Low-frequency, high-power ultrasound

Low-frequency, high-power ultrasound, with frequencies between 20 kHz and 1 MHz, can be employed directly in a wide variety of food processing applications (see Table 1). 

There are many possible stimuli, both physical and chemical, which can trigger the release of the capsule content. The physical ones are, for example, high-frequency ultrasounds, " magnetic fields, microwaves, " or near-infrared radiation. " " The capsules may also respond by changing size indeed, some could both shrink and expand when heated. " However, some of these stimuli, such as low-frequency high-power ultrasounds, are not applieable under in vivo conditions. " ... 

High-frequency or diagnostic ultrasound in clinical imaging (3-10 MHz) Medium-frequency or therapeutic ultrasound in physical therapy (0.7-3.0 MHz) Low-frequency or power ultrasound for lithotripsy, cataract emulsification, liposuction, tissue ablation, cancer therapy, dental descaling, and ultrasonic scalpels (18-100 kHz)... 

In the literature we can now find several papers which establish a widely accepted scenario of the benefits and effects of an ultrasound field in an electrochemical process [13-15]. Most of this work has been focused on low frequency and high power ultrasound fields. Its propagation in a fluid such as water is quite complex, where the acoustic streaming and especially the cavitation are the two most important phenomena. In addition, other effects derived from the cavitation such as microjetting and shock waves have been related with other benefits reported for this coupling. For example, shock waves induced in the liquid cause not only an enhanced convective movement of material but also a possible surface damage. Micro jets of liquid, with speeds of up to 100 ms-1, result from the asymmetric collapse of cavitation bubbles at the solid surface [16] and contribute to the enhancement of the mass transport of material to the solid surface of the electrode. Therefore, depassivation [17], reaction mechanism modification [18], surface activation [19], adsorption phenomena decrease [20] and the mass transport enhancement [21] are effects derived from the presence of an ultrasound field on electrode processes. We have only listed the main phenomena referring to the reader to the specific reviews [22, 23] and reference therein. 

High-frequency, low-power ultrasound generally within the frequency range 0.5-20 MHz can be used to evaluate foodstuffs in terms of physical characteristics such as the degree of emulsification or the concentration of solids or gas. In... 

Active teehniques require external power, sueh as eleetronie or acoustic fields and vibration sourees. In the eleetrostatie field teehnique, both direct current and alternative eurrent ean be applied to a dieleetrie fluid. That causes a better bulk mixing of the fluid in the vieinity of the heat transfer surface [2]. Vibration teehniques are elassified as surfaee vibration and fluid vibration teehniques. Surface vibration impinges small droplets onto a heated surface to promote spray eooling. Both low and high frequeneies are used in surface vibration, espeeially for single-phase heat transfer. However, fluid vibration is a more practieal vibration enhaneement, due to the mass of most heat exehangers. Surface vibration eovers the frequency range from 1 Hz to ultrasound. 

In contrast, high-power or low-frequency ultrasound can be used to compatibi-lize immiscible polymer pairs by the high shear and degradation of the polymer chains. Moreover, in many cases there was no deterioration in mechanical properties, despite a lowering of both the molecular weight and viscosity. Dramatic improvements in the weld line strength of immiscible PS/HDPE blends were... 

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