Experimental Study on Ultrasonic Cavitation Intensity Based on Fluorescence Analysis

Practical applications of ultrasonic cavitation have been found for ultrasonic cleaning, atomization, emulsification, crystallization and so on since 1927, when Loomis first observed the chemical effects of ultrasound. It is quite popular and essential to do ultrasonic cavitation studies for a detailed understating of sonochemistry. The quantitative characterization of ultrasonic cavitation intensity has always been a difficulty. Hydrophones have been normally used to measure sound pressure, which is used to calculate the ultrasonic cavitation intensity. Although various hydrophones, such as Polyvinylidene fluoride (PVDF) and fiber optic hydrophones, are available for ultrasonic field mapping, many hydrophones suffer from a lack of uniform response over a wide range of frequencies while still maintaining sensitivity, particularly below 200 kHz where power ultrasonic applications operate. Furthermore, any measurement device must be robust enough to withstand the vibration associated with high power ultrasound measurement. To attain increased levels of sensitivity, spatial resolution, and non-invasiveness, hydrophone devices are generally quite delicate and fragile, which can be easily damaged by power ultrasound. Therefore, it is necessary to conduct the measurements using non-invasive techniques.

Ultrasonic cavitation phenomenon refers to a high-intensity nonlinear dynamic process in which the cavitation nuclear undergoes various stages in the liquid medium, such as primary growth, expansion, shock, compression, and collapse, accompanied by secondary physicochemical phenomena such as strong shock wave, high-speed micro-jet, acoustically induced free radical, sonoluminescence, and acoustically induced heating [1, 2]. The main ways to induce cavitation include ultrasound, hydraulic, particle, laser, etc. Among them, ultrasonic-induced cavitation has become the main method because it is convenient to adjust the location and intensity of cavitation, and it has a stronger cavitation effect than hydraulic cavitation [3] and will lead to the generation of hydroxyl radical ·OH [4]. With the progress of modern science and technology, ultrasonic cavitation technology has been applied in many fields such as mechanical processing, biomedicine, surface cleaning, chemical engineering, and so on [5‐8], and its research is of great significance.

Scholars had done a series of studies on the effect and function of ultrasonic cavitation [9, 10]. In 2017, Ye et al. [11] established a three-dimensional fluid-structure coupling model to discuss the impact effect of micro-jet produced by cavitation collapse near the wall on a 1060 aluminum plate. In 2021, Chowdhury et al. [12] connected 6063 aluminum alloy and C26000 copper alloy through friction stir welding and ultrasound-assisted friction stir welding and analyzed the mechanical properties. They observed that when ultrasonic vibration of 10−14 kHz was added to the welding, the mechanical properties increased from 10% to 25%. This indicated that more scholars had been using the cavitation effect to make the processing effect better. In 2021, Liang et al. [13] deduced a new theoretical model to describe the translation, pulsation, and deformation of double bubbles in the ultrasonic field. Du et al. [14] used the numerical simulation method to predict the volume and pressure pulsation of the cavitation cavity, and the consistency between the results and the experimental data was about 90%. In 2020, Kalmár et al. [15] studied the chemical generation of bubbles excited by harmonics. Based on considering a total of 44 different chemical reactions and the evaporation, condensation, heat transfer, and diffusion of 9 different chemical substances, a mathematical model was established. Without considering the chemical kinetics, the chemical activity could be accurately predicted. Researchers had studied cavitation bubbles and chemical products through theoretical models or numerical simulations, but they made a lot of assumptions, which may lead to deviation from reality. In 2020, Ye et al. [5] used the ultrasonic cavitation effect to improve the surface properties of AZ31B magnesium alloy, and studied the effects of ultrasonic amplitude, treatment time, and distance from the sample on the Vickers hardness and grain size. The Vickers hardness was increased by 1.5−3 times and the grain size was refined to 2−10 μm. In addition, ultrasonic cavitation can produce residual compressive stress on the metal surface. Hitoshi Soyama et al. [16] measured that the surface residual stress after cavitation shot peening is about − 300 MPa by using two-dimensional X-ray diffraction method. After 100 times of cyclic fatigue tests with bending stress of 400 MPa, the residual stress is still about − 168 MPa. Gu et al. [17] studied the influence of ultrasonic cavitation on the surface strengthening of Q235 steel by combining simulation and experiment. The research showed that the surface had plastic deformation after cavitation shot peening, and the residual stress could reach − 126 MPa. Soyama et al. [18] introduced residual stress of about − 220 MPa into Ti6Al4V material by using cavitating micro abrasive jet, reducing surface roughness and improving fatigue life strength. These researches not only showed that ultrasonic cavitation would produce residual compressive stress on the metal surface, but also expounded the gain effect of ultrasonic cavitation on materials. The above showed that the study of ultrasonic cavitation intensity had very important practical significance, and the quantitative analysis and measurement of ultrasonic cavitation intensity had always been a difficulty. Some scholars directly measured the degree of cavitation reaction through high-speed photography and aluminum foil corrosion [19‐21], but they could not quantitatively describe cavitation.

In 2008, Hu et al. [22] studied the ultrasonic field by detecting hydroxyl radicals through the chemiluminescence method, and pointed out that the content of hydroxyl radicals increased with the increase of ultrasonic power. In 2012, Capocelli et al. [23] used the consumption of nitrophenol to estimate the production of hydroxyl radicals, and indicated that the content of hydroxyl radicals gradually increased and there was an extreme point between 40−1000 kHz. Ding et al. [24] quantified the fluorescence peak area in the emission wavelength range of 220−600 nm to characterize the generation of ·OH. In 2006, Sahni et al. [25] used dimethyl sulfoxide (DMSO) and disodium terephthalate (NaTA) as chemical fluorescence probes, set the excitation wavelength to 315 nm, collected the fluorescence spectrum of the sample in the range of 340 to 625 nm, and quantitatively determined the hydroxyl radical through the fluorescence peak area. The essence of fluorescence peak area is to select the appropriate excitation wavelength and calculate the fluorescence intensity integrally in a certain range. In 2015, Jimenez-Relinque et al. [26] used terephthalic acid as a fluorescent probe to quantitatively analyze the hydroxyl radical produced by several photocatalytic building materials. The standard TAOH solution had a linear relationship between fluorescence spectrometry intensity and concentration, which is independent of specific pollutants. Moreover, this method has the advantages of fast, simple, cheap, and has a wide application range. In 2010, Hasanzadeh et al. [27] proved that the fluorescence analysis method had the advantages of low cost, high repeatability, and strong real-time performance compared with the subharmonic spectral analysis method to characterize the intensity of cavitation.

In the engineering application of ultrasonic cavitation, the quantitative characterization of cavitation intensity is an important problem. Some scholars use polyvinylidene fluoride (PVDF) needle hydrophone [28, 29] to measure the pressure in the cavitation field and then characterize the cavitation intensity. However, the essence of this method is to measure the pressure at the point through the pressure sensor, which can only characterize the point cavitation intensity and cannot characterize the global cavitation intensity. Fluorescence analysis can characterize the overall cavitation intensity. In addition, it has the advantage of being able to describe steady-state and transient cavitation directly and quantitatively. Using TA as a fluorescent probe has been widely used in sonochemical research to detect ·OH [26, 30]. In a liquid medium, ultrasound causes cavitation collapse to release energy, causing instantaneous high temperature and high pressure (5000 K and 100 MPa) locally. In this case, the molecular bond in the medium to break and recombine alternately. The reactions caused by chemical bond breaks, such as the opening of water molecular bonds to form hydroxyl radicals [9, 22, 31], the combination of oxygen ions to generate oxygen, and the combination of hydrogen ions and oxygen ions to generate water, continue to occur [32, 33]. The increase in fluorescence intensity is caused by the strong fluorescence substance TAOH generated by ·OH and TA, as shown in Eq. (1). Capocelli et al. [23] gave the concentration of TAOH in cavitation, as shown in Eq. (2).where C_TAOH represents the concentration of TAOH in the bubble, V is the bubble volume, α_TAOH is the stoichiometric weight, and v' represents the reaction rate.

In addition, the reaction of ·OH with terephthalic acid is almost not affected by other cavitation products or active substances [32]. When detecting fluorescence, the excitation wavelength was selected as 315 nm and the emission wavelength was selected as 425 nm. The concentration of TAOH within the range of PH 6−11 could be characterized by fluorescence intensity, which had a linear relationship [26]. Therefore, fluorescence intensity and fluorescence peak area could be used to characterize the concentration of TAOH to reflect the production of ·OH, and then the ultrasonic cavitation intensity was characterized.

In conclusion, the ultrasonic cavitation effect has been applied in many fields, and the quantitative characterization of cavitation intensity is the research focus. Based on this, in this paper, we took TA as the fluorescence probe, used F-7000 to measure the fluorescence intensity and fluorescence peak area of the treated samples to characterize the cavitation intensity, and then analyzed the effects of ultrasonic power, irradiation time, and ultrasonic frequency, to provide a basis for the quantitative characterization of cavitation intensity in theory and experiment.

The principle of fluorescence spectrometry is that the non-fluorescent substance TA reacts with ·OH formed by the molecular bond fracture in the solution after ultrasonic treatment to form a strong fluorescent substance TAOH. TAOH receives external energy (i.e., exciting light in the experiment) and jumps from the ground state to the excited state. When it returns to the ground state, the energy is emitted as electromagnetic radiation (luminescence). The output of TAOH can be characterized by the change of fluorescence intensity, and the amount of ·OH can be determined to characterize the ultrasonic cavitation intensity. After ultrasonic treatment, the sample was left standing at room temperature for 1 h, and the fluorescence intensity and fluorescence peak area of the sample were measured by the Hitachi F-7000 fluorescence spectrophotometer shown in Figure 1. The amount of ·OH generated is proportional to the fluorescence intensity and fluorescence peak area, and the yield of ·OH in TA solution can be used to characterize ultrasonic cavitation intensity. The basic properties of fluorescent probe TA are shown in Table 1. The pH value of the sample was measured before and after the experiment, which fluctuated in the range of 10.6 to 10.8, reaching the technical indicators of using fluorescence analysis.

The ultrasonic cavitation experiment device is shown in Figure 2. It is composed of an ultrasonic generator, an ultrasonic transducer, an ultrasonic vibration device, a loading support, a working platform, a lifting mechanism, and a beaker. The alternating current is converted into a high-frequency current signal through the ultrasonic generator, matched with the transducer, and converted into mechanical vibration with the same vibration frequency as the tool head by the transducer. The preparation method for TA solution is as follows. The TA used in the experiment is almost insoluble in water but soluble in strong alkali. Accurately measure 25.0 mL NaOH solution (1.0 mol/L) to dissolve 0.25 g TA, and then add distilled water to prepare 2 L terephthalic acid solution. Three variables were selected in the experiment, namely ultrasonic power (200 W, 400 W, 600 W, and 800 W), irradiation time (4 min, 6 min, 8 min, 12 min, 16 min, 25 min, and 30 min), and ultrasonic frequency (20 kHz, 30 kHz, and 40 kHz). In this paper, the fluorescence intensity and fluorescence peak area of the treated samples were quantified, and the effects of the above three factors on them were analyzed respectively.

Fluorescence intensity and fluorescence peak area are important indexes to evaluate ultrasonic cavitation intensity, and directly proportional to it, which means that the larger the value, the stronger the ultrasonic cavitation intensity, and ultrasonic cavitation intensity is an important reference index to evaluate the effect of ultrasonic cavitation. The spectrum of TA solution under ultrasonic irradiation was measured by the Hitachi F-7000 fluorescence spectrophotometer. Before that, the instrument parameters were adjusted. The PMT voltage was 400 V, the slit width was 10 nm, and the scan speed was set to 2400 nm/min. When the fluorescence intensity was measured, the excitation wavelength and the emission wavelength were 315 nm and 425 nm, respectively. When the fluorescence peak area was measured, the wavelength scan mode was selected, the excitation wavelength was 315 nm, and the fluorescence peak area was quantified in the range of 365−600 nm. The fluorescence intensity and fluorescence peak area of each group experiment were measured three times, and their average value was taken as the final experimental results.

For spectral analysis, the TA solution samples after ultrasonic treatment were sub-packed with a 50 mL transparent screw glass reagent bottle. After 1 h at room temperature, a 2 mL disposable Pasteur pipette was used to transfer the appropriate solution to a four-side transparent glass dish as a fluorescence observation target, and the fluorescence spectrophotometer was used for sample detection and analysis.

In this paper, the ultrasonic cavitation experiment platform was equipped, and ultrasonic treatment experiments were carried out in TA solution. The effects of ultrasonic power, irradiation time, and ultrasonic frequency on ultrasonic cavitation intensity were studied by measuring fluorescence intensity and fluorescence peak area with the F-7000 fluorescence spectrophotometer. The fluorescence intensity and fluorescence peak area of the samples increased by 2−20 times after ultrasonic treatment. The research conclusions were as follows: