Ultrasound 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. 

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],... 

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]. 

Gold sulphide had a featureless non-crystalline plain surface with very sharp and smooth edges of the dried mass. However, the smoothness of the surface was destroyed completely with blisters appearing as a result of cavitation, after 30 min of ultrasound irradiation. A closer look at the surface at x 7,500 magnification reveals extensive surface erosion as a result of the micro-jets streaming. 

Low power ultrasound offers the possibility of enhancing the effects of chlorine. The results of a study of the combined effect of low power ultrasound and chlorination on the bacterial population of raw stream water are shown (Tab. 4.2). Neither chlorination alone nor sonication alone was able to completely destroy the bacteria present. When sonication is combined with chlorination however the biocidal action is significantly improved [10]. The effect can be ascribed partly to the break-up and dispersion of bacterial clumps and floes which render the individual bacteria more susceptible to chemical attack. In addition cavitation induced damage to bacterial cell walls will allow easier penetration of the biocide. 

A. Kabalnov, D. Klein, T. Pelura, E. Schutt, J. Weers, Dissolution of multicomponent microbubbles in the blood stream 1. Theory, Ultrasound Med. Biol. 24 (1998) 739-749. 

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. 

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. 

Mixing can be achieved by ultrasound using lead zirconate titanate (PZT), a piezoelectric ceramic, operated in the kHz region [22], In this way, liquid streams can be moved and even turbulent-like eddies are induced. Favorably, ultrasonic action is coupled into a closed volume, a micro chamber. Here, the creation of standing waves has been reported. 

The delivery of macromolecules into the central nervous system (CNS) via the blood stream is seriously limited by the blood-brain barrier (BBB). Noninvasive, transient, and local image-guided blood-brain barrier disruption (BBBD) can be accomplished using focused ultrasound exposure with intravascular injection of pre-formed microbubbles. A detailed description of the method for MRI-guided focal BBBD in animals will be described in this chapter. The method may open a new era in CNS macromolecular drug delivery. 

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