Ultrasound cavitational effects

Fig. 2 shows the entrance die pressure and power consumption for various wt% loadings of CNTs as a function of ultrasonic amplitude. The measured pressure is before the ultrasonic treatment of PEI/MWNT composites. A continuous decrease in pressure with increasing ultrasonic amplitude was observed. This is from a combination of heating from dissipated energy from ultrasound, cavitational effect from ultrasonic waves leading to some thixotropic and permanent changes in polymer, reduction in friction at die walls and horn surfaces due to ultrasonic vibrations and possible shear thinning effect created by ultrasound waves. The die... 

To find the effect of reaction temperature and ultrasoimd for the preparation of nickel powders, hydrothermal reductions were performed at 60 °C, 70 °C and 80 °C for various times by using the conventional and ultrasonic hydrothermal reduction method. Table 1 shows that the induction time, when starts turning the solution s color to black, decreases with increasing the reaction temperature in both the method. The induction time in the ultrasonic method was relatively shorter, compared to the conventional one. It assumes that hydrothermal reduction is faster in the ultrasonic method than the conventional one due to the cavitation effect of ultrasound. 

To examine the cavitational effect of ultrasound on bright-red complex of aluminon adsorbed on Al(OH)3, in our experiments, 25 ml of 0.005 M aluminium sulphate was treated with 5 ml of 1% aluminon (triammonium aurine-tricarboxylate). This adsorption complex was sonicated for 10, 20 and 30 min, while the control sample was agitated for the same duration with a magnetic stirrer. The turbidity in sonicated sample increased, as time of sonication increased compared to the unsonicated condition (Table 9.18 and Fig. 9.4). In sonicated sample, the colour of the adsorption complex was dark compared to the control sample and the settlement of the adsorption complex was also slower due to the smaller size particle of the complex. 

Sonawane et al. [90] investigated the affect of ultrasound and nanoclay for the adsorption of phenol. Three types of nanoclay tetrabutyl ammonium chloride (TBAC), N-acetyl-N,N,N trimethyl ammonium bromide (CTAB) and hexadecyl trimethyl ammonium chloride (HDTMA), modified under sonication, were synthesized which showed healthier adsorption of phenol within only 10 min in waste water. The interlamellar spacing of all the three clay increased due to incorporation of long chain quaternary salts under cavitational effect. 

In case of crystals of Cu-Dy composite formed under sonication, the concentration of dysprosium increased while in case of the crystals of Mn-Dy and Co-Dy composites, the concentration of dopant, Dy, decreased indicating a strong attraction of Dy for Cu compared to its weak interaction for Mn and Co ions. Nevertheless, the possibility of some of the Dy having been ejected out due to forceful cavitational effect of the ultrasound from the lattice of Mn and Co cannot be ruled out. Higher percentage of Cu, Mn, and Ce in case of Cu-Ce, Co-Ce and Mn-Ce composites, synthesized under sonication compared to normal crystals, could be attributed to the change in the composition of the lattice pattern due to the mechanical impact of ultrasound, whereas, such an effect has not been found in Co salts. These can be seen in Table 11.1. 

This facilitates the flow of degradable species through these tunnels onto the surface of the TiC>2 where electron could be donated to the holes of the anatase phase and the photocatalytic action in combination with the cavitational effect of the ultrasound can accelerate the fragmentations of pollutants. The details of this mechanism are however discussed at the end of chapter. Ultrasound also breaks TiC>2 particles to still smaller size and increases the active surface area manifold. 

Power ultrasound also has an additional property which is particularly beneficial in crystallisation operations namely that the cleaning action of the cavitation effectively stops the encrustation of crystals on cooling elements in the crystallisation vat and thereby ensures continuous efficient heat transfer. 

Researdi into tlie use of ultrasound in environmental protection has received a considerable amount of attention with the majority of investigations focusing on the harnessing of cavitational effects for the destruction of biological and chemical pollutants in water. The field is not restricted to these two topics however, it is much broader (Tab. 4.1), and in this chapter we will review several aspects in addition to the decontamination of water... 

However, this commonly accepted theory is incomplete and applies with much difficulty to systems involving nonvolatile substances. The most relevant example is metals. For a heterogeneous system, only the mechanical effects of sonic waves govern the sonochemical processes. Such an effect as agitation, or cleaning of a solid surface, has a mechanical nature. Thus, ultrasound transforms potassium into its dispersed form. This transformation accelerates electron transfer from the metal to the organic acceptor see Chapter 2. Of course, ultrasonic waves interact with the metal by their cavitational effects. 

Cavitation is the formation of gaseous cavities in a medium upon ultrasound exposure. The primary cause of cavitation is ultrasound-induced pressure variation in the medium. Cavitation involves either the rapid growth and collapse of a bubble (inertial cavitation) or the slow oscillatory motion of a bubble in an ultrasound field (stable cavitation). Collapse of cavitation bubbles releases a shock wave that can cause structural alteration in the surrounding tissue [13]. Tissues contain air pockets trapped in the fibrous structures that act as nuclei for cavitation upon ultrasound exposure. The cavitational effects vary inversely with ultrasound frequency and directly with ultrasound intensity. Cavitation might be important when low-frequency ultrasound is used, when gassy fluids are exposed, or when small gas-filled spaces are exposed. 

Ultrasound-based degassing Involves removing gases from solutions without the need for heat or vacuum. The cavitational effects underlying sonochemical action are also the basis of the extremely effeotlve use of US to degas liquids. Onoe cavitation bubbles have... 

Ultrasound can be applied either in a continuous or a pulsed mode. A pulsed mode of ultrasound application is used many times because it reduces the severity of adverse side effects of ultrasound, such as thermal effects. However, pulsed application of ultrasound may have a significant effect on the efficacy of sonophoresis. As will be discussed later, cavitational effects, which play a crucial role in sonophoresis,... 

Since cavitational effects in fluids vary inversely with ultrasound frequency, it is likely that cavitational effects should play an even more important role in low-frequency sonophoresis. Tachibana et al. hypothesized that application of low-frequency ultrasound results into acoustic streaming in the hair follicles and sweat ducts of the skin, thus leading to enhanced transdermal transport. Mitragotri et al. hypohesized that transdermal transport during low-frequency sonophoresis occurs across the keratinocytes rather than hair follicles. They provided the following hypothesis for the higher efficacy of low-frequency sonophoresis. 

As described earlier, ultrasound affects biological tissues via three main effects, thermal effects, cavitational effects, and acoustic streaming. Conditions under which these effects become critical are given below. ... 

The degradation of methyl cellulose and its copolymerization with AN under sonication has been studied [33]. This degradation resulted from the cavitational effects of ultrasound but the degradation rate was not found to increase with irradiation time. A water-soluble copolymer was obtained by irradiating methyl cellulose and AN in a mixed H20/HCOOH solvent at 15 °C for 30 min using 21.5-kHz ultrasound. The copolymer was identified as both block and branch with joints mainly located at methoxyl groups of methyl cellulose by means of differ-... 

A study on the cavitation effect of 28-kHz ultrasound on sonic intensity and sonication time was completed using the electrical method [89]. The experimental... 

Another fruitful area of research has been that of the sonochemical activation of immobilized enzymes where ultrasound appears to be particularly useful in increasing the transport of substrate to the enzyme. Using a-chymotrypsin (on agarose gel) and casein as substrate, a two-fold increase in activity was observed at 20 kHz [12]. Here the origin of the enhancement was thought to be associated with increased penetration of the casein into the support gel induced by cavitational effects close to the surface. However an increase in the activity of a-amylase (on porous polystyrene) was produced on irradiation with 7 MHz ultrasound [13]. This is a very significant result since at this high-frequency cavitation cannot occur and... 

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