Intensity zones, ultrasonic field

Cavitation is the phenomenon of bubble formation and collapse in liquid flows of high velocity or in intensive ultrasonic fields. It results from the evaporation of the liquid phase or from the release of dissolved gases at zones of low pressure (Webster 1963). It occurs when the cavitation number falls below a critical value ... 

The maximum stress intensities acting on particles during ultrasonication are considered for energy and for force quantities. Some authors estimated the specific rate of dissipation (power density) based on caloric data, which yielded values in the range of 10 -10" kW/m (Pohl et al. 2004) to 10 kW/m (Kuntzsch 2004, p. 100). The problem here is the definition of the dispersion zone, i.e. the volume where the ultrasonic field significantly contributes to deagglomeration. This zone... 

The power density Py is the characteristic quantity of turbulent flow. It determines the size of the smallest eddies and the intensity of microturbulence. In addition, it is a measure of the shear intensity in laminar flows or the intensity of cavitation in ultrasonic fields (see above). The power input P in the dispersion zone can be derived from the pressure drop (e.g. in pipes and nozzles) or can be measured caloricafly (e.g. for rotor-stator systems and ultrasonication Pohl 2005 Kuntzsch 2004). Additionally, P can be roughly approximated by the electric power consumption of the dispersion machine (e.g. for ultrasonication Mandzy et al. 2005 Sauter et al. 2008), even though the real values may be lower by a factor of 2 to 5. A further source of uncertainty is the volume of the dispersion zone (Vdisp). since the stress intensities are not uniformly distributed in dispersion apparatuses. In particular, this applies to agitated vessels, where the highest dissipation rates are obtained in the vicinity of the stirring instmment (Henzler and Biedermann 1996),... 

When operated in a batchwise mode, the induced turbulent field should be intensive enough to ensure the mixing of the complete sample volume, since only a small zone beneath the sonotrode tip actively contributes to the dispersion process. Alternatively, there are flow cells for continuous operation, in which the suspension flows axially onto the sonotrode tip and where the dispersion zone is usually defined by the cell geometry (Fig. 5.4). As a result of the high power consumption and, thus, energy dissipation, the samples should be cooled during ultrasonication. 

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