Physical effects of ultrasound

The physical effects of ultrasound in relation to analytical chemistry have not yet been rationalized. 

Apphcations of ultrasound to electrochemistry have also seen substantial recent progress. Beneficial effects of ultrasound on electroplating and on organic synthetic apphcations of organic electrochemistry (71) have been known for quite some time. More recent studies have focused on the underlying physical theory of enhanced mass transport near electrode surfaces (72,73). Another important appHcation for sonoelectrochemistry has been developed by J. Reisse and co-workers for the electroreductive synthesis of submicrometer powders of transition metals (74). 

The purpose of this chapter will be to serve as a critical introduction to the nature and origin of the chemical effects of ultrasound. We will focus on organo-transition metal sonochemistry as a case study. There will be no attempt to be comprehensive, since recent, exhaustive reviews on both organometallic sonochemistry Q) and the synthetic applications of ultrasound (2) have been published, and a full monograph on the chemical, physical and biological effects of ultrasound is in press (3). 

Ultrasound spans the frequencies of roughly 20 KHz to 10 MHz (human hearing has an upper limit of <18 KHz). Since the velocity of sound in liquids is 1500 m/sec, ultrasound has acoustic wavelengths of roughly 7.5 to 0.015 cm. Clearly no direct coupling of the acoustic field with chemical species on a molecular level can account for sonochemistry. Instead, the chemical effects of ultrasound derive from several different physical mechanisms, depending on the nature of the system. 

The first area involves low amplitude (higher frequency) sound and is concerned with the physical effect of the medium on the wave and is commonly referred to as low power or high frequency ultrasound . Typically, low amplitude waves are used for analytical purposes to measure the velocity and absorption coefficient of the wave in a medium in the 2 to 10 MHz range. Information from such measurements can used in medical imaging, chemical analysis and the study of relaxation phenomena and this will be dealt with later. 

In this chapter we will deal with those parts of acoustic wave theory which are relevant to chemists in the understanding of how they may best apply ultrasound to their reaction system. Such discussions tvill of necessity involve the use of mathematical concepts to support the qualitative arguments. Wherever possible the rigour necessary for the derivation of the basic mathematical equations has been kept to a minimum within the text. An expanded treatment of some of the derivations of key equations is provided in the appendices. For those readers who would like to delve more deeply into the physics and mathematics of acoustic cavitation numerous texts are available dealing with bubble dynamics [1-3]. Others have combined an extensive treatment of theory with the chemical and physical effects of cavitation [4-6]. 

According to Mason, mutual connections between the three strands can help strengthen and expand research. He has identified three areas where such connections exist, namely (i) the use of focused ultrasound in cancer therapy, where the development of transducer arrays is linked to the physical effect of power ultrasound and the sonochemically improved performance of chemotherapeutic agents ... 

Cavitation, which is the source of the main effects of ultrasound, is also the origin of a common problem with probe systems tip erosion, which occurs despite the fact that most probes are made of a titanium alloy. There are two unwanted side effects associated with erosion, namely (a) metal particles eroded from the tip will contaminate the system and (b) physical shortening of the horn reduces efficiency — eventually, the horn will be too short to be efficiently tuned. The latter problem is avoided by... 

Bates, D. The Effect of Ultrasound and Other Physical Parameters on the Reactivity of Powders and Catalysts, Coventry University, Ph.D. Thesis (1992). 

For recent reviews on the effect of ultrasound on heterogenous reactions, see (a) P. Boudjouk, in Ultrasound its Chemical, Physical and Biological Effects-, K. S. Suslick (ed.), VCH Publishers, New York, 1988 (b) P. Boudjouk, in High Energy Processes in Organtme-tallic Chemistry ACS Symposium Series No. 333, K. S. Suslick (ed.), American Chemical Society, Washington, DC, 1987 (c) P. Boudjouk, J. Chem Educ., 63, 427 (1986). 

The dimensions associated with ultrasound are not on the molecular scale. Thus the chemical effects of ultrasound cannot be attributed to any direct interaction of the acoustic wave with chemical species on a molecular level. Indeed, it is well known now that these effects are the result of the physical processes associated with ultrasonic waves. The most important of these is cavitation. 

An aim of this volume is to highlight rapidly developing areas of electroanalyt-ical chemistry and electrochemistry. In this context, the application of ultrasound on electrochemical processes is a topic of particular interest. In a series of three chapters, Compton and coworkers provide a treatment of the underlying physical aspects connected with the coupling of ultrasound to electrochemical systems (Chapter 2.8) and applications in electroanalysis (Chapter 2.9). The first of these chapters considers the effect of ultrasound on mass transport, on the electrode surface and on chemical reactions in solution, while the second chapter looks at the use of sonoelectrochemical methods in... 

If cavitation is responsible for the acknowledged effects of ultrasound, the associated physical phenomena are still not well defined. The complexity of the problem is apparent in Ch. 1, to which the reader interested in the physics of cavitation should refer. At the time of writing, the majority of practitioners accept a rationale based on the "hot-spot" interpretation, provided this expression is not taken literally, but rather with the meaning of a "high-energy state in a small volume". One should also recall that only a very small part (10 ) of the acoustic energy absorbed by the system is used to produce a chemical activity. ... 

A. Shanmugam, J. Chandrapala, M. Ashokkumar, The effect of ultrasound on the physical and functional properties of skim milk. Innov. Food Sci. Emerg Technol. 16, 251-258 (2012)... 

Kentish, S., Ashokkumar, M., 2011. The physical and chemical effects of ultrasound, in Ultrasound technolo es for food and... 

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