Theoretical study of single-bubble sonochemistry

Numerical simulations of bubble oscillations in liquid water irradiated by an ultrasonic wave are performed under the experimental condition for single-bubble sonochemistry reported by Didenko and Suslick [Nature (London) 418, 394 (2002)]. The calculated number of OH radicals dissolving into the surrounding liquid from the interior of the bubble agrees sufficiently with the experimental data. OH radicals created inside a bubble at the end of the bubble collapse gradually dissolve into the surrounding liquid during the contraction phase of an ultrasonic wave although about 30% of the total amount of OH radicals that dissolve into the liquid in one acoustic cycle dissolve in 0.1 micros at around the end of the collapse. The calculated results have indicated that the oxidant produced by a bubble is not only OH radical but also O atom and H2O2. It is suggested that an appreciable amount of O atom is produced by bubbles inside a standing-wave-type sonochemical reactor filled with water in which oxygen is dissolved as in the case of air.     

Correlation between acoustic cavitation noise and yield enhancement of sonochemical reaction by particle addition

The mechanism of the effect of particle addition on sonochemical reaction is studied through the measurements of frequency spectrum of sound intensity for evaluating the cavitation noise and the absorbance for the liberation of iodine from an aqueous solution of KI as an index of oxidation reaction by ultrasonic irradiation in the presence or absence of alumina particles. As it is expected that both the acoustic noise and a rise in temperature in the liquid irradiated by intense ultrasound will increase with the number of collapsing bubbles, these are supposed to be the best tools for evaluating the relative number of bubbles. In the present investigation, it has been shown that the addition of particles with appropriate amount and size results in an increase in the absorbance when both the acoustic noise and the rise in the liquid temperature due to cavitation bubbles also increase. This suggests that the enhancement in the yield of sonochemical reaction by appropriate particle addition comes from an increase in the number of cavitation bubbles. The existence of particle in liquid provides a nucleation site for cavitation bubble due to its surface roughness, leading to the decrease in the cavitation threshold responsible for the increase in the number of bubbles when the liquid is irradiated by ultrasound. Thus, from the present investigation, it is clarified that the particle addition has a potential to enhance the yield in the sonochemical reaction.     

两种不同类型的声场与声化学产额的关系

采用声源频率为1.8 MHz的连续声波在声强0～5 W•cm－2间,研究了连续混响声场中声化学产领(以溶液的电导率改变、溶液pH值改变和KI溶液的I2析出量以及空化水中的水合负电子()表征)与声强和超声辐照时间之间的关系.理论推论与实验结果均表明,连续声场中声化学产额与声强、超声辐照时间之间呈线性关系.并证明了声场中声化学反应动力学方程具有简单线性关系,与Henglein等人对脉冲声场中声化学产额与声场参数之间呈非线性关系的研究结果比较,证明对实际工业化运行的声化学反应器,采用连续声波更利于目标反应生成物的控制。     

Cavitation mechanism in cyanobacterial growth inhibition by ultrasonic irradiation

To prevent cyanobacterial bloom in eutrophic water by ultrasonic method, ultrasonic irradiations with different parameters were tested to inhibit Spirulina platensis from growth. The experimental result based on cyanobacterial growth, chlorophyll a and photosynthetic activity showed that, the ultrasonic irradiation inhibited cyanobacterial proliferation effectively, furthermore the inhibition effectiveness increased in the order: 200 kHz>1.7 MHz>20 kHz and became saturated with the increased power. The inhibition mechanism can be mainly attributed to the mechanical damage to the cell structures caused by ultrasonic cavitation, which was confirmed by light microscopy and differential interference microscopy. The optimal frequency of 200 kHz in cavition and sonochemistry was also most effective in cyanobacterial growth inhibition. The higher frequency of 1.7 MHz is weaker than 20 kHz in cavitation, but has more effective inhibition because it is nearer to the resonance frequency of gas vesicle. The inhibition saturation with ultrasonic power was due to the ultrasonic attenuation induced by the acoustic shielding of bubbles enclosing the radiate surface of transducer.     

First results from coupled acousto-ultrasonics and electrochemical noise techniques applied to gas evolving electrodes

This paper presents the first results obtained from the coupling of acoustic emission, ultrasonic testing and electrochemical noise measurements for the real-time monitoring of metal–electrolyte interfacial processes of stochastic nature. In this first paper, this combined approach is applied to gas-evolving electrodes (hydrogen and oxygen) to verify the ability of acoustic–ultrasonic measurements to complement the purely electrochemical information brought by conventional potential and current electrochemical noise measurements. Results have been treated both in the time and frequency domains indicating that acoustic–ultrasonic measurements actually sense the dynamic bubble evolution behaviour. It is hence shown that this new approach is a promising efficient tool that can be potentially extended to the study of other complex multiphase phenomena.     

Picosecond acoustic transmission measurements. II. Probing high frequency structural relaxation in supercooled glycerol

The high frequency acoustic response of liquids is measured in a manner directly analogous to conventional ultrasonic measurements. Two thin metal films act as acoustic transducer and receiver for a liquid layer between them. Pulsed optical excitation generates high bandwidth wave packets in the transducer, and these are detected in the receiver after damping and dispersion by the liquid. This initial measurement probes structural relaxation dynamics of glycerol in the frequency range 2-20 GHz, for temperatures between 235 and 291 K. The analysis presented here demonstrates the presence of excess relaxation, not accounted for by either the alpha or beta relaxation of the mode-coupling theory, and suggests the presence of constant loss in the susceptibility spectrum of supercooled glycerol.     

Monitoring of a heterogeneous reaction by acoustic emission

The feasibility of monitoring the reaction of itaconic acid and 1-butanol by non-invasive acoustic emission measurements has been assessed. A piezoelectric transducer with a resonant mode at 90 kHz was attached to the external wall of a 1 L jacketed glass reactor. Acoustic emission from the oil jacket, stirrer and toluene was insignificant in comparison to that produced by the itaconic acid particles, which was transmitted through the glass walls and heating oil to the transducer. The transducer responded to acoustic emission from itaconic acid up to approximately 300 kHz, with the region around 90 kHz having the highest sensitivity. The effect of particle concentration and size on the acoustic emission generated has also been investigated, with higher concentrations and larger particles giving the greater signals. The detection limit for itaconic acid particles was 14 g dm(-3) of toluene. The effect of 1-butanol concentration and temperature on the progression of reactions was monitored using acoustic emission. It was possible to detect differences in the rate and extent of the reaction under different conditions, and also to identify when a combination of the concentration and/or size of itaconic acid particles had reached a steady state. However, it was not possible to differentiate between changes in particle size and concentration using the resonant transducer.     

Thermal and sonochemical studies on the Diels-Alder cycloadditions of masked o-benzoquinones with furans: new insights into the reaction mechanism

What does a Diels-Alder cycloaddition look like? This question is here addressed in the case of the increasingly significant cycloadditions of masked o-benzoquinones (MOBs), which serve as versatile dienes for the construction of complex and functionalized structures. So first, what the mechanism is not: It is not (by and large) a classical, concerted [4 + 2] cycloaddition. Experimental evidence is now supported by sonochemical studies, which were instrumental in elucidating the pathway. Reactions with furans are accelerated and improved under sonication, even when conducted at -10 degrees C. Variation of the acoustic energy, temperature, and solvent composition allows us to optimize the yields and provide insights into the mechanism. Ultrasound does not cause sonochemical switching, as an alternative radical pathway should be ruled out. Results are consistent with a polar mechanism as claimed recently in a theoretical study. Moreover, this also does justice to a series of seminal papers, largely ignored, that gave a clue to the crucial issue of furan regioselectivity based on a nucleophilic addition. Most effects caused by ultrasonic agitation are of mechanochemical nature and suggest the existence of a perfectly stirred reactor with enhanced mass transfer.     

Acoustic wave detection of chemical species electrokinetically transported within a capillary tube

For the first time, we report the acoustic wave detection of chemical species being transported in a capillary tube to a region where acoustic coupling occurs. The measured parameter was a change in phase, which was originally only attributed to a change in solution density as the analyte passed by the detection region. Accordingly, we report the detection of change in phase as various chemical species (e.g. Cy5 dye, Cy5-derivatized glycine and underivatized glycine) were introduced into and migrated along a capillary tube through electrokinetic processes. To improve detection sensitivity, we modified various experimental parameters, such as run buffer concentration, capillary wall thickness and transducer frequency. Although acoustic wave detection was feasible, the peak width and detection limit were inadequate as compared to conventional detection methods for HPLC or CE. Nevertheless, the effects of various physical and chemical relaxation processes on acoustic wave absorption were discussed, and this has shed some light on explaining some observations, which cannot be explained by density differences alone. Accordingly, the acoustic wave method is suggested to investigate these processes, as studied in ultrasonic relaxation spectroscopy, in a flow system.     

纳米金红石型TiO2催化超声降解亚甲基蓝溶液

水溶性染料是印染工业中污染治理的主要对象,由于它们大多含有苯环,目前所使用的化学和生物等处理方法效果均不佳。而以半导体为催化剂的光催化处理方法对单一染料和实际印染废水处理效果较好。本文采用高温处理过的纳米TiO2作催化剂对亚甲基蓝溶液进行降解,大大缩短了降解时间,提高了降解率,60min降解率即可达66．8％,120min降解率则可达80％以上。而且发现利用超声波来替代紫外光对亚甲基蓝进行降解时,     

Factors affecting broadband acoustic emission measurements of a heterogeneous reaction

Broadband acoustic emission signals were obtained by attaching a piezoelectric transducer, sensitive up to 750 kHz, to the external wall of a 1 L jacketed glass reactor. Measurements were acquired of itaconic acid particles mixing in toluene; the total area of the acoustic emission signal from 55-500 kHz increased when the particle concentration, particle size or stir rate were increased. Signals at frequencies above 200 kHz were less sensitive to changes in particle size than those at lower frequencies. From calculation of the area of the signal in the range 55-200 kHz as a percentage of the signal area over the range 55-500 kHz, for mixtures of different size ranges of itaconic acid, it was possible to obtain an estimate of the mean particle size of a mixture. The heterogeneous esterification reaction of itaconic acid and 1-butanol was monitored non-invasively. A decrease in the overall acoustic signal area between 60 and 500 kHz was observed as the reaction progressed. Particle size and concentration information were contained in the amplitude of the acoustic emission signal, while the emission frequency yielded information on changes in the mean particle size.     

Fabrication of Cd(OH)2 nanorings by ultrasonic chiselling on Cd(OH)2 nanoplates

Cd(OH)2 nanorings were successfully fabricated from Cd(OH)2 nanoplates by ultrasonic chiselling, a unique effect of the sonochemical process that has never been reported before.     

Acoustic emission from cavitating solutions: implications for the mechanisms of sonochemical reactions

The acoustic emission from collapsing cavitation bubbles generated using ultrasound of 20 kHz and 515 kHz frequencies in water has been measured and correlated with sonoluminescence and hydroxyl radical production to yield further information on the frequency dependence of sonochemical reactions. A reasonable correlation was found, and the results suggest differences in the predominant types of cavitation observed under laboratory conditions.     

Effect of ultrasound frequency on pulsed sonolytic degradation of octylbenzene sulfonic acid

It has been shown that pulsed ultrasound can influence the amount of surfactant that can adsorb to and decompose at the surface of cavitation bubbles. However, the effect of ultrasound frequency on this process has not been considered. The current study investigates the effect of ultrasound frequency on the pulsed sonolytic degradation of octyl benzenesulfonate (OBS). Furthermore, the effect of pulsing and ultrasound frequency on the rate of *OH radical formation was determined. OBS degradation rates were compared to the rates of *OH radical formation. In this way, conclusions were made regarding the relative importance of accumulation of OBS at cavitation bubble surfaces versus sonochemical activity to the sonochemical decomposition of OBS under different conditions of sonolysis. Comparisons of the data in this way indicate that sonolytic degradation of OBS depends on both the sonochemical activity (i.e., *OH yield) and the accumulation of OBS on cavitation bubble surfaces. However, under a certain set of pulsing and ultrasound frequency exposure conditions, enhanced accumulation of OBS at the gas/solution interface of cavitation bubbles is the sole mechanism of enhanced degradation due to pulsing. On the basis of this finding, conclusions on how pulsing at various ultrasound frequencies affects cavitation bubbles were made.     

Sonochemical Electrodeposition of Copper Coatings

Potentiodynamic polarization method was used to study kinetic specific features of the electrodeposition of copper under a sonochemical treatment of a sulfuric acid electrolyte. The working current densities, scattering power of the electrolyte, the structure, microhardness, and luster of the formed coatings were determined depending ultrasonic field power and the presence of a thiourea additive in the electrolyte. It was found that it is advisable to perform the sonochemical electrodeposition of copper from a sulfuric acid electrolyte with addition of 0.003 g dm^–3 of thiourea at a current density of 2 A dm^–2 and ultrasonic vibration power of 8 W dm^–3.     

Preparation of nanosized TiO2 supported on activated alumina by a sonochemical method: observation of an increased photocatalytic decolourisation efficiency

Nanosized TiO₂ supported on activated alumina was prepared using a sonochemical method. To prepare this supported material, direct immersion horn and cup horn ultrasonic systems were used. The efficiency of the sonochemical coating process was determined by UV photodecolourisation of the aqueous methyl orange solution. It was found that TiO₂ supported on activated alumina prepared by the sonochemical method gave the maximum decolourisation efficiency. In that method itself, direct immersion horn gave a higher photodecolourisation activity as compared to the cup horn. The obtained results suggest that the direct immersion horn results in uniform, as well as a maximum number of TiO₂ particles to be adsorbed on an activated alumina surface.     

The range of ambient radius for an active bubble in sonoluminescence and sonochemical reactions

Numerical simulations of nonequilibrium chemical reactions inside an air bubble in liquid water irradiated by ultrasound have been performed for various ambient bubble radii. The intensity of sonoluminescence (SL) has also been calculated taking into account electron-atom bremsstrahlung, radiative attachment of electrons to neutral molecules, radiative recombination of electrons and ions, chemiluminescence of OH, molecular emission from nitrogen, etc. The lower bound of ambient radius for an active bubble in SL and sonochemical reactions nearly coincides with the Blake threshold for transient cavitation. The upper bound is in the same order of magnitude as that of the linear resonance radius. In actual experiments, however, the distribution of ambient radius for active bubbles may be narrow at around the threshold ambient radius for the shape instability. The threshold peak temperature inside an air bubble for nitrogen burning is higher than that for oxidant formation. The threshold peak temperatures depend on ultrasonic frequency and acoustic amplitude because chemical reactions inside a bubble are in nonequilibrium. The dominant emission mechanism in SL is electron-atom bremsstrahlung except at a lower bubble temperature than 2000 K, for which molecular emissions may be dominant.     

磁性蛋白质微胶囊的声化学法构筑

通过声化学法制备了具有生物相容性的磁性蛋白质微胶囊, 利用高强度超声波辐照含有油酸改性磁性的Fe₃O₄纳米粒子的油相与蛋白质水溶液的两相界面, 只需几分钟即可得到磁性蛋白质微胶囊. 这种制备蛋白质微胶囊的方法快速简便, 高效环保, 可将分散于油相的疏水性药物直接装载, 不破坏药物. 在药物靶向传输等领域具有应用性.     

Acoustofluidics 4: Piezoelectricity and application in the excitation of acoustic fields for ultrasonic particle manipulation

Piezoelectric materials are widely used in the excitation of MHz frequency vibrations in devices for ultrasonic manipulation. An applied electrical voltage is transformed into mechanical stress, strain and displacement. Piezoelectric elements can be used in either a resonant or non-resonant manner. Depending on the desired motion the piezoelectric longitudinal, transverse or shear effects are exploited. Because of the coupling between electrical and mechanical quantities in the constitutive law the modelling of devices turns out to be quite complex. In this paper, the general equations that need to be used are delineated. For a one-dimensional actuator the underlying physics is described, including the consequences resulting for the characterization of devices. For a practical setup used in ultrasonic manipulation, finite element models are used to model the complete system, including piezoelectric excitation, solid motion and acoustic field. It is shown, how proper tailoring of transducer and electrodes allows selective excitation of desired modes.     

Ultrasonic agitation in microchannels

This paper describes an acoustic method for inducing rotating vortex flows in microchannels. An ultrasonic crystal is used to create an acoustic standing wave field in the channel and thus induce a Rayleigh flow transverse to the laminar flow in the channel. Mixing in microchannels is strictly diffusion-limited because of the laminar flow, a transverse flow will greatly enhance mixing of the reactants. This is especially evident in chemical microsystems in which the chemical reaction is performed on a solid phase and only one reactant is actually diffusing. The method has been evaluated on two different systems, a mixing channel with two parallel flows and a porous silicon micro enzyme reactor for protein digestion. In both systems a significant increase of the mixing ratio is detected in a narrow band of frequency for the actuating ultrasound.