Acoustic streaming

The ultrasound intensity and the distance between the hom and the electrode may be varied at a fixed frequency, typically of 20 kHz. This cell set-up enables reproducible results to be obtained due to the fonuation of a macroscopic jet of liquid, known as acoustic streaming, which is the main physical factor in detenuinmg tire magnitude of the observed current. 

Cavitations generate several effects. On one hand, both stable and transient cavitations generate turbulence and liquid circulation - acoustic streaming - in the proximity of the microbubble. This phenomenon enhances mass and heat transfer and improves (micro)mixing as well. In membrane systems, increase of fiux through the membrane and reduction of fouling has been observed [56]. 

An example of enhancement in mass transfer by acoustic cavitation is the increase in the limiting current density in electrolysis [79], The electrochemistry with ultrasound is called sonoelectrochemistry. Another example is ultrasonic cleaning [80], Soluble contaminants on a solid surface dissolve into the liquid faster with acoustic cavitation. Insoluble contaminants are also removed from a solid surface with ultrasound. This is also induced by acoustic cavitation in many cases, but in some other cases it is by acoustic streaming [81-85],... 

Mitome H, Kozuka T, Tuziuti T, Wang L (1997) Quasi acoustic streaming induced by generation of cavitation bubbles. IEEE Ultrason Sympo Proc 1 533-536... 

Kumar A, Gogate PR, Pandit AB (2007) Mapping of acoustic streaming in sonochemical reactors. Ind Eng Chem Res 46 4368 1373... 

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]. 

KKma J, Bernard C (1999) Sonoassisted electrooxidative polymerisation of salicylic acid. Role of acoustic streaming and microjetting. J Electroanal Chem 462 181-186... 

Enhanced chemical reactivity of solid surfaces are associated with these processes. The cavitational erosion generates unpassivated, highly reactive surfaces it causes short-lived high temperatures and pressures at the surface it produces surface defects and deformations it forms fines and increases the surface area of friable solid supports and it ejects material in unknown form into solution. Finally, the local turbulent flow associated with acoustic streaming improves mass transport between the liquid phase and the surface, thus increasing observed reaction rates. In general, all of these effects are likely to be occurring simultaneously. 

Mass Transport. Cavitation improves mixing but, on a macroscopic scale, it is probably less effective than a high speed stirrer. On a microscopic scale, however, mass transport is improved at solid surfaces in motion as a result of sound energy absorption. This effect is called acoustic streaming and contributes to increasing reaction rates. 

Acoustic perturbation methods, 14 617 Acoustic streaming, 9 59, 81 Acoustic wave gravimetric technique, acoustic wave sensors and, 22 270. 

Acoustic streaming (which aids mass transport) is the movement of the liquid induced by the sonic wave which can be considered to be simply the conversion of sound to kinetic energy and is not a cavitation effect. 

Acoustic streaming leading to the disturbance of the diffusion layer on the surface... 

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. 

Clarke, L., A. Edwards, and E. Graham. 2004. Acoustic streaming An in vitro study. Ultrasound Med Biol 30 559. 

Nightingale, K.R., P.J. Kornguth, and G.E. Trahey. 1999. The use of acoustic streaming in breast lesion diagnosis A clinical study. Ultrasound Med Biol 25 75. 

Shi, X., et al. 2001. Color doppler detection of acoustic streaming in a hematoma model. Ultrasound Med Biol 27 1255. 

---