Ultrasonic intensity

Transient cavitation is generally due to gaseous or vapor filled cavities, which are believed to be produced at ultrasonic intensity greater than 10 W/cm2. Transient cavitation involves larger variation in the bubble sizes (maximum size reached by the cavity is few hundred times the initial size) over a time scale of few acoustic cycles. The life time of transient bubble is too small for any mass to flow by diffusion of the gas into or out of the bubble however evaporation and condensation of liquid within the cavity can take place freely. Hence, as there is no gas to act as cushion, the collapse is violent. Bubble dynamics analysis can be easily used to understand whether transient cavitation can occur for a particular set of operating conditions. A typical bubble dynamics profile for the case of transient cavitation has been given in Fig. 2.2. By assuming adiabatic collapse of bubble, the maximum temperature and pressure reached after the collapse can be estimated as follows [2]. 

The bubble formed in stable cavitation contains gas (and very small amount of vapor) at ultrasonic intensity in the range of 1-3 W/cm2. Stable cavitation involves formation of smaller bubbles with non linear oscillations over many acoustic cycles. The typical bubble dynamics profile for the case of stable cavitation has been shown in Fig. 2.3. The phenomenon of growth of bubbles in stable cavitation is due to rectified diffusion [4] where, influx of gas during the rarefaction is higher than the flux of gas going out during compression. The temperature and pressure generated in this type of cavitation is lower as compared to transient cavitation and can be estimated as ... 

Klima J, Frias-Ferrer A, Gonzalez-Garcy J, Ludvyk J, Saez V, Iniesta J (2007) Optimisation of 20 kHz sonoreactor geometry on the basis of numerical simulation of local ultrasonic intensity and qualitative comparison with experimental results. Ultrason Sonochem 14 19-28... 

In order to achieve complete destruction of biological contaminants in water through sonication very high ultrasonic intensities are necessary. Unfortunately this makes the technique expensive to use for general microbiological decontamination. However over the last two decades some conventional disinfection techniques involving chemicals, ultraviolet light and heat treatment have become less effective as some bacteria become more resistant. Such processes have become a focus for the use of sonication as an adjunct to other techniques. 

A similar combination of ultrasound and photocatalysis has also been reported to destroy 2,4,6-trichlorophenol in aqueous solution [39]. An ultrasonic probe (22 kHz) with a uv light source (15 W) was used to examine the effect of changing such operating conditions as ultrasonic intensity, reaction temperature and uv transmission. The experiments involved using 2,4,6-trichlorophenol (100 ppm) and TiOj (0.1 g L ) and showed that the degradation rates increased with the temperature of the solution. The cumulative effect was more pronounced at lower ultrasonic intensities with little additional benefit derived at increased ultrasonic powers. 

J.A. Gallego-Juarez, G. Rodriguez-Corral, J.L. San Emeterio, and E. Montoya-Vitini, Electroacoustic unit for generating high sonic and ultrasonic intensities in gases and interphases, 1989, Spanish Patent 8903371, European Patent noe EP.450.030.A1 (1991), USA Patent noe 5299175 (1994). 

One of the earliest reports on the effect of frequency was by Schmid and Poppe [29]. Working at a constant ultrasonic intensity of 1 W cm and irradiation frequencies of... 

Similar increases in k with ultrasonic intensity have been found for other polymers such as polystyrene [44], poly(methyl methacrylate) [45], poly(dimethylsiloxane) [46], poly(ethyleneoxide), hydroxyethyl cellulose, poly(vinyl acetate), poly(acrylamide)... 

Figures 5.24, 5.25 and 5.26 also show that the limiting molar masses are lower the higher the intensity. Whilst Okuyama [50] and Thomas et al. [51] predicted, and several workers observed [52], that the limiting molar mass is invariant with intensity, most workers now agree that decreases with increase in ultrasonic intensity. Price [39] found that the results of the ultrasonic degradation of polystyrene in toluene fitted equation (Eq. 5.22).

For example, we showed in Chapter 2 that that the solvent velocity, V, was proportional to the acoustic pressure, P, (Eq. 2.11), and that the acoustic pressure was proportional to the ultrasonic intensity, I (Eq. 2.13). 

Recently Biggs [74] has investigated the emulsion polymerisation of styrene using ultrasonic irradiation as the initiation source (i. e. in the absence of a chemical initiator). Similar to Lorimer and Mason using a thermally initiated system, Biggs found both a marked increase in monomer conversion rate as a function of time as the ultrasonic intensity was increased but remarkable constancy in the resultant latex particle... 

In general they found both enhanced reaction rates and polymers with lower poly-dispersities in the presence of ultrasound provided by both bath and probe systems. Higher ultrasonic intensities resulted in narrower molar mass distributions. 

The overall conclusion for thermoplastics was that ultrasonic intensity was more influential than time in promoting fusion and that the depth of fusion increased with increase in pressure. For thermosets ultrasonic intensity and time have equal influence in promoting fusion and the depth of fusion decreased with an increase in pressure. 

For large scale (200 L capacity plating tank) it proved impossible to apply airborne ultrasound and therefore the ultrasound was introduced into the plating tank (Fig. 6.13). The transducers consisted of two banks of three mounted in dummy tanks and delivered in total 1.4 kW into 135 L making the overall ultrasonic intensity 0.01 W cm The cathode consisted of a steel bar (5 cm diameter and 20 cm length) and 4 large lead anodes (diameter 3.7 cm and length 39 cm) were used. On sonication... 

The ultrasonic intensity and size of sample to be irradiated can be matched fairly accurately for optimum effect. 

Li, C. Z., Yoshimoto, M., Ogata, H., Tsukuda, N., Fukunaga, K., and Nakao, K. 2005. Effects of ultrasonic intensity and reactor scale on kinetics of enzymatic saccharification of various waste papers in continuously irradiated stirred tanks. Ultrasonics Sonochem.,12, 373-384. 

Ultrasonic baths will be familiar from their everyday use in the laboratory where they are commonly used for cleaning surfaces and to aid dissolution. A bath essentially comprises a number of transducers of fixed frequency, commonly 20-100 kHz, attached beneath the physical exterior of the bath unit. Baths typically deliver ultrasonic intensities between 1 and 10 W cm to the reaction medium. For sonovoltammetry (or sonoelectrosynthesis) the bath may be filled with distilled water and a conventional electrochemical cell is placed inside the bath at a fixed position (Walton et al., 1995) so that the cell is electrically isolated from the sound source. Alternatively, the internal metal casing of the bath can be coated so that the full volume is available to use as an electrochemical cell (Huck, 1987). For both arrangements results can be highly sensitive to positioning and/or cell geometry effects. 

An ultrasonic horn transducer consists of a transducer unit attached to a horn (rod) usually made from titanium alloy and which has a length a multiple of half-wavelengths of the sound wave. For the commonly encountered 20-kHz horn this corresponds to 12.5 cm. The horn is then partially inserted into the fluid medium of interest and intense ultrasound is generated at its tip so that, for adequately large intensities, a cloud of cavitation bubbles is visible. This arrangement permits significantly higher ultrasonic intensities (10-1000 W cm ) to be applied than are achievable with a bath. 

The quantity S can be interpreted as the mean thickness of a diffusion layer at the electrode surface as schematically depicted in Fig. 37. In this simple picture, the electrode is separated from the turbulent bulk by a laminar sub-layer inside of which the concentration of the electroactive species depletes from the bulk value to that at the electrode surface across the (physically smaller) diffusion layer. Within this model the effects of the ultrasonic intensity and the horn-to-electrode separation emerge through their effects on the size of 8. 

Recentiy this phenomenon was found to be induced at a reiatively low ultrasonic intensity, even with progressive waves by the second harmonic overlapping the fundamental one [10]. This finding is of paramount importance as regards the clinical applicability of US, and also in sonochemical uses involving systems sensitive to high ultrasound intensities. 

This section discusses the potential of sonoelectroanalysis, expansion of which is currently at a standstill owing to the few groups working on it. With few exceptions involving baths, probes are the ultrasonic sources used to assist electroanalytical processes with US. Some authors have pointed that the low, spatially variable distribution of ultrasonic intensity provided by baths is a major hindrance for using these devices with electroanalytical techniques [131]. Therefore, most of the examples described in this section involve the use of probes as US sources. 

The distance between the probe tip and the electrode is a key variable here as it affects the diffusion layer thickness (see Fig. 8.14A). The diffusion layer thickness tends to approximately the same limit for two different ultrasonic intensities at short horn-electrode... 

Figure 8.14. (A) Variation of diffusion iayer thiokness as calculated from Eq. 8.2 with the probe tip-eiectrode separation for two different ultrasonic intensities. The solution was 1 mM [Fe(CN)ef 0.1 mM KCi and the working electrode a 4-mm-diameter Ft disc. (B) infiuence of the addition of 40% heptane in the electrolyte (aqueous 0.1 M KCI) on the limiting current of 1 mM N,N,N, N -tetramethyl-p-phenylenediamine. (Reproduced with permission of Elsevier, Refs. [153,156].)...

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