Ultrasound intensities

The ultrasound intensity and the distance between the hom and the electrode may be varied at a fixed frequency, typically of 20 kHz. This cell set-up enables reproducible results to be obtained due to the fonuation of a macroscopic jet of liquid, known as acoustic streaming, which is the main physical factor in detenuinmg tire magnitude of the observed current. 

In an alternative design, the actual tip of the ultrasonic hom may be used as the working electrode after insertion of an isolated metal disc [77, 78 and 79]. With this electrode, known as the sonotrode, very high limiting currents are obtained at comparatively low ultrasound intensities, and diflflision layers of less than 1 pm have been reported. Furdiemiore, the magniPide of the limiting currents has been found to be proportional to D, enabling a parallel to be drawn with hydrodynamic electrodes. 

In these last researches, a continuous feedback between the process study and the prototype design and development was established. In this way FEM (Finite element method) simulation has provided useful information about geometry, ultrasound intensity distribution and structural material coupling [37, 48, 49] for the design of an optimized sonoelectrochemical reactor. 

Sonoelectrochemistry has also been used for the efficient employment of porous electrodes, such as carbon nanofiber-ceramic composites electrodes in the reduction of colloidal hydrous iron oxide [59], In this kind of systems, the electrode reactions proceed with slow rate or require several collisions between reactant and electrode surface. Mass transport to and into the porous electrode is enhanced and extremely fast at only modest ultrasound intensity. This same approach was checked in the hydrogen peroxide sonoelectrosynthesis using RVC three-dimensional electrodes [58]. 

The chemical reactions induced by ultrasonic irradiation are generally influenced by the irradiation conditions and procedures. It is suggested that ultrasound intensity , dissolved gas , distance between the reaction vessel and the oscillator and ultrasound frequency are important parameters to control the sonochemical reactions. 

Figure 5.5 shows the changes in the concentration of Au(III) at different ultrasound intensities [29], where the intensities are determined by the calorimetric method. It can be seen that the concentration of Au(HI) decreases with increasing irradiation time and the reduction behavior is clearly dependent on the ultrasound intensities. At more than 1.20 W cm-2, the reduction of Au(III) was completely finished within the 20 min irradiation. On the other hand, it was also observed that no reduction occurred in a conventional ultrasonic cleaning bath (Honda Electric Co., W-113, 28 kHz, 100 W, bath-volume ca. 2 L) [29]. 

Based on these results, the reduction of Au(III) requires the formation of hot cavitation bubbles which cause pyrolysis of water and 1-propanol molecules. In addition, it is suggested that the number of hot cavitation bubbles and/or the bubble temperatures increase with increasing ultrasound intensity in the irradiation system. 

Fig. 5.8 Rate of Au(III) reduction as a function of ultrasound frequency. Conditions Au(III) 0.2 mM, 1 -propanol 20 mM, atmosphere Ar, ultrasound intensity 0.1 W mlA1 [33]...

The sonochemical reduction of Au(III) has been investigated under Ar in the presence of 20 mM 1-propanol at different frequencies, where two types of ultrasound irradiation systems were used one is a horn type sonicator (Branson 450-D, frequency 20 kHz, diameter of Ti tip 19 mm) and the other is a standing wave sonication system with a series of transducers operating at different ultrasound frequencies (L-3 Communication ELAK Nautik GmbH, frequency 213, 358, 647, and 1,062 kHz, diameter of oscillator 55mm) [33]. All experiments were performed at a constant ultrasound intensity ((0.1+/—0.01 W mL-1), which was determined by calorimetry. 

Fig. 5.10 Relation between the rate of Au(III) reduction and the average size of the formed gold particles. Each error bar corresponds to the standard deviation of the size of the gold particles. Closed circles correspond to the dependence of the ultrasound intensities, in which the rate of reduction increases with increasing the intensity. Conditions 200 kHz.

Price was also to confirm the dependence of the initial rate of polymerisation on the square root of the ultrasound intensity as have other workers, working in initiator free systems. 

The absorption of ultrasound increases the temperature of the medium. Materials that possess higher ultrasound absorption coefficients, such as bone, experience severe thermal effects as compared to muscle tissue, which has a lower absorption coefficient [5]. The increase in the temperature of the medium upon ultrasound exposure at a given frequency varies directly with the ultrasound intensity and exposure time. The absorption coefficient of a medium increases directly with ultrasound frequency resulting in temperature increase. 

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. 

Fig. 21. (a) Cyclic voltammogram obtained for the reduction of 1 mM Ru(NH3)63+ in 0.1 M KC1 at a polished polycrystalline diamond electrode with potential scan rate of 100 mV s-1 (b) Sonovoltammo-gram obtained under the same conditions with 90 W cm 2 ultrasound intensity. Reprinted from [109], Copyright (1999), with permission from Elsevier Science. 

An often-adopted sonovoltammetric design is that shown in Fig. 35 built around a conventional three-electrode cell and which allows the ultrasound intensity and the distance between the horn and electrode to be continuously varied at a fixed ultrasound frequency of typically 20 kHz. This arrangement is much less sensitive to the shape and dimensions of the electrochemical cell than when a sonic bath is utilized. A further and important point of contrast is that the direct contact of the (metallic) horn with the electrochemical system may dictate the use of a bipotentiostat to control its electrical potential relative to that of the reference electrode (Marken and Compton, 1996). Alternatively, the horn may be electrically isolated (Huck, 1987 Klima et al., 1994). A significant merit of the design shown in Fig. 35 is that the mass transport characteristics may be empirically but reliably established. It is to this essential topic we next turn. 

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. 

By contrast, ultrasonic probes have the advantage over ultrasonic baths in that they deliver their energy on a localized sample zone, thereby providing more efficient cavitation in the liquid. Also, they are not subjected to any exhaustion restrictions, so they are much more suitable for use in analytical chemistry than are ultrasonic baths. However, probes tend to uncouple as a consequence, cavitation occurs only at the radiating surface and only marginal ultrasound intensity can be detected elsewhere in the surrounding liquid. 

Effect of ultrasound intensity, power and horn tip size... 

The ultrasound intensity employed in US-based detection is so low (typically <1 W/cm ) that it causes no physical or chemical alteration of the properties of the material through which the wave propagates the amplitude of deformations is therefore extremely small, so ultrasonic spectroscopic techniques can be deemed non-destructive. 

Various ultrasound intensities in the range of 0.1-2W/cm have been used for sonophoresis. In most cases, use of higher ultrasound intensities is limited by thermal effects. Several investigations have been performed to assess the dependence of sonophoretic enhancement on ultrasound intensity. Miyazaki, Mizuoka, and Takada. foimd a relationship between the plasma concentrations of indomethacin transported across the hairless rat skin by sonophoresis (therapeutic conditions) and the ultrasoimd intensity used for this purpose. Specifically, the plasma indomethacin concentration at the end of three hours after sonophoresis (0.25 W/cm ) was about 3-fold higher than controls at the same time. However, increasing intensity by 3-fold (to 0.75 W/cm ) further increased sonophoretic enhancement only by 33%. Mortimer, Trollope, and Roy found that application of ultrasoimd at IW/cm increased transdermal oxygen transport by 40%i while that at 1.5 W/cm and 2 W/cm induced an enhancement by 50%i and 55 /o, respectively. 

In the very low-frequency ultrasound region (20 kHz), Mitragotri, Blankschtein, and Langer have reported that permeability of human skin in vitro to insulin increased by more than 100-fold as the ultrasound intensity increased from 12.5 to 125mW/cm. This variation of sonophoretic skin permeability with ultrasound intensity is quite different from that observed in therapeutic frequency region described above. 

In a recent systematic study of the dependence of 20 kHz sonophoresis on ultrasound parameters, Mitragotri et al. showed that the enhancement of skin permeability varies linearly with ultrasound intensity and ultrasound on-time (for pulsed ultrasound, ultrasound on-time equals the product of total ultrasound application time and duty cycle), while is independent of the ultrasound duty cycle. Based on those findings, fhe authors reported that there is a threshold energy dose for ultrasound induced transdermal drug transport. Once the threshold value is crossed, the enhancement of skin permeability varies linearly with the ultrasound energy dose (J/cm ), which is calculated as the product of ultrasound intensity and ultrasound on-time. This result indicates that ultrasound energy dose can be used as a predictor of the effect of 20 kHz sonophoresis. The authors also indicated that it is important to determine the threshold energy dose for each individual sonophoresis system, for example, the real in vivo situation, because it may vary from system to system. Specifically, it may vary between different skin models, as well as with the ultrasound frequency and the distance of the transducer from the skin surface, etc. 

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