Microbubbles cavitation, acoustic

Other interesting processes include cryocrushing (Chakraborty et al., 2005) and ultrasonication (Zhao et al., 2007). Cryocrushing consists of pretreating libers with a laboratory beater PFI-type mill and lieezing the liber aqueous suspension with liquid nitrogen. The libers are nanolibrillated by the mechanical action of a cast iron mortar and pestie. In ultrasonication, ultrasound waves are applied to the liber aqueous suspension, and fibrillation is accomplished by the creation and collapse of microbubbles by acoustic cavitation. 

Cavitations generate several effects. On one hand, both stable and transient cavitations generate turbulence and liquid circulation - acoustic streaming - in the proximity of the microbubble. This phenomenon enhances mass and heat transfer and improves (micro)mixing as well. In membrane systems, increase of fiux through the membrane and reduction of fouling has been observed [56]. 

The ultrasonic irradiation of a solution induces acoustic cavitation, a transient process that promotes chemical activity. Acoustic cavitation is generated by the growth of preexisting nuclei during the alternating expansion and compression cycles of ultrasonic waves. For example, in aqueous liquid, temperatures as high as 4300 K and pressures over 1000 atm are estimated to exist within each gas- and vapor-filled microbubble following an adiabatic collapse (Didenko et. al. 1999). 

Sirotyuk (ref. 25) found that the complete removal of solid particles from a sample of water increased the tensile strength by at most 30 percent, indicating that most of the gas nuclei present in high purity water are not associated with solid particles. Bernd (ref. 15,16) observed that gas phases stabilized in crevices are not usually truly stable, but instead tend to dissolve slowly. This instability is due to imperfections in the geometry of the liquid/gas interface, which is almost never exactly flat (ref. 114). Medwin (ref. 31,32) attributed the excess ultrasonic attenuation and backscatter measured in his ocean experiments to free microbubbles rather than to particulate bodies this distinction was based on the fact that marine microbubbles in resonance, but prior to ultrasonic cavitation (ref. 4), have acoustical scattering and absorption cross sections that are several orders of magnitude greater than those of particulate bodies (see Section 1.1.2). 

There is other acoustical evidence to support the belief of Sirotyuk and other investigators that stable microbubbles serve as cavitation nuclei in fresh water. As noted by Sirotyuk (ref. 25), numerous experiments have disclosed that the cavitation threshold of water is increased by degassing of the liquid or by the... 

A detailed knowledge of the predominant physicochemical/biochemical mechanism by which gas microbubbles are stabilized in aqueous media is of practical importance to numerous and varied fields acoustic and hydrodynamic cavitation, commercial oil recovery, hydraulic and ocean engineering, waste-water treatment, chemical oceanography, meteorology, marine biology, food technology, echocardiography, and the continual medical problem of decompression sickness. Many of these applications... 

Therefore, sonochemistry can be described as the result of acoustic cavitation consisting of three events creation, expansion, and implosive collapse of bubbles in ultrasonically irradiated liquids (Apfel, 1981 also see Suslick, 1986). The microbubble, or cavitational, field is characterized by spherical bubbles partially filled with noncondensable gases. The radius of the bubble rf, at any time deviates from its initial value /-, o in a periodic fashion. This dynamic behavior of the bubble is well described by the Raleigh-Plesset equation given by... 

The optoacoustic properties of plasmon-resonant gold nanoparticles originate from photoinduced cavitation effects. This process can be summarized as follows (i) thermalization of conduction electrons on the subpicosecond timescale/ (ii) electron-phonon relaxation on the picosecond timescale and thermalization of the phonon lattice, with a subsequent rise in temperature by hundreds to thousands of degrees (iii) transient microbubble expansion upon reaching the kinetic spinodal of the superheated medium, initiated on the nanosecond timescale (iv) microbubble collapse, resulting in shockwaves and other forms of acoustic emission. The expansion and collapse of a cavitation bubble takes place on a microsecond timescale, and are easily detected by ultrasonic transducers. 

Sonochemical hybrid AOPs such as sonoelectro-Fenton (SEE) also possess a high efficacy for water remediation [1]. The main effect of ultrasounds applied to aqueous solutions is cavitation, which consists of the formation, growth, and collapse of microbubbles that concentrate the acoustic energy into microreactors. The consequent extreme conditions of temperature and pressure, along with the action of OH formed from water sonolysis via reaction (12), cause the pyrolysis of organic matter ... 

Fig. 2.10 Schematic representation of air-filled microsphere formation in an ultrasonic field. Partially denatured protein molecules adsorb at the ultrasonically-generated bubble solution interface. Superoxide radicals generated during acoustic cavitation leads to inter-molecular cross-linking of proteins resulting in the formation of stable protein-sheUed microbubbles [74]...

The particular and interesting characteristics of ultrasound waves in chemical reactions arise from the physical phenomenon known as acoustic cavitation. Cavitation is the production, growth, and collapse of microbubbles in a liquid when a large negative pressure is applied to it [10,11]. As can be seen in Figure 3, the formation of cavitation bubbles is initiated during the rarefaction cycle. 

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