Ultrasound mechanical effects

In a biphasic solid-liquid medium irradiated by power ultrasound, major mechanical effects are the reduction of particles size leading to an increased surface area and the formation of liquid jets at solid surfaces by the asymmetrical inrush of the fluid into the collapsing voids. These liquid jets not only provide surface cleaning but also induce pitting and surface activation effects and increase the rate of phase mixing, mass transfer and catalyst activation. 

Ultrasound frequency has revealed as the most important operational variable. Low frequency (20-60 kHz) has been most used to obtain mechanical effects such mass transport enhancement, shock waves, microjetting and surface vibration, especially used in the nanostructure preparation. It has been reported [118] that... 

Ultrasonic irradiation of a liquid leads to the generation of cavitation phenomenon which comprised of unique reaction fields in addition to physical and mechanical effects the formation of micro-meter sized bubbles, formation of bubbles with high temperature and high pressure conditions, formation of shock waves, and strong micro-stirring effects are produced. Table 5.1 shows representative ultrasound techniques to synthesize inorganic and metal nanoparticles and nanostructured materials. 

From the above one might be tempted to attribute ultrasonically enhanced chemical reactivity mainly to the mechanical effects of sonication. However this cannot be the whole reason for the effect of ultrasound on reactivity because there are a variety of homogeneous reactions which are also affected by ultrasonic irradiation. How, for example, can we explain the way in which power ultrasound can cause the emission of light from sonicated water (sonoluminescence), the fragmentation of liquid alkanes, the liberation of iodine from aqueous potassium iodide or the acceleration of homogeneous solvolysis reactions ... 

Photochemical decomposition can also be carried out in the presence of a suspension of photoactive material such as Ti02 where substrate absorption onto the uv activated surface can initiate chemical reactions e. g. the oxidation of sulphides to sul-phones and sulphoxides [37]. This technology has been adapted to the destruction of polychlorobiphenyls (PCB s) in wastewater and is of considerable interest in environmental protection. Using pentachlorophenol as a model substrate in the presence of 0.2 % TiOj uv irradiation is relatively efficient in dechlorination (Tab. 4.5) [38]. When ultrasound is used in conjunction with photolysis, dechlorination is dramatically improved. This improvement is the result of three mechanical effects of sonochemistry namely surface cleaning, particle size reduction and increased mass transport to the powder surface. 

Sonophotocatalysis is photocatalysis with ultrasonic irradiation or the simultaneous irradiation of ultrasound and light with photocatalyst. Tnis method includes irradiation with alternating ultrasound and light. Ultrasound effects on heterogeneous photocatalytic reaction systems have been demonstrated by Mason,1 Sawada et al.,2) Kado et al.,3) and Suzuki et al.4) In these papers, not only acceleration of photocatalytic reactions but increase in product selectivity by ultrasonic irradiation has also been reported. It was postulated that ultrasound effects, such as surface cleaning, particle size reduction and increased mass transfer, were the result of the mechanical effects of ultrasound.1,5) Lindley reviewed these and other effects.5)... 

Recently, liquid water was decomposed to hydrogen and oxygen stoichiometrically and continuously by irradiations of ultrasound and light with particulate photocatalyst.n) This reaction system is thought to be a joint one for sonolysis and photocatalysis. Furthermore, this system also is a hybrid of mechanical effects and chemical effects. In this chapter, the effect of ultrasound on photocatalytic reaction is considered. The joint system of sonochemical and photocatalytic reactions, in particular s explained. 

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. 

Biological Effects of Ultrasound Mechanisms and Clinical Implications (1983)... 

Luche et al.2 point out that soaia to mechanical effects such as aguu mechanism of a reaction. They note ation of single electron transfers i obtained in the absence of sonkatim ultrasound. 

Luche et al.2 point out that sonication can result in an overall rate increase owing to mechanical effects such as agitation, but that sonication can also influence the mechanism of a reaction. They note that in the latter case sonication leads to acceleration of single electron transfers and can result in a different product than that obtained in the absence of sonication. In general, ionic reactions are not sensitive to ultrasound. 

Tests involving new, more sophisticated measurement tools have provided new interpretations and equations for the cavitation phenomenon [14,15]. The thermal and non-thermal effects of non-inertial cavitation, and the chemical and mechanical effects of Inertial cavitation in relation to their impact on ultrasound safety have recently been Investigated [16]. 

Somewhat similar measurements could be based on solid disruption [18], polymer degradation [7], or accelerated dissolution. These well-known mechanical effects of ultrasound also derive from cavitation. Thus one might measure the rate of particle size reduction under sonication of some standard solid dispersed in a given fluid. Alternatively one could measure the rate of dissolution of a standard solid in a solvent, or the reduction in molecular weight of polymer chains. Here again the initial particle size and surface conditions, together with pressure and temperature, should be carefully monitored. 

The preceding methods are all in some way related to the mechanical effects of ultrasound. It is also possible to make direct measurements of mass transfer... 

The preceding methods are mainly based on the primary mechanical effects of ultrasound during which cavitation is most often present. In contrast to this, the following methods are connected with either the secondary effects of cavitation and/or with the tremendous local accelerations reported in Table 5. 

The reaction can easily be monitored by HPLC, NMR, or by simple weighing of the addition product [197]. The rate increase with ultrasound not only depends on the mechanical effects (mass transfer improvement) but also on some electronic effects as it has recently been shown that the reaction mechanism involves a single electron transfer step which can be stimulated by ultrasound [198]. Hence the development of this chemical probe could provide a very good dosimetry system since it involves both the mechanical and sonochemical effect of ultrasound. 

The classical techniques for the solvent extraction of materials from vegetable sources are based upon the correct choice of solvent coupled with the use of heat and/or agitation. The extraction of organic compounds contained within the body of plants and seeds by a solvent is significantly improved by the use of power ultrasound. The mechanical effects of ultrasound provide a greater penetration of solvent into cellular materials and improves mass transfer. There is an additional benefit for the use of power ultrasound in extractive processes which derives from... 

NCRP Report No. 74. Biological Effects of Ultrasound Mechanisms and Clinical Applications. National Council on Radiation Protection and Measurements Bethesda, MD, 1983. 

Keywords Cavitation Mechanical effects Mechanotransduction Self-assembly Streaming Ultrasound-responsive systems... 

More productive chemical results, which stiU harness the destructive action of ultrasound on certain bonds, can be attained when sonication is applied to biological fluids (e.g. protein solutions) en route to bionanomaterials [15], A conspicuous example can be found in sonochemically-prepared protein microspheres, in which the interplay of mechanical effects (emulsification) and chemical effects (formatiOT of transient species) is noticeable. A protein emulsion is readily created at the interface between two immiscible liquid phases, while radicals generated by water sonolysis promote disulfide bond cross-linking between cysteine residues. Surface modifications, via conjugation with monoclonal antibodies or RGD-containing peptides, can also be carried out [102, 103]. The sonochemical preparation of chitosan microspheres also exploits the intermolecular cross-linking of imine bonds from the sugar precursor [104]. 

Tran KVB, Kimura T, Kondo T, Koda S (2014) Quantification of frequency dependence of mechanical effects induced by ultrasound. Ultrason Sonochem 21 716-721... 

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