Cavitation viscosity effect

The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

The important liquid phase physicochemical properties which affect the cavitation phenomena and hence the extent of cavitational effects for the given application include vapor pressure, viscosity and surface tension. 

The effect, though not insignificant, is hardly dramatic. Taking corn and castor oils as examples, a ten-fold increase in viscosity has only led to a 30% increase in the acoustic pressure needed to bring about cavitation. 

Let us now consider the effect of solvent viscosity on the cavitation threshold. According to Tab. 2.1, an increase in the solvent viscosity required the application of a... 

Whilst vapour pressure may be the major solvent factor involved in the degradation process, there could also be a contribution from solvent viscosity or even, yet less likely, from surface tension. It has already been argued (see Section 2.6.2) that although an increase in viscosity raises the cavitation threshold, (i. e. makes cavitation more difficult), provided cavitation occurs, the pressure effects resulting from bubble collapse... 

Such observations have been interpreted in terms of the increase in viscosity of the solution - i. e. the higher the viscosity the more difficult it becomes to cavitate the solution, at a given intensity, and the smaller is the degradation effect. 

Colloidal potassium has recently been proved as a more active reducer than the metal that has been conventionally powdered by shaking it in hot octane (Luche et al. 1984, Chou and You 1987, Wang et al. 1994). To prepare colloidal potassium, a piece of this metal in dry toluene or xylene under an argon atmosphere is submitted to ultrasonic irradiation at ca. 10°C. A silvery blue color rapidly develops, and in a few minutes the metal disappears. A common cleaning bath (e.g., Sono-clean, 35 kHz) filled with water and crushed ice can be used. A very fine suspension of potassium is thus obtained, which settles very slowly on standing. The same method did not work in THF (Luche et al. 1984). Ultrasonic waves interact with the metal by their cavitational effects. These effects are closely related to the physical constants of the medium, such as vapor pressure, viscosity, and surface tension (Sehgal et al. 1982). All of these factors have to be taken into account when one chooses a metal to be ultrasonically dispersed in a given solvent. 

Viscosity of dissolved polymers drops irreversibly under acoustic treatment65 A8). The depolymerization process us rather fast during the first minutes of the treatment and then it becomes slow and ceases completely when the equilibrium molecular mass (MM) M is reached. The higher the polymer s initial molecular mass N0, the higher the rate of destruction. The majority of authors associate polymer destruction in solution with cavitation effects occurring under acoustic treatment. 

Finally, it is worth mentioning that the cavitation phenomenon observed in low viscosity liquids is also caused by (explosive) boiling induced by sudden reduction of pressure, such as that occurring in regions behind moving surfaces, such as impellers, or as the result of flow acceleration (Bernoulli effect) (23). 

Characteristics of the solvent. Solvent properties affect US-assisted digestion as they impose the cavitation threshold above which sonochemical effects are felt by the medium. Also, any phenomenon altering some solvent property can modify such a threshold. Thus, any change in temperature results in a change in solvent properties such as the vapour pressure, viscosity or surface tension, which affect cavitation and their effects as a result. 

The effect of the viscosity of the continuous phase was studied theoretically in o/w emulsions containing water-soluble stabilizers and also in w/o emulsions of various oils [32]. In the former, droplets were larger in the absence of a stabilizer than in its presence. However, there was no clear-cut correlation of the viscosity of the continuous phase with droplet size. This can be ascribed to the increased amount of energy dissipated in the immediate vicinity of the droplets relative to the bulk liquid, which may result in more efficient disruption than if the energy dissipation occurs evenly throughout the continuous phase. The addition of a stabiiizer possibly alters and partly suppresses cavitation in the bulk liquid, the cavitation threshoid and viscosity being related similarly as in pure liquids [58]. The energy may subsequently dissipate preferentially at the surface of droplets and result in more efficient use in terms of droplet disruption. 

As in other oxidation processes, increased temperatures have an adverse effect which aiso ascribed to the resuiting increased vapour pressure ieading to easier cavitation, but iess vioient coiiapse, as a consequence of the decreased viscosity and surface tension. As the temperature approaches the soivent boiiing point, a iarge number of cavitation bubbies are formed concurrentiy that act as a barrier to sound transmission and dampen the effective US energy from the source to enter the iiquid medium. A temperature dose to room ievei is easy to maintain and ensures proper deveiopment of the process. 

Dissolved air is not readily drawn out of solution. It becomes a problem when temperatures rise rapidly or pressures drop. Petroleum oils contain as much as 12% dissolved air. When a system starts up or when it overheats, this air changes from a dissolved phase into small bubbles. If the bubbles are very small in diameter, they remain suspended in the liquid phase of the oil, particularly in high viscosity oils. This can cause air entrainment, which is characterized as a small amount of air in the form of extremely small bubbles dispersed throughout the bulk of the oil. Air entrainment is treated differently than foam and is typically a separate problem. Some of the potential effects of air entrainment include pump cavitation, spongy and erratic operation of hydraulics, loss of precision control, vibrations, oil oxidation, component wear due to reduced lubricant viscosity, equipment shutdown when low oil pressure switches trip, microdieseling... 

It is postulated that the role of the HBP in toughening will be to act similarily to a coreshell particle that is, the core of the HBP will act to cavitate and promote shear yielding, and the shell will be able to be tailored to control aggregation and interactivity with the epoxy-resin matrix. Increases in core should promote cavitation, and shell-chemistry functionalization should increase dissolution and reactivity with the epoxy resin. However, unlike with the core-shell particles, the inherently greater number of shell sites and low viscosity of the HBP will enable the toughening to occur without deleterious effects on other properties. 

Any increase in temperature will raise the vapor pressure of a medium and so lead to easier cavitation but less violent collapse (see above). This effect will be accompanied by a decrease in viscosity and surface tension. However, at temperatures approaching the solvent boiling point, a large number of cavitation bubbles are generated concurrently. These will act as a barrier to sound transmission and dampen the effective ultrasonic energy from the source which enters the liquid medium. The combination of all these effects shows a shape of maximum and the optimum temperature depends on the experimental conditions used and reaction studied. 

In addition to the important factors previously considered which influence sono-chemical reactions, a few others need to be considered as well. Among these are surface tension, viscosity, and solubility. The viscosity of a liquid increases as the pressure is increased or the temperature is decreased. Solvents with higher viscosity require higher amplitudes (or power) for cavitation to occur. In other words, cavitation becomes difficult to induce in high viscosity liquids. This is a situation that enhances the ultrasonic effect, and hence higher viscosities should normally lead to greater rate enhancements. 

High viscosity of a sonicated liquid lowers the cavitation threshold markedly. Viscous liquids generate bubbles only at high sound pressures. Bubble motion is damped by the dissipative effect of the viscosity and the smaller maximum bubble radii, and the lower inward wall velocities terminate most sonochemical effects. 

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