Ultrasound is simply sound Essay Example for Free

Ultrasound is simply sound Essay Ultrasound is simply sound pitched above human hearing. Ultrasound is the part of the sonic spectrum which ranges from about 20 kHz to 10 MHz and can be roughly subdivided in three main regions: (A) low frequency, high power ultrasound (20-100 kHz), (B) high frequency, medium power ultrasound (100 kHz-1 MHz), (C) and high frequency, low power ultrasound (1-10 MHz). The frequency level is inversely proportional to the power output. High power, low frequency ultrasound does alter the state of the medium and is the type of ultrasound typically used for sonochmical applications. Table A shows the application of ultrasound (1). 2. 3. 2 Theory Two theories exist to explain the chemical effects due to cavitation: hot-spot theory (2) and the electrical theory (3 4). Hot spot theory has been experimentally shown that the cavitational collapse creates drastic conditions inside the medium for an extremely short time: temperatures of 2000-5000 K and pressures up to 1800 atm inside the collapsing cavity. A remarkable event during the cavitation collapse is the emission light under certain conditions (sonoluminescence). Furthermore, the collapse causes a couple of strong physical effects outside the bubble: shear forces, jets and shock waves. The electrical theory postulates that an electrical charge is created on the surface of a cavitation bubble, forming enormous electrical field gradients across the bubble which are capable of bond breakage upon collapse. 2. 3. 3 Sonochemistry in Aqueous Solution The reactive species formed during the sonolysis of water are similar to those observed radiolysis (Table B). Among the most extensively studies species are OH radical and hydrogen peroxide (H2O2), produced by the thermolysis of water molecules in the gas phase of the bubble, and recombination of the resulting free radicals H2O2 is formed in the cooler interfacial area of the cavitation bubble (5). 2. 3. 3. 1 Kinetic Analysis The chemical transformation which occur during sonolysis may occur in several different regions of the cavitation bubble (Fig C). Three regions of sonochemical activity in sonicated systems (6). Attack by oxidizing species such as hydroxyl radical (OH) or oxygen atom or thermolysis of chemical bonds within the substrate can occur in either the gas phase or interfacial region. OH is most concentrated in the gas phase of the cavitation bubble. It is presumed that aromatic substrates are attacked by addition of OH whereas non-aromatic molecules are attach by hydrogen atom abstraction (7) due to much stronger C-H bond in aromatic system. 2. 3. 4 Acoustic cavitation Bubble collapse in liquids results in an enormous concentration of energy from the conversion of the kinetic energy of liquid motion into heating of the contents of the bubble. The high local temperatures and pressures, combined with extraordinarily rapid cooling, provide a unique means for driving chemical reactions under extreme conditions. The origin of sonochemical effects in liquids is the phenomenon of acoustic cavitation. Ultrasonic waves traveling through a solution impose upon the liquid a sinusoidal pressure variation, alternately compressing the liquid molecules or pulling them apart by overcoming the intermolecular forces. As an ultrasonic frequency of 20 kHz, the liquid will be compressed and rarefied each second. Therefore, the distance among the molecules vary as the molecules oscillate around their mean position. If the intensity of ultrasound in a liquid is increased, a point is reached at which the intramolecular forces are not able to hold the molecular structure intact. Consequently, it breaks down and a cavity is formed. This cavity is called cavitation bubble as this process is called cavitation and the point where it starts cavitation threshold. A bubble responds to the sound field in the liquid by expanding and contracting, i. e. it is excited by a time-varying pressure (1 4). Two forms of cavitation are known: stable and transient. Stable cavities oscillate for several acoustic cycles before collapsing, or never collapse at all. Transient cavities, conversely, exist for only a few acoustic cycles (10). 2. 3. 5 Sonoluminescence (SL) During the acoustic cavitations, the emission of light was referred by the SL. During the underwater exposure of photographic plates, it was first observed, when these plates were irradiated with ultrasound in the solutions (11). Generally, the Hot Spot theory explains the origin of the sono-luminescence and sono-chemistry, which is widely accepted by the scholars. It simplifies the expansion of potential energy of a bubble, when it is concerntrated into the core of a heated gas, and which makes the implosion of that bubble. Sono-luminescence has been divided into two forms; single-bubble SL (12 13) and the multi-bubble SL (14), through which the information is gathered regarding the conditions during the implosion of the cavitation bubbles. Commonly, shock waves are the usually proposed mechanisms that are used in the SL during the implosion of the bubbles. Through this, the bubbles are converged at the center by these mechanisms (15). Hydroxyl radical is another mentioned that is being used in the SL, which produces the emission from the chemical species during their excited state (16). However, small changes in bulk parameters can bring significant influences in the nature of the emissions. The moment, at which the air bubbles glow in the water was observed by the first researchers and was then, known as the multiple-bubble sono-luminescence (MBSL). However, it is advised to observe this glowing condition in a darkened room due to the fainting characteristic of these bubbles. In 1990, two researchers; Crum and Gaitan observed the SBSL in its perfect conditions (17). Placing of a single bubble of gas was done in the liquid, in order to create SBSL. In this regard, an air bubble was injected in the liquid, which created the SBSL. Normally, the bubble was arisen after in the center of the cylindrical flask after its injection. However, the sound waves were bombarded that kept it in its place. Nowadays, elaborate setups are available and practiced by the researchers in their labs. Sono-luminescence requires around 100 decibels of sound waves, which can cause deafness in a normal person. Another factor that is imperative for the SL is the frequency of sound, which a human hearing cannot reach in its range. During the acoustic cavitations, the SL referred the release of light waves. Sono-luminescence is divided into two forms; single-bubble SL (12 13) and the multi-bubble SL (14), through which the information is gathered about the conditions during the implosion of the air bubbles. Commonly, shock waves are the regularly proposed methods that are used in the SL during this process. Through this, the bubbles meet at the center by these methods (15). The moment, at which the air bubbles glow in the water was observed by the first researchers and was then, known as the multiple-bubble sono-luminescence (MBSL). In 1990, two researchers; Crum and Gaitan observed the SBSL in its perfect conditions (17). A single bubble of gas was placed in the liquid to create SBSL. In this regard, an air bubble was inserted in the liquid, which created the SBSL. Sono-luminescence requires around 100 decibels of sound waves, which can also cause deafness in a normal person. 2. 3. 6 Heterogeneous systems: liquid liquid interface Ultrasound forms very fine emulsions in systems with two immiscible liquids, which is very beneficial when working with biphasic systems or phase transfer catalyzed. When very fine emulsions are formed, the surface area available for reaction between the two phases is significantly increased, enhance the mass transfer in the interfacial region, thus increasing the rate of the reaction. Ultrasound cavitation creates reactive intermediates that shorten the reaction time (18). 2. 3. 7 Ultrasonic System Types Generally 3 types: Ultrasonic bath, Probe system and Planar Transducers. 2. 3. 7. 1 Ultrasonic Bath Ultrasonic bath: originally manufactured for cleaning purposes (1). Fig Y shows the bath that has transducers attached to the bottom. The reaction vessel is typically immersed in the coupling fluid contained in the bath. When indirect sonication is used, the ultrasonic power which reaches the reaction vessel is relatively low as compare to other ultrasonic systems, such as a probe. In addition, obtaining reproducible results may be difficult because the amount of power reaching the reaction mixture is highly dependent upon the placement of the sample in the bath. 2. 3. 7. 2 Probe System Probe systems are being more frequently used for sonochemical research in the laboratory. This may be because manufactures are aware that this type of research is increasing and are providing equipment to meet the demand (19). Probe sytems are capable of delivering large amounts of power directly to the reaction mixture which can be regulated b varying the amplitude delivered to the transducer. Disadvantages in using a probe system include erosion and pitting of the probe tip, which may contaminate the reaction solution. Figure Z shows the probe type sonoreactor. 2. 9. 7. 3 Planar transducers This type of setup is typically made in the laboratory and consists of a planar transducer connected to a vessel which contains either the reaction mixture (direct sonication) or a coupling fluid (indirect sonication) into which the reaction vessel is immersed. Planar transducers capable of delivering higher powers than ultrasonic bath systems (1). However, they are difficult to scale-up.