空化或沸腾产生的气泡对液滴链喷泉中的雾化有显著贡献。
