Cavitation probe systems

Cavitation, which is the source of the main effects of ultrasound, is also the origin of a common problem with probe systems tip erosion, which occurs despite the fact that most probes are made of a titanium alloy. There are two unwanted side effects associated with erosion, namely (a) metal particles eroded from the tip will contaminate the system and (b) physical shortening of the horn reduces efficiency — eventually, the horn will be too short to be efficiently tuned. The latter problem is avoided by... 

These methods have several advantages over the methods previously described including (a) the absence of distortions of the ultrasonic field which might be engendered by an invasive probe system (b) they can be used in a wide range of frequency and ultrasonic power, below or above the cavitation threshold and (c) they can even be used with solid materials by studying the reflected beam at the surface of the material [140]. 

Methods for measuring ultrasonic power have been reviewed [42] but, in short, there does not seem to be a simple method for the quantitative measurement of local ultrasonic intensity when cavitation is present. Pugin has developed a number of methods for the characterisation of sound fields in a variety of reactors [43]. These were used to develop profiles of the acoustic intensity for both cleaning probes and probe systems with a view to examining the reproducibility of reactions. 

The uses of inorganic metal compounds and rare gases to probe the conditions of cavitation collapse have become some of the most important methods available in fundamental ultrasonics. Quantitative determination of collapse temperatures and pressures, and qualitative determination of fundamental aspects of the nature of the cavitation field have been achieved, largely through SL spectroscopic methods. The presence of salts has a marked influence on properties on the acoustic systems, such as the extent of coalescence and bubble size, and the sonochemical activity and SL intensity. 

Ultrasounds can be applied to chemical systems by using ultrasonic baths or probes. Although baths are more widely used, probes are more efficient as a result of (a) the lack of uniformity in the transmission of ultrasounds (in baths, only a small fraction of the total liquid volume in the immediate vicinity of the ultrasound source experiences the effects of cavitation) and (b) the decline in power with time, which leads to exhaustion of the energy applied to baths. Both phenomena result in substantially decreased experimental repeatability and reproducibility. For this reason, the use of baths should be restricted to cleaning operations and removal of dissolved gases, their intended applications. A wide variety of commercially available ultrasonic baths exists ranging from laboratory to industrial-scale models. 

As noted earlier, most applications of ultrasound-assisted leaching involve discrete systems using a bath or an ultrasonic probe. Although ultrasonic baths are more common, ultrasonic probes have the advantage that they focus their energy on a localized sample zone, thereby providing more efficient cavitation in the liquid. 

In a recent study, Crosby et al. [31] have discussed the different possible initial failure mechanisms of a thin adhesive elastic layer in a probe test and have extracted two geometrical parameters which couple with the two material parameters, E and % . the degree of confinement of the adhesive layer (represented by the ratio of a lateral dimension over a thickness of the layer) and a characteristic ratio between the size of a preexisting internal flaw, ac, and a lateral dimension of the system, a. They distinguished among three main types of initial failure bulk cavitation, internal crack and external crack as shown in Fig. 6. 

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