Ultrasound acoustic streaming effects

In the literature we can now find several papers which establish a widely accepted scenario of the benefits and effects of an ultrasound field in an electrochemical process [13-15]. Most of this work has been focused on low frequency and high power ultrasound fields. Its propagation in a fluid such as water is quite complex, where the acoustic streaming and especially the cavitation are the two most important phenomena. In addition, other effects derived from the cavitation such as microjetting and shock waves have been related with other benefits reported for this coupling. For example, shock waves induced in the liquid cause not only an enhanced convective movement of material but also a possible surface damage. Micro jets of liquid, with speeds of up to 100 ms-1, result from the asymmetric collapse of cavitation bubbles at the solid surface [16] and contribute to the enhancement of the mass transport of material to the solid surface of the electrode. Therefore, depassivation [17], reaction mechanism modification [18], surface activation [19], adsorption phenomena decrease [20] and the mass transport enhancement [21] are effects derived from the presence of an ultrasound field on electrode processes. We have only listed the main phenomena referring to the reader to the specific reviews [22, 23] and reference therein. 

As we have mentioned before, acoustic streaming, cavitation and other effects derived from them, microjetting and shock waves take also relevance when the ultrasound field interacts with solid walls. On the other hand, an electrochemical process is a heterogeneous electron transfer which takes place in the interphase electrode-solution, it means, in a very located zone of the electrochemical system. Therefore, a carefully and comprehensive read reveals that all these phenomena can provide opposite effects in an electrochemical process. For example, shock waves can avoid the passivation of the electrode or damage the electrode surface depending on the electrode process and/or strength of the electrode materials [29]. 

Significant attention has thus been given to investigating the effects of ultrasound on biological tissues. Ultrasound affects biological tissues via three main effects thermal, cavita-tional, and acoustic streaming. 

Abstract In this paper the effect of ultrasound on flow through porous media has been investigated both experimentally and theoretically. Ultrasounds (20 and 40 kHz) have been proved to increase the flow rate through porous media. Two effects have been found of relevance. Decrease in viscosity due to dissipation of acoustic waves and acoustic streaming. The two effects have been modeled and those models compared with experimental data. 

Keywords Ultrasounds, temperature effect, acoustic streaming. 

Since cavitational effects in fluids vary inversely with ultrasound frequency, it is likely that cavitational effects should play an even more important role in low-frequency sonophoresis. Tachibana et al. hypothesized that application of low-frequency ultrasound results into acoustic streaming in the hair follicles and sweat ducts of the skin, thus leading to enhanced transdermal transport. Mitragotri et al. hypohesized that transdermal transport during low-frequency sonophoresis occurs across the keratinocytes rather than hair follicles. They provided the following hypothesis for the higher efficacy of low-frequency sonophoresis. 

As described earlier, ultrasound affects biological tissues via three main effects, thermal effects, cavitational effects, and acoustic streaming. Conditions under which these effects become critical are given below. ... 

The electrodeposition process is complicated, but can be simply thought of as the transfer of ions to and/or from the metal surface [58], It is well known that when a metal is immersed in an aqueous solution a diffusion layer (Nemst diffusion layer) forms at the metal/solution interface. If an electrochemical reaction is to occur at the metal surface it is therefore necessary for ions to be transported across this diffusion layer. Any process which can affect this layer will therefore influence the electrochemical process. Ultrasound is known to reduce the thickness of this diffusion layer [26] but is unlikely to completely remove it as was suggested by early Russian workers. Ultrasound can also effect electrochemical reactions since it produces surface cavitation and acoustic streaming both of which assist diffusion to and from the metal surface, this movement often being the rate-controlling step in electrochemical processes such as deposition. 

Acoustic streaming has also been applied to electrochemistry. Compton s group [9] has tested the use of ultrasound to study the effect of the various configurations of the acoustic streaming... 

Marken F, Akkermans RP, Compton RG (19%) Voltammetry in the presence of ultrasound the limit of acoustic streaming induced diffusion layer thinning and the effect of solvent viscosity. J Electroanal Chem 415 55-63... 

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