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Ultrasonic atomization of liquids in drop-chain acoustic fountains
Published online by Cambridge University Press: 02 February 2015
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.
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References
Simon et al. supplementary movie
Movie showing figures 4 and 5 in video format. The effect of ultrasonic frequency on water atomization in the drop-chain fountain was investigated for three different focused ultrasound transducers with operational frequencies of 1.04 MHz, 155 kHz, and 2.165 MHz. Atomization appears relatively similar between the three frequencies. The experimental setup is shown in figure 2 and consists of a focused ultrasound transducer in water focused at an air interface. All movies are played back at 10 fps.
Simon et al. supplementary movie
Movie showing figures 4 and 5 in video format. The effect of ultrasonic frequency on water atomization in the drop-chain fountain was investigated for three different focused ultrasound transducers with operational frequencies of 1.04 MHz, 155 kHz, and 2.165 MHz. Atomization appears relatively similar between the three frequencies. The experimental setup is shown in figure 2 and consists of a focused ultrasound transducer in water focused at an air interface. All movies are played back at 10 fps.
Simon et al. supplementary movie
Movie showing figure 8 in video format. This movie shows the formation and atomization of drop-chain fountains of olive oil and n-propanol. It is apparent that olive oil atomization appears qualitatively quite different from water atomization, while n-propanol atomization appears very similar to water atomization. The experimental setup is shown in figure 2, with the addition of a custom-designed holder with an acoustically transparent, thin film bottom. The container is partially submerged in the water tank for coupling to the transducer while maintaining a free, liquid-air interface. All movies are played back at 10 fps.
Simon et al. supplementary movie
Movie showing figure 8 in video format. This movie shows the formation and atomization of drop-chain fountains of olive oil and n-propanol. It is apparent that olive oil atomization appears qualitatively quite different from water atomization, while n-propanol atomization appears very similar to water atomization. The experimental setup is shown in figure 2, with the addition of a custom-designed holder with an acoustically transparent, thin film bottom. The container is partially submerged in the water tank for coupling to the transducer while maintaining a free, liquid-air interface. All movies are played back at 10 fps.
Simon et al. supplementary movie
Movie showing figure 9 in video format. In this movie (recorded at 10,000 fps and played back at 10 fps), water atomization is shown for a 2.127 MHz transducer operating at 850 W/cm² for four different static pressure levels - 0.1 MPa, 2.41 MPa, 8.27 MPa, and 13.79 MPa. Water atomization appears very dramatic at atmospheric
Simon et al. supplementary movie
Movie showing figure 9 in video format. In this movie (recorded at 10,000 fps and played back at 10 fps), water atomization is shown for a 2.127 MHz transducer operating at 850 W/cm² for four different static pressure levels - 0.1 MPa, 2.41 MPa, 8.27 MPa, and 13.79 MPa. Water atomization appears very dramatic at atmospheric
Simon et al. supplementary movie
Movie showing figure 10 in video format. In this movie (recorded at 10,000 fps and played back at 10 fps), water atomization is shown for a 2.127-MHz transducer operating at 1200 W/cm² for four different static pressure levels - 0.1 MPa, 2.41 MPa, 8.27 MPa, and 13.79 MPa. Atomization, which is very dramatic at atmospheric pressure is reduced as the static pressure level increases up to 8.27 MPa, but when the pressure reaches 13.79 MPa droplets are again released.
Simon et al. supplementary movie
Movie showing figure 10 in video format. In this movie (recorded at 10,000 fps and played back at 10 fps), water atomization is shown for a 2.127-MHz transducer operating at 1200 W/cm² for four different static pressure levels - 0.1 MPa, 2.41 MPa, 8.27 MPa, and 13.79 MPa. Atomization, which is very dramatic at atmospheric pressure is reduced as the static pressure level increases up to 8.27 MPa, but when the pressure reaches 13.79 MPa droplets are again released.
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