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Ultrasound Irradiation: Fundamental Theory, Electromagnetic Spectrum, Important Properties, and Physical Principles

Sumit Kumar1, Amrutlal Prajapat2, Sumit K. Panja2, and Madhulata Shukla3

1Magadh University, Department of Chemistry, Bodh Gaya 824234, Bihar, India

2Uka Tarsadia University, Tarsadia Institute of Chemical Science, Maliba Campus, Gopal Vidyanagar, Bardoli, Mahuva Road, Surat 394350, Gujarat, India

3Veer Kunwar Singh University, Gram Bharti College, Department of Chemistry, Ramgarh, Kaimur 821110, Bihar, India

1.1 Introduction

US, also referred to as ultrasonic treatment or sonication, employs high frequency sound waves to agitate particles in a liquid or solid medium [1]. This process relies on the phenomenon of cavitation, which happens when high-intensity sound waves create small bubbles in a liquid. These bubbles rapidly expand and collapse, producing pressure and temperature gradients that can break down particles and disrupt chemical bonds. This is known as acoustic cavitation, and it can be utilized for various purposes, including emulsification, dispersion, mixing, and extraction. Additionally, US can increase the surface area of reactants and enhance chemical reactions by promoting mass transfer between phases. It can also induce the formation of free radicals, which can react with target compounds and break them down. US is widely used in a range of fields, such as wastewater treatment, food processing, pharmaceuticals, and materials science [2-4]. The effectiveness of US depends on several factors, such as the frequency and intensity of the sound waves, the duration of exposure, and the characteristics of the medium being treated. Cavitation can be generated either by passing ultrasonic energy in the liquid medium or by utilizing alterations in the velocity/pressure in hydraulic systems. The intensity of cavitation, and hence the net chemical/physical effects, relies heavily on the operating and design parameters, including reaction temperature, hydrostatic pressure, irradiation frequency, acoustic power, and ultrasonic intensity. To increase the extent or rate of reaction, cavitation can be combined with one or more irradiations or some additives can be utilized, which can be solids or gases and can sometimes have catalytic effects. The free radicals generated during the oxidation process consist of hydroxyl (·OH), hydrogen (·H), and hydroperoxyl (HO2·) radicals. Overall, the theory behind US is based on the principles of acoustic cavitation, which can be harnessed to achieve a variety of physical, chemical, and biological effects.

US refers to the application of high-frequency sound waves to a target material or medium. Here are some properties of US:

• Frequency: Ultrasound waves have frequencies above the upper limit of human hearing, typically between 20?kHz and several MHz (megahertz). The frequency determines the energy and penetration depth of the ultrasound waves.
• Wavelength: The wavelength of ultrasound waves is inversely proportional to the frequency. Higher frequencies result in shorter wavelengths. This property allows ultrasound waves to interact with small-scale structures and particles.
• Intensity: Ultrasound intensity refers to the amount of energy carried by the sound waves per unit area. It determines the strength of the ultrasound waves and their effect on the target material. Ultrasound intensity is typically measured in units of watts per square centimeter (W/cm2).
• Propagation: Ultrasound waves propagate through materials as longitudinal waves, causing the particles of the medium to vibrate in the direction of wave propagation. This enables the transmission of energy and information through the medium.
• Absorption: Ultrasound waves can be absorbed by materials they pass through. The extent of absorption depends on the properties of the material, such as its density, viscosity, and composition. Absorption leads to the conversion of ultrasound energy into heat, which can be utilized in various applications.
• Reflection and refraction: When ultrasound waves encounter an interface between two different media, such as air and a solid object, some of the waves are reflected back and some are transmitted into the new medium. The angles of reflection and refraction obey the laws of physics similar to those governing light waves.
• Cavitation: US can induce a phenomenon known as cavitation, where the rapid changes in pressure cause the formation and implosion of tiny bubbles in a liquid medium. Cavitation can generate localized high temperatures and pressures, which can be utilized in processes like sonochemistry and ultrasonic cleaning.
• Noninvasiveness: Ultrasound waves can be transmitted through the body noninvasively, making them useful in medical imaging techniques like ultrasound scans and sonograms. They provide real-time visualization of internal organs, tissues, and structures without the need for surgery or ionizing radiation.
• Doppler effect: The Doppler effect occurs when there is relative motion between the source of ultrasound waves and the target. This effect causes a shift in the frequency of the reflected waves, enabling the measurement of blood flow, velocity, and direction in medical applications like Doppler ultrasound [5, 6].
• Safety: US is generally considered safe for medical and industrial applications, as it does not involve ionizing radiation like X-rays or gamma rays. However, high-intensity ultrasound can cause thermal effects, and prolonged exposure to certain intensities may have biological effects. Safety guidelines and standards are in place to ensure the safe use of ultrasound in different applications.

1.2 Cavitation History

The phenomenon of cavitation was first observed by Thornycroft and Barnaby in 1895 when the propeller of their submarine became pitted and eroded over a short operating period. This was due to collapsing bubbles caused by hydrodynamic cavitation, which generated intense pressure and temperature gradients in the surrounding area [7]. In 1917, Rayleigh published the first mathematical model describing a cavitation event in an incompressible fluid [8]. It was not until 1927, when Loomis reported the first chemical and biological effects of ultrasound, that researchers realized the potential of cavitation as a useful tool in chemical reaction processes [9]. One of the earliest applications of ultrasound-induced cavitation was the degradation of a biological polymer [10]. Since then, the use of acoustic cavitation has become increasingly popular, particularly as a novel alternative to traditional methods for polymer production, enhancing chemical reactions, emulsifying oils, and degrading chemical or biological pollutants [11]. The advantage of utilizing acoustic cavitation for these applications is that it allows for much milder operating conditions compared to conventional techniques, and many reactions that may require toxic reagents or solvents are not necessary.

1.2.1 Basics of Cavitation

Ultrasound is a type of sound wave with a frequency above 20?kHz, and when it propagates through a liquid medium, it can create conditions for cavitation. Ultrasound has been extensively used as an intensifying approach in various fields, including chemical synthesis, electrochemistry, food technology, environmental engineering, materials, and nanomaterial science, biomedical engineering, biotechnology, sonocrystallization, and atomization [2, 12-21]. The use of ultrasound can lead to greener intensified processing with significant economic savings [22, 23]. Ultrasound-induced cavitation, also known as acoustic cavitation, is mainly due to the alternate compression and rarefaction cycles that drive the various stages of cavity inception, growth, and final collapse, as shown in Figure 1.1 [12].

When cavities collapse, a significant amount of energy is released, leading to the formation of acoustic streaming associated with turbulence resulting from the continuous generation and collapse of cavities in the system. Moreover, chemical effects, such as the occurrence of local hotspots in the interfacial region between the bubble and adjacent liquid, can generate free radicals [24]. The primary reactions that occur during sonication can be considered the initiator of a series of radical reactions depending on the species:

Figure 1.1 Schematic representation of the mechanism of generation of acoustic cavitation.

Source: Reproduced from Gogate et al. [12]/John Wiley & Sons.

When ultrasound is applied to water, it causes the generation of ·OH and H· radicals, which subsequently leads to the production of hydrogen peroxide (H2O2). Both of these agents are strong oxidizing agents. As the cavitation bubble collapses, it generates tremendous local pressure gradients, temperature, and microjets in the liquid at the collapse point [25]. The release of the accumulated energy during bubble collapse in the form of shock waves and hot spots can significantly enhance the reaction rate...