Ultrasound frequencies

Ultrasound frequencies can be introduced into the walls of the vacuum system. If a source of ultrasound is placed on the wall of an ultrahigh vacuum system, a large hydrogen peak is observed. Related phenomena, presumably from frictional effects, are observed if the side of a vacuum system is tapped with a hammer a desorption peak can be seen. Mechanical scraping of one part on another also produces desorption. 

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... 

Saez V, Gonzalez-Garcia J, Iniesta J et al (2004) Electrodeposition of Pb02 on glassy carbon electrodes influence of ultrasound frequency. Electrochem Commun 6 757-761... 

Various irradiation parameters were investigated, such as the types of organic additives, intensity of the ultrasound, dissolved gas and distance between the reaction vessel and the oscillator. In the case of frequency effects, other irradiation systems were used. The details are described in the section of effects of ultrasound frequency on the rate of reduction. 

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. 

When the ultrasound frequency used changes, the following factors would change (1) the temperature and pressure inside the collapsing cavitation bubbles, (2) the number and distribution of bubbles, (3) the size and lifetime of bubbles, (4) the... 

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. 

The rate of Au(III) reduction is also affected by ultrasound frequency as seen in Fig. 5.8. From TEM analyses, the size of the formed Au nanoparticles was found to be affected by ultrasound frequency. Figure 5.11 shows the relationship between the average size of the formed Au nanoparticles and the rate of Au(III) reduction. It is found that, even in the case of different frequency, the rate of Au(III) reduction affects the size of the formed Au nanoparticles. Based on the obtained results, the nucleation process is important in determining the size of Au particles, because the nucleation process should be closely related to the rate of reduction [29, 33],... 

Okitsu K, Ashokkumar M, Grieser L (2005) Sonochemical synthesis of gold nanoparticles effects of ultrasound frequency. J Phys Chem B 109 20673-20675... 

Tauber et al. [23] following the same method as Hart et al. but using tert-butanol as the methyl radical source, obtained a temperature of 3,600 K in 10 3 M /(77-butanol and reported, similar to Hart et al. that this temperature decreased with increasing /( / /-butanol concentration. More recently, this method was adopted by Rae et al. [24] and Ciawi et al. [25, 26] in aqueous solutions. Rae et al. examined the effect of concentration of a series of aliphatic alcohols, extrapolating a maximum temperature of about 4,600 K at zero alcohol concentration [24]. They also observed a decrease in temperature with increasing alcohol concentration, which correlated well with the alcohol surface-excess and SL measurements obtained in the same system. Ciawi et al. investigated the effects of ultrasound frequency, solution temperature and dissolved gas on bubble temperature [26],... 

Ciawi E, Rae J, Ashokkumar M, Grieser F (2006) Determination of temperatures within acoustically generated bubbles in aqueous solutions at different ultrasound frequencies. J Phys Chem B 110 13656-13660... 

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. 

This technique appears particularly attractive because the high frame rate allows the dynamics of fast changing liquid flows to be studied and the spatial resolution is significantly reduced using a high ultrasound frequency (Manneville et al., 2005). This technique can be adapted to small-scale, high-speed gas-liquid two-phase flows that are not presently subject to ultrasound-based techniques. 

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. 

Sonophoresis (or phonophoresis) is a technique to enhance percutaneous penetration via the application of ultrasonic energy. Review of the sonophoresis literature, however, shows that the effects of ultrasound have been variable. For example, largely unsuccessful earlier studies with various common drugs used ultrasound frequencies of 1-3 MHz at 1-3 W/cm [67,68], whereas a... 

Fig. 8 Change in blood glucose level following insulin delivery by ultrasound exposure to alloxan-diabetic rabbits. Filled bars (mean SD) show the relative blood glucose level following ultrasound treatment for 1.5 h. The area of skin exposed was 7cm the ultrasound frequency used was 105 kHz. Open bars indicate the corresponding control values. (Redrawn from Ref. l)...

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