Microjetting

Microjet Formation during Cavitation at Liquid—Solid Interfaces... 

Cavitation has three negative side effects in valves—noise and vibration, material removal, and reduced flow. The bubble-collapse process is a violent asymmetrical implosion that forms a high-speed microjet and induces pressure waves in the fluid. This hydrodynamic noise and the mechanical vibration that it can produce are far stronger than other noise-generation sources in liquid flows. If implosions occur adjacent to a solid component, minute pieces of material can be removed, which, over time, will leave a rough, cinderlike surface. 

Figure 12.2 Disintegration of protective corrosion product by impacting microjet torpedo.

Second, deformation twins were observed in metal grains at the damaged surfaces. Deformation twinning cannot result from corrosion but is the consequence of shock loading of the metal, precisely the effects of microjets of water impacting on the metal surface. 

Where air bubbles and other gases are entrained in turbulent FW and an abrupt reduction in pressure takes place, cavitation may occur. The result of the extremely rapid formation and collapse of steam bubbles on the suction side of feed pumps or the discharge side of valves produces erosive microjets that over time may promote severe cavitation-al metal wastage. 

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... 

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

Asymmetric cavitation bubble collapse in the vicinity of the solid surface leading to the formation of high speed microjets targeted at the solid surface. The microjets can enhance transport rates and also increase surface area through surface pitting. 

Microjet system, which uses vibrating walls in the inkjet channel to emit jets, and whose ease of constrnction makes it a strong contender in wide-format machines. 

Tezel and Mitragotri [66] describe a theoretical analysis of the interaction of cavitation bubbles with the stratum corneum lipid bilayers. Three modes were evaluated—shock-wave emission, microjet penetration into the stratum corneum, and impact of microjet on the... 

FIGURE 16.2 Three possible modes through which inertial cavitation may enhance SC permeability, (a) Spherical collapse near the SC surface emits shock waves, which can potentially disrupt the SC lipid bilayers, (b) Impact of an acoustic microjet on the SC surface. The microjet possessing a radius about one tenth of the maximum bubble diameter impacts the SC surface without penetrating into it. The impact pressure of the microjet may enhance SC permeability by disrupting SC lipid bilayers, (c) Microjets may physically penetrate into the SC and enhance the SC permeability. (From Mitragotri, S., and Kost J., Adv. Drug Deliv. Rev., 56, 589, 2004. With permission.)... 

As seen in Figure 5.1, there are two main types of drop-on-demand technologies. The thermal ink jet or bubble jet has a heating element that causes a vapor bubble to eject an ink droplet from the nozzle. The piezo ink jet uses a piezoelectric transducer for ejecting ink droplets. The microjet technology is a further... 

Mixer 92 [M 92] Frontal-collision Impinging Jet Micro Mixer, Microjet Reactor ... 

In fact, depending on particle size, simultaneous microstreaming and microjetting or some other effect can determine the efficiency of US-assisted digestion to a variable extent. 

Cavitation problems are common when handling cryogenic liquids. Cavitation can occur when the pressure at the vena contracta (P. ,) is less than both the vapor pressure (P ) and the outlet pressure of the valve (P2). Under such conditions, vapor bubbles can form at the vena contracta, and these bubbles can implode and release powerful microjets that will damage any metallic surface as the pressure rises downstream (Figure 2.77). 

Cavitation occurs when the pressure rises downstream of the vena contracta. When it reaches the vapor pressure of the process fluid, the vapor bubbles implode and release powerful microjets that will damage any metallic surface in the area. 

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