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Acoustic cavitation bubbles are considered to be generated due to heterogeneous nucleation because a smaller amount of energy is required in this case compared with homogeneous nucleation. A typical example of such nucleation sites is tiny cracks filled with gas, commonly termed as "gas pocket." Such microcracks can exist in abundance on the surface of reactor walls or sonotrodes used for the cavitation treatment. Eventually, the heterogeneously nucleated cavitation bubble is detached from the solid surface; however, the detachment mechanism has been unclear so far. Therefore, the present study numerically investigated the dynamic behavior of the cavitation bubble originated from the heterogeneous nucleation at wall cracks. During propagation of an ultrasound wave through a liquid, gas pockets in cracks oscillate sinusoidally when the sound pressure amplitude and crack radius are small. In this case, the bubble cannot detach from the surface. On the other hand, as the sound pressure amplitude, crack size, and/or contact angle become larger, the bubble begins to oscillate non-linearly that causes liquid jetting. The liquid jetting can cause the bubble detachment from the surface due to the interaction between the liquid jet and the bottom wall. Stronger liquid jetting makes many small detached bubbles. [ABSTRACT FROM AUTHOR]

Sound waves have the ability to propagate through a gas phase and, thus, to supply the acoustic energy from a sound generator to materials being processed. This offers an attractive tool, for example, for controlling the rates of interfacial reactions in steelmaking processes. This study investigates the kinetics of decarburization in molten Fe-C alloys, the surface of which was exposed to sound waves and Ar-O gas blown onto the melt surface. The main emphasis is placed on clarifying effects of sound frequency, sound pressure, and gas flow rate. A series of water model experiments and numerical simulations are also performed to explain the results of high-temperature experiments and to elucidate the mechanism of sound wave application. This is explained by two phenomena that occur simultaneously: (1) turbulization of Ar-O gas flow by sound wave above the melt surface and (2) motion and agitation of the melt surface when exposed to sound wave. It is found that sound waves can both accelerate and inhibit the decarburization rate depending on the Ar-O gas flow rate and the presence of oxide film on the melt surface. The effect of sound waves is clearly observed only at higher sound pressures on resonance frequencies, which are defined by geometrical features of the experimental setup. The resonance phenomenon makes it difficult to separate the effect of sound frequency from that of sound pressure under the present experimental conditions. [ABSTRACT FROM AUTHOR]

Recently, there has been increasing interest in developing the coating of metal components with ceramic materials. Although considerable progress has been made in this area, there still remain many practical [...]