Ultrasound microjetting

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]. 

Ultrasound frequency has revealed as the most important operational variable. Low frequency (20-60 kHz) has been most used to obtain mechanical effects such mass transport enhancement, shock waves, microjetting and surface vibration, especially used in the nanostructure preparation. It has been reported [118] that... 

It is well known that high-intensity ultrasound causes the formation of cavitation bubbles which in turn collapse in a manner that in the presence of a solid surface form high-velocity fluid microjets directed toward the surface. This microjetting, or intense fluid agitation at the surface, enhances transport of the electroanalyte, causes heat, and influences the behavior and integrity of surface films on metal electrodes. It also leads to the erosion or corrosion of the surface. 

Recently Perusich and Alkire [105] have proposed a mathematical model to determine the reaction and transport between liquid microjets and a reactive solid surface. Conditions were established under which oxide depassivation and repassivation occurs as a function of ultrasonic intensity, surface film thickness, and fluid microjet surface coverage. The model was based on the concept that cavitation induces sufficient momentum and mass transfer rates (water hammer pressures as described earlier) at a surface to create oxide film stresses leading to depassivation. The model was used to evaluate experimental data on the corrosion behavior of iron in sulfuric acid [106,107], Focused ultrasound was used to investigate processes that influence depassivation and repassivation phenomena on pure and cast iron in 2N H2S04 at two ultrasound frequencies and at power intensities of up to 7.8 kW/cm2. 

Figure 8.1.23 depicts the ultrasound enhancement factor Eus as a function of the surface activation degree with parameter f2. The circles represent measured values for Eus and . In the case of magnesium as the treated sohd with tetrahy-drofurane as solvent and different chlorobutanes as reactants a mechanical activation degree of approximately 10% has been observed. This means that 10% of the necessary activation energy has been delivered by the mechanical treatment by impinging liquid microjets. This is a typical value that can be found in other devices like reaction mills, too. This value of 4 to 6kJ/mol for mechanically treated... 

The mass transport can also be enhanced by ultrasound [62]. An ultrasonic source immersed in the solution produces a radial flux and the formation of bubbles [69]. When a bubble collapses at an electrode surface, a microjet of solution is formed [70]. This form of transport is called microstreaming. It can be utilized in ultrasound-enhanced electroanalysis [71-73]. 

A major factor in ultrasound-induced processes is the presence of cavitation both in the bulk solution and at interfaces. The phenomenon caused by voids or gas bubbles in the solution phase being coupled to the oscillating pressure field, is responsible for hot spot processes and microjetting. There are different types of cavitation, notably stable cavitation (violently oscillating bubbles), or transient cavitation (collapsing bubbles) [34]. The ultrasound frequency and intensity determine the type and violence of the process. Cavitation occurs more readily in the vicinity of the electrode surface... 

A physical model has been proposed [35-38], which may be used to describe the processes that occm within the electrochemicaUy important layer of ca. 100 pm adjacent to the electrode surface in the presence of power ultrasound. The current may be separated into a steady component and a transient component suggesting a significant local fluctuation of the current density. The physical process responsible for the fluctuations is the collapse of cavitational bubbles and the formation of microjets. The number... 

Birch conducted the first electrochem-icaUy driven reduction process in liquid ammonia and demonstrated that the same type of products are obtained compared with the alkali metal dissolution method [33]. However, a common problem when carrying out electrochemical studies in liquid ammonia is the passivation of the working electrode by trace water present in the system [34]. This problem can be overcome by applying ultrasound. Asymmetric cavitation and collapse at the electrode surface casts microjets of solution towards it, thus depassivating by removing blocking material. In a typical procedure applied to 3-methylanisole in... 

Ultrasound has also been employed to accelerate chemical reactions, including breakdown of polymers in solution, and catalytic reactions [55]. Ultrasound is capable of producing high local temperatures and pressures unlike any other apparatus, and can drive unique chemistries as a result. The principle mechanism is cavitation of the sonic agitated fluid and the resulting bubbles collapse/explode at surfaces, which in turn produce high velocity microjets of liquid. This produces both physical and chemical changes. 

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