Keywords

Functional materials are integral part of human life [1, 2]. Considering the health of human beings, drug delivery vehicles and materials possessing antimicrobial activities and nutritional properties come under this category. Delivering nutritional compounds and pharmaceuticals through commonly consumed food matrix is in practice. For example, non-polar nutrients are dispersed as emulsions in water-based drinks including milk [3]. Similarly, inorganic and organic nano- and micro materials with functional properties are increasingly being used in various applications. For example, metal and semiconductor nanoparticles are used as catalytic materials [4, 5].

Novel methodologies for synthesising functional materials have been constantly developed. Nanomaterials for energy conversion and biological applications could be synthesised by sol-gel methods and conventional chemical reactions to precisely control their size and size distribution [6–10]. Layer-by-layer method to generate core-shell materials for drug delivery applications is a well-known procedure [11] that primarily works on electrostatic or hydrophobic interactions between functional molecules. Emulsion polymerisation is another commonly used method to generate core-shell functional materials [12–14]. Strong shear forces are applied and surfactant/polymer based stabilisers are used to stabilise for example an oil-in-water emulsion droplet. The emulsion droplet itself could be a nutrient dispersed in a food or drink matrix [15]. Such emulsions are also used to generate functional polymer latex particles [16].

Ultrasonic synthesis of functional materials has been widely reported and is a growing methodology that has significant potential to generate large-scale functional materials. This Brief is aimed at providing the fundamental science involved in ultrasonic synthesis of functional materials with specific examples. The advantages, disadvantages and challenges of this technology have also been highlighted.

1.1 Sound

Animal kingdom communicate with each other using soundwaves. Soundwaves can be divided into various frequency range based on their uses/applications [17]. Accordingly, below 20 Hz frequency range is referred as subsonic or infrasonic waves. Human ear cannot detect this frequency range. Sometimes, such frequency range can be felt as shock waves, for example during earth quakes. About 20 Hz–20 kHz is in the range of human hearing. Soundwaves above 20 kHz frequency are referred as ultrasound, which are subdivided into power ultrasound, used in a variety of physical and chemical applications and magasonic, above 1 MHz, used in medical diagnostics. Most of the studies dealing with material synthesis use 20 kHz–1 MHz frequency range.

The speed of sound varies depending upon the medium. In air, sound travels at a speed of about 320 m/s and its speed in water is 1500 m/s. Sound travels relatively faster in solids—for example, the speed of sound in steel is about 5000 m/s. The mechanical vibrations generated by sound waves have been used in various applications [18, 19]. For example, rate of dehydration of specific materials could be enhanced by ultrasonic vibration [20]. Acoustic reflection and scattering techniques have been used in quality control of food and other materials [21, 22]. Ultrasonic imaging is a well-known technique in diagnostic medicine [23]. Ultrasonic cleaning of small and large equipment is a well-known industrial application [24]. Ultrasonic cleaning is also a common practice in dental clinics [25]. Ultrasonic extraction and emulsification are used in food and pharmaceutical industries [26–30].

While soundwaves are commonly used for communication through various media, some unique events occur in liquids when soundwaves interact with the medium. In particular, when ultrasound passes through a liquid medium, it strongly interacts with small gas bubbles that exist in the liquid. Such interaction between ultrasound and gas bubbles leads to a phenomenon known as acoustic cavitation [31, 32].

1.2 Acoustic Cavitation

Cavitation literally means creation of a cavity. Hence, acoustic cavitation literally refers to the formation of a cavity using acoustic (sound) energy. Such a process requires significant amount of energy. For example, to separate water molecules apart overcoming intermolecular forces to a distance in the order of nanometres requires a negative pressure of hundreds of atmospheres [17]. Equation 1.1 can be used to calculate the critical pressure (PB) required to create a bubble of radius, Re.

$$ P_{B} \sim P_{h} + \frac{{0.77 \upsigma}}{{R_{e} }} $$
(1.1)

\( \upsigma \) is surface tension of the liquid and Ph is hydrostatic pressure (could be approximated to atmospheric pressure under normal experimental conditions). Note that this equation is valid when \( 2\upsigma/{\text{R}}_{\text{e}} \ll {\text{P}}_{\text{h}} \).

However, cavities could be formed from pre-existing gas nuclei at much lower power levels [33–35]. Gas nuclei, inherently present in liquids, are forced to oscillate under the fluctuating pressure field when ultrasound passes through a liquid. Gas bubbles formed from such oscillations grow over a period by a process known as rectified diffusion [36, 37]. Bubble growth by rectified diffusion can be understood by area and shell effects. During the growth phase of a bubble, solvent vapour and dissolved gas molecules diffuse into the bubble. Since the surface area of the bubble wall increases during the growth phase, more molecules diffuse into the bubble. The opposite process occurs during the compression phase—molecules diffuse out of the bubble. However, the surface area decreases in the compression process and hence a relatively lower amount of molecules diffuse out of the bubble. For each expansion/compression cycle, more vapour/gas molecules stay inside the bubble resulting in a net growth of the bubble size. Another aspect that needs to be considered is the time scale involved for the expansion and compression phases. While an acoustic cycle has equal time for rarefaction and compression half cycles, single bubble dynamics experiments have shown that the growth phase of a bubble is a relatively slow process resulting in a higher amount of gas/vapour molecules diffusing into a bubble. In addition, dissolved gas concentration in the liquid shell surrounding an oscillating bubble varies significantly. In the compressed state of a bubble, gas molecules diffuse out of the bubble into the liquid shell surrounding a bubble. The diffusion of gas molecules becomes relatively slow and hence less gas molecules diffuse out of the bubble. The process works favourably during expansion process. Gas molecules can freely diffuse into the bubble. The shell effect is a very complex process than what is described above. A combination of area and shell effects and the time scale involved in these processes lead to rectified diffusion growth of bubbles in an acoustic field. Detailed models have been developed to theoretically estimate the growth of bubbles by rectified diffusion process [38].

Bubble growth by rectified diffusion is not an infinite process. The growth of the bubble is restricted by the applied frequency. The wall of a free gas bubble in a liquid oscillates at a given frequency depending upon the size of the bubble. The relationship between oscillation frequency and radius of a bubble is given by a simplified form of the Minnaert’s Equation (Eq. 1.2) [39].

$$ {\text{f }}*{\text{ R}}\sim 3 $$
(1.2)

where, f is frequency in Hz (s−1) and R is resonance radius of the bubble in m. For example, the wall of a 150 μm radius bubble oscillates at a resonance frequency of 20 kHz.

Note that despite a single resonance size is expected theoretically for a given frequency, a size range exists [40]. Using pulsed sonoluminescence and sonochemiluminescence methods, the resonance size range of SL and sonochemically active bubbles have been estimated [41, 42]. Lee et al. developed a novel pulsed sonoluminescence (SL) technique [41] to determine the resonance size range of cavitation bubbles. By systematically increasing the pulse off time, they could see a decrease in the SL intensity due to the dissolution of active cavitation bubbles below the resonance size during this period. Beyond a certain pulse off time, no more SL intensity could be observed indicating the dissolution of all cavitation bubbles below the resonance size. This is schematically shown in Fig. 1.1.

Fig. 1.1
figure 1

Schematic representation [Adapted from Ref. 41] of pulsed sonoluminescence technique to determine the resonance size of cavitation bubbles. Bubbles grow during pulse on (T) and dissolve during pulse off (To). With increasing To, steady-state SL intensity decreases (top right) eventually to zero—the corresponding To is used to calculate the bubble size using Eq. 1.3

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Taking the pulse off time at which SL disappeared, the resonance size of the cavitation bubbles was calculated. Epstein Plesset equation, (Eq. 1.3) which relates bubble dissolution time to its radius, was used for this purpose.

$$ \left( {\frac{{DC_{s} }}{{\rho_{g} R_{o}^{2} }}} \right) t = \frac{1}{3}\left( {\frac{{RT\rho_{g} R_{o} }}{2M\gamma }} \right) + 1 $$
(1.3)

D—diffusion coefficient; Cs—dissolved gas concentration; ρg—density of gas; Ro—initial bubble radius; t—dissolution time; M—molar mass of gas; γ—surface tension of the liquid; R—gas constant and T—solution temperature. Replacing t with To, Ro (assumed to be equal to resonance size) can be determined. The resonance size of cavitation bubbles at 515 kHz was found to be in the range 2.8–3.8 μm.

A follow up work by Brotchie et al. [42] noted that the resonance sizes of sonoluminescence and sonochemically active bubbles are different. The sonochemiluminescence, resulting from the reaction between OH radicals generated within cavitation bubbles and luminol molecules, intensity was used to determine the sonochemically active (SCL) bubbles. The resonance size of SL bubbles are found to be relatively larger than that of SCL bubbles. In addition, Eq. (1.2) shows that the resonance size decreases with an increase in ultrasonic frequency. Brotchie et al. [42] have also confirmed this experimentally. The sizes were found to be 3.9, 3.2, 2.9, 2.7 and 2 μm at 213, 355, 647, 875 and 1056 kHz frequency, respectively. Another important aspect that needs to be mentioned is the difference between theoretical and experimentally determined resonance sizes of the cavitation bubbles. Equation (1.2) provides a theoretical value of 14 μm at 213 kHz whereas the experimental value is found to be 3.9 μm. This is also known from single bubble work at 20 kHz where the experimental resonance size was found to be about 5 μm compared to the theoretical value of 150 μm [43]. The difference between the resonance size determined by Eq. (1.2) and experimental value is due to the fact that Eq. (1.2) is a very simplified one that does not consider the physical properties of the liquid or bubble contents.

When bubbles reach the resonance size range, they grow to a maximum size within one acoustic cycle and implode. Bubble implosion/collapse is a near adiabatic process. In simple thermodynamic terms, the volume of the bubble decreases instantaneously resulting in the generation of extreme heat within the bubble. Theoretical estimates predict greater than 15,000 K [44, 45]. However, experimental methods estimate about 1000–5000 K [46–50]. A number of techniques have been used to calculate the bubble temperatures. First, the bubble temperature could be theoretically calculated using Eq. (1.4).

$$ T_{max} = T_{m} \left( {\frac{{P_{m} (\gamma - 1)}}{{P_{v} }}} \right) $$
(1.4)

Tmax—bubble temperature on collapse; Tm and Pm are solution temperature and pressure, respectively, Pv is pressure inside the bubble and γ is heat capacity ratio of the gas inside the bubble. A theoretical temperature of about 12,700 K could be calculated by using γ = 1.66 (ideal gas), Tm = 298 K, Pm = 2 atm, Pv = 0.031 atm. Replacing γ of an ideal gas by that of water (1.32), the temperature drops to 6150 K highlighting the importance of the heat capacity ratio of the gas contained in the collapsing bubbles. Suslick and coworkers [46, 47] have used sonoluminescence spectra to calculate bubble temperatures in multibubble systems and found to be in the order of 1000–5000 K. Henglein and coworkers [48] have used methyl radical recombination method and determined the cavitation bubble temperatures to be in a similar range. Independent of the accuracy of these methods, it could be realised that the extreme temperature conditions are generated within cavitation bubbles.

1.3 Physical and Chemical Effects of Acoustic Cavitation

When bubble collapse occurs, a number of physical and chemical events are generated. High intensity shock waves are generated when bubbles collapse symmetrically [51, 52]. The energy associated with shock waves are extremely high that could be used to increase mass transfer processes in liquids. Suslick and coworkers have shown [53] that shockwaves could drive particle-particle collision generating temperatures as high as 3500 K on the surface of the colliding particles. Another significant force that is generated during the asymmetric collapse of a cavitation bubble is microjet [54]. When a bubble is near a boundary, it experiences uneven acoustic force around it and undergoes asymmetric collapse with a liquid jet rushing towards the middle of the bubble hitting the surface/boundary with a speed of greater than 100 miles per second. Such high-speed jets possess enormous kinetic energy that could make pits or holes on the surface of a metal plate or particle. Other physical forces that are generated during acoustic cavitation are microstreaming, agitation, turbulence, etc. that could be used to enhance mass transfer effects in a medium [55]. Ultrasonic synthesis of polymer latex particles and core-shell biomaterials need strong shear forces and a small amount of radicals to initiate polymerisation or cross-linking reactions. 20 kHz horn system is found to be suitable for such applications [56–60].

The generation of very high temperatures on bubble collapse has already been mentioned. The heat generated can raise the temperature of the core of the bubbles to thousands of degrees for a short period (micro- to nanoseconds). Such extreme thermal conditions lead to light emission from the bubbles, referred to as sonoluminescence [61], which will not be discussed in this book. Another consequence of the high temperature conditions within the core of the cavitation bubble is the induction of a variety of chemical reactions [62–70]. In organic solvents, the heat generated has been used to synthesise amorphous metal nanoparticles (see Sect. 2.2). In aqueous solutions, water and other volatile compounds could diffuse into the bubble and pyrolysed/decomposed by the extreme temperatures of the bubble. If it is pure water saturated with argon, only H and OH radicals are predominantly generated and are referred to as primary radicals (Reaction 1).

$$ {\text{H}}_{ 2} {\text{O}} \to {\mathbf{H}} + {\mathbf{OH}} $$
(Reaction 1)

The primary radicals may undergo recombination reactions to for molecular products (Reactions 2–4).

$$ {\mathbf{H}} + {\mathbf{OH}} \to {\text{H}}_{ 2} {\text{O}} $$
(Reaction 2)
$$ {\mathbf{OH}} + {\mathbf{OH}} \to {\text{H}}_{ 2} {\text{O}}_{ 2} $$
(Reaction 3)
$$ {\mathbf{H}} + {\mathbf{H}} \to {\text{H}}_{ 2} $$
(Reaction 4)

In air-saturated water, a variety of radicals and molecular products are generated (Reactions 5–8)

$$ {\mathbf{H}} + {\text{O}}_{2} \to {\mathbf{HO}}_{{\mathbf{2}}} $$
(Reaction 5)
$$ {\mathbf{HO}}_{{\mathbf{2}}} + {\mathbf{HO}}_{{\mathbf{2}}} \to {\text{H}}_{ 2} {\text{O}}_{ 2} + {\text{O}}_{ 2} $$
(Reaction 6)
$$ {\text{O}}_{2} \to 2{\mathbf{O}} $$
(Reaction 7)
$$ {\text{O}}_{2} + {\mathbf{O}} \to {\text{O}}_{ 3} $$
(Reaction 8)

The reaction between N2 and O2 within cavitation bubbles has been shown to produce nitric acid, responsible for lowering the solution pH [71]. When an organic solute such as alcohol is dissolved in water, secondary reducing radicals are generated [72] (discussed in Sect. 2.1).

In simple terms, a variety of redox radicals are generated within the cavitation bubbles and hence each cavitation bubble could be compared to an electrochemical cell. For certain applications, only reducing radicals could be preferred: for example, for the reduction of metal ions to form metal nanoparticles (discussed in Sect. 2.2). In this case, the addition of a small amount of organic solute could convert all oxidising radicals (OH for example) to secondary reducing radicals. For reactions involving oxidation only, purging the solution with oxygen could convert the reducing radicals into oxidising radicals (Reaction 5).

The primary and secondary radicals could also be used in polymerisation and other chemical reactions [72–80]. Specific examples are discussed in Chap. 2. Prior to looking at such examples, it should be highlighted that the choice of right frequency is important to achieve optimal efficiency for every reaction. For this purpose, reactions/processes could be grouped into 3 categories: reactions that primarily depend upon the physical effects of ultrasound, reactions that primarily depend upon the chemical effects of ultrasound and reactions that depend upon both physical and chemical effects. For example, ultrasonic depolymerisation reactions, ultrasonic emulsification, ultrasonic extraction and ultrasonic cleaning primarily depend upon the physical forces (shear forces, microstreaming, microjetting, shock waves, agitation, etc.) generated during acoustic cavitation [81–85]. Ultrasonic synthesis of nanomaterials in aqueous solutions, sonochemical degradation of pollutants, etc., depend primarily on the amount of primary and secondary radicals generated during acoustic cavitation [86–90]. Ultrasonic synthesis of polymer latex particles and core-shell biomaterials depend upon both physical and chemical effects [91–95]. The mechanism behind such reactions/processes will be discussed later. However, it is worth noting that the physical forces generated at 20 kHz in a horn-type sonicator is significantly stronger. For this reason, applications such as extraction, cleaning and emulsification use a horn-type sonicator. The amount of radicals generated in such system is significantly lower compared to that generated at high frequencies where plate-type transducers are used.

At 20 kHz, most of the cavitation activity occurs at the tip of the horn and hence the number of active bubbles generated is relatively lower. Despite the amount of radicals generated per bubble is higher, the overall yield is lower. At 20 kHz, the heat generated within the bubble could be significantly higher compared to that generated at higher frequencies since the bubble size is larger. Equation 1.2 shows the relationship between resonance bubble size and frequency. Accordingly, the amount of radicals generated per bubble decreases with increasing frequency. On the other hand, the number of bubbles generated increases with an increase in frequency (for a given volume and power input) leading to an increase in the amount of primary and secondary radicals with an increase in frequency. The increase in number of active cavitation bubbles is due to an increase in the number of standing waves as schematically and photographically shown in Fig. 1.2.

Fig. 1.2
figure 2

Left Schematic representations of standing waves leading to an increase in the number of cavitation bubbles; Right photographic images of SL observed at 37 and 440 kHz

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Hence, for achieving redox reactions, higher frequencies are found useful. It should also be noted that the radical yield reaches a maximum level in the frequency range, 200–800 kHz beyond which the yield is found to decrease [96–100]. The amount of primary radicals generated during acoustic cavitation could be quantified using a relatively simple iodide oxidation process. OH radicals generated react among themselves to produce hydrogen peroxide (Reaction 2). In the absence of any additive (in pure water), the amount of H2O2 produced remain reasonably stable for a short period of time. However, in the presence of iodide ions, H2O2 could be used to oxidise iodide ions to molecular iodine, which is useful to quantify the amount of OH radicals generated [100].

Figure 1.3 shows that the amount of OH radicals generated is the highest at 355 kHz among the three frequencies investigated [100]. The decrease observed at the highest frequency is due to a relatively lower bubble temperature and a lower amount of water vapour that could evaporate into cavitation bubbles during the expansion phase (due to relatively less time available for rarefaction cycle) at very high frequencies. A detailed discussion on this is available elsewhere [101].

Fig. 1.3
figure 3

OH radical yield as a function of sonication time at 20, 355 and 1056 kHz Adapted from Ref. [100]

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In brief, for a given solution volume and acoustic power, a change in acoustic frequency results in an increase in the number of active bubbles and a decrease in the resonance size of the bubble. This would have two opposing effects. A decrease in bubble size means a decrease in collapse intensity and hence lower bubble temperature. This leads to a decrease in the amount of primary and secondary radicals generated per bubble. In the meantime, an increase in the number of bubbles (due to an increase in the number of standing waves) leads to an increase in the amount of radicals generated. It has been shown in many studies [96–100] that the sonochemical reaction yield peaks around 200–800 kHz beyond which a decline in the yield is observed.

Despite an increase in the number of bubbles, a decrease in bubble temperature and very short time available for volatile molecules to diffuse into the bubbles during the expansion cycle leading to the observed decrease in radical yield. This has been theoretically demonstrated in Fig. 1.4. It is known that solvent molecules adsorb on the surface of cavitation bubbles. Using the resonance radius of the bubbles at each frequency, the amount of water molecules in a monolayer on the surface of bubbles could be calculated [101]. Also, there is a finite time required for the evaporation process to occur. Using the number of molecules at the interface and the time required for evaporation and expansion cycle, it could be shown that there is enough time available for the evaporation of one monolayer of water at low frequencies, whereas not enough time is available at high frequencies for the evaporation of one monolayer to be completed.

Fig. 1.4
figure 4

Theoretical data shown mass of evaporated from bubble surface during a single expansion phase at various frequencies. It could be seen that the mass that could evaporate exceeds the amount present in a monolayer on the bubble surface at lower frequencies. At higher frequencies, the amount that could evaporate is less than a monolayer, which is due to very short expansion time available during bubble oscillations. Further details are available in Ref. [101]

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It could be seen in Fig. 1.4 that the transition occurs around 200 kHz and gap widens at higher frequencies. However, the decrease in radical yield per bubble is compensated by the increase in number of bubbles until about 800 kHz as observed in many studies.

In summary, this chapter provides a simple overview of the various physical and chemical effects generated during acoustic cavitation and how different experimental parameters could be manipulated to control such effects.