Ultrasonic source

Reaction After the Ultrasonic Source Was Stopped (Slowly Reverting to the Original Composition) ... 

The sonication intensity is directly proportional to the square of the vibration amplitude of the ultrasonic source. As a rule, increasing the intensity increases the sonoohemioal effeots however, the ultrasonic energy a system can take is limited. Thus, cavitation bubbles, which are initially difficult to create at the higher frequencies as a result of the shorter duration of rarefaction cycles, are now possible by virtue of the collapse time, temperature and pressure on collapse being mutually dependent. However, the sonication intensity cannot be increased indefinitely as the maximum possible bubble size is also dependent on the pressure amplitude. As the pressure amplitude is increased, bubbles may grow so large on rarefaction that the time available for collapse will be inadequate. In fact, it has been unequivocally established that ... 

Both ultrasonic baths and probes are useful for US-assisted digestion. When the ultrasonic energy provided by a bath suffices to accelerate dissolution of the sample, its use is preferred to that of a probe because baths are more commonplace in analytical laboratories. However, the increased power output of ultrasonic probes is often required to shorten digestion times. No comparison of the efficiency of the two types of ultrasonic source for US-assisted digestion appears to have been reported to date. 

That soft digestion enhanced by US was first demonstrated by Kannan and Pathan [15], who soft-digested aqueous benzoic add In the experimental device shown in Fig. 3.3. The cylindrical geometry for the sample was intended to ensure uniformity in the solid surface and improved accessibility by the liquid phase. The solid was immersed in a water bath with temperature control at 31 °C. The ultrasonic source was a probe that was inserted vertically into the water bath. The device was rated at 500 W and 20 kHz frequency. Trial experiments of USASD without temperature control of the water bath... 

One mechanism similar to that of capillary waves is based on the oscillation and subsequent disruption of droplets under US action. The corresponding resonance radius at a frequency of 20 kHz (common for ultrasonic sources) is about 10 xm. This mechanism must be considered as one source of US-assisted emulsification, but can only be applied to immiscible liquid-liquid systems with a diameter within the established range for most of the droplets. In fact, most immiscible liquid-liquid systems are formed by droplets with... 

Position of the ultrasonic source with respect to the liquid-liquid interface... 

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 frequency, shape of the wave, nature of the ultrasonic source used. 

McLean and Mortimer [187] have studied the variations in HO free radical production during the sonication of aqueous solutions at different powers at 970 kHz. A typical curve is given in Figure 36. From this it is clear that a threshold exists for radical production, after which there is a linear correlation with acoustic power up to a limiting value which probably corresponds with surface cavitation . Acoustic power was calibrated with a radiation balance and a PVDF hydrophone. Repeatability on experiments performed on the same day was less than 15%, but day-to-day variations could be as much as 50%, probably mainly due to small uncontrolled changes in the alignment of the reaction chamber (a test tube dipped in a water tank) with the ultrasonic source which was an acoustic horn. 

Mason et al. reported for the first time the response of the TA dosimeter with different ultrasonic sources and frequencies. They employed an ultrasonic cleaning bath (Kerry Pulsatron 55 operating at 38 kHz) with different immersed reactors (flat bottom Erlenmeyer and round bottom flask) and the Undatim Sonoreactor with 20-, 40-, or 60-kHz horns. Ultrasonic power measurements were monitored using the calorimetric method described previously. 

Pugin [37] monitored the formation of an organolithium compound (butyl lithium in THF) and compared it to thermal and erosion measurements. He found a linear correlation between the rate of reaction for this process and the temperature rise of a coated thermocouple. This is an interesting result, but the slope of the line will almost certainly depend on the surface condition of the lithium pieces, on their size, and on the location of the solid relative to the ultrasonic source. These parameters should be carefully controlled in order to get reproducible results using this dosimeter. 

Figure 39. Normalized beam patterns obtained from a single ultrasonic source (980 kHz) using three different probes.

The efficiency of ultrasonic degassing in a melt flow corresponds to the completeness of the processes of cavitation nucleation, growth and evolution of hydrogen bubbles. So with increasing flow of a melt through an ultrasonic setup, the increase in a number of working ultrasonic sources and in the duration of the residence time of the melt in the cavitation region are required. 

Ultrasonic Source Nucleation Time (h) Completion Time (h)... 

This has come about because of modem developments in both ultrasonic technology and in electrochemistry. Thus ultrasonic sources are now readily available in a variety of powers, frequencies, and geometries, capable of various pulse-styles, duty cycles, and other sophistications. Baths, horns, probes, and other reactors are now all available to the experimentalist. 

The development of a cylindrical ultrasonic transducer or vibrator has revolutionized ultrasonic wire cleaning. The wire or narrow metal strip continuously passes through the ultrasonic source while being submerged in the cleaning solution with increased cleaning rate, improved cost effectiveness, and reduced pollution hazards [54,55,56]. 

Ultrasonic nozzles are designed to specifically operate from a vibration energy source. In ultrasonic atomization, a liquid is subjected to a sufficiently high intensity of ultrasonic field that splits it into droplets, which are then ejected from the liquid-ultrasonic source interface into the surrounding air as a fine spray (Rajan and Pandit 2001). A number of basic ultrasonic atomizer types, like capillary wave, standing wave, bending wave, fountain, vibrating orifice, and whistle, etc., exist. 

The mass transport can also be enhanced by ultrasound [62]. An ultrasonic source immersed in the solution produces a radial flux and the formation of bubbles [69]. When a bubble collapses at an electrode surface, a microjet of solution is formed [70]. This form of transport is called microstreaming. It can be utilized in ultrasound-enhanced electroanalysis [71-73]. 

Immerse an ultrasonic source directly in the reaction medium... 

With triethylbenzylammonium bromide (TEBA) as the catalyst on a macro-porous styrene support cross-linked with 12% of divinylbenzene, the following kinetic rate constants k have been obtained (Table 6). It is noteworthy to observe that the apparatus FB-UM gives a rate constant very close to that of the SR-UM with the advantage of avoiding pulverization of the catalyst. Therefore, the FB-UM apparatus can be proposed for any reaction using a solid catalyst and an ultrasonic source to increase the reactivity with respect to silent conditions. ... 

Figure 47. Interdigilal metal pattern of a uniform transducer the IDT behaves like a sequence of ultrasonic sources or receivers [247]...

The IDT behaves as a sequence of ultrasonic sources. For an applied sinusoidal voltage, all vibrations interfere constructively only if the distance all between two adjacent fingers is equal to half the elastic wavelength. The frequency Fo = Vaw/>1 = Faw/o that corresponds to this cumulative effect is called the synchronous frequency or the resonance frequency. The bandwidth of an IDT is narrower, when there are more fingers. When IDT is used the AW velocity is determined by the plate material and orientation, while wavelength depends only on the ITD periodicity. 

Cholesteric liquid crystals can also be used to display sound fields, especially ultrasonic fields [13, 14]. The ultrasonic source is immersed in water and directed at a black foil on the water surface. The upper side of the foil is coated with a cholesteric liquid crystal film. The acoustic irradiation of the indicator foil generates colored images of the ultrasonic source and also various interference patterns. The reason for colors appearing can be attributed to conversion to heat of the ultrasonic energy absorbed by the foil. 

Besides measuring the attenuation of acoustic waves, another type of acoustic spectroscopy has been demonstrated to be able to size particles in the rang from 0.1 to 30 pm. In this technique, the transit time (hence, the velocity) of pulsed multiple frequency ultrasonic waves passing through a concentrated suspension (up to 10 v%) is measured. The frequencies applied (50 kHz- 50 MHz) in this technique are lower than those in the attenuation measurement so that a longer operational distance between the ultrasonic source and detector can be used [37]. The zeta potential of the particles in suspension can also be determined using an acoustic instrument if additional devices are used to measure the colloid vibration potential in the acoustic field. 

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