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Concept: Sonochemistry

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Megasonic cleaning as applied in leading edge semiconductor device manufacturing strongly relies on the phenomenon of acoustic cavitation. As the occurrence of acoustic cavitation is incorporating a multitude of interdependent effects, the amount of cavitation activity in the cleaning liquid strongly depends on the sonication conditions. It is shown that cavitation activity as measured by means of ultraharmonic cavitation noise can be significantly enhanced when pulsed sonication is applied to a gas supersaturated liquid under traveling wave conditions. It is demonstrated that this enhancement coincides with a dramatic increase in particle removal and is therefore of great interest for megasonic cleaning applications. It is demonstrated that the optimal pulse parameters are determined by the dissolution time of the active bubbles, whereas the amount of cavitation activity depends on the ratio between pulse-off and pulse-on time as well as the applied acoustic power. The optimal pulse-off time is independent of the corresponding pulse-on time but increases significantly with increasing gas concentration. We show that on the other hand, supersaturation is needed to enable acoustic cavitation at aforementioned conditions, but has to be kept below values, for which active bubbles cannot dissolve anymore and are therefore lost during subsequent pulses. For the applicable range of gas contents between 100% and 130% saturation, the optimal pulse-off time reaches values between 150 and 340 ms, respectively. Full particle removal of 78 nm-diameter silica particles at a power density of 0.67 W/cm(2) is obtained for the optimal pulse-off times. The optimal pulse-off time values are derived from the dissolution time of bubbles with a radius of 3.3 μm and verified experimentally. The bubble radius used in the calculations corresponds to the linear resonance size in a 928 kHz sound field, which demonstrates that the recycling of active bubbles is the main enhancement mechanism. The optimal choice of the pulsing conditions however is constrained by the trade-off between the effective sonication time and the desire to have a sufficient amount of active bubbles at lower powers, which might be necessary if very delicate structures have to be cleaned.

Concepts: Fundamental physics concepts, Fluid dynamics, Pulse, Wave, Ultrasound, Sound, Superheating, Sonochemistry

28

Various industrial processes such as sonochemical processing and ultrasonic cleaning strongly rely on the phenomenon of acoustic cavitation. As the occurrence of acoustic cavitation is incorporating a multitude of interdependent effects, the amount of cavitation activity in a vessel is strongly depending on the ultrasonic process conditions. It is therefore crucial to quantify cavitation activity as a function of the process parameters. At 1 MHz, the active cavitation bubbles are so small that it is becoming difficult to observe them in a direct way. Hence, another metrology based on secondary effects of acoustic cavitation is more suitable to study cavitation activity. In this paper we present a detailed analysis of acoustic cavitation phenomena at 1 MHz ultrasound by means of time-resolved measurements of sonoluminescence, cavitation noise, and synchronized high-speed stroboscopic Schlieren imaging. It is shown that a correlation exists between sonoluminescence, and the ultraharmonic and broadband signals extracted from the cavitation noise spectra. The signals can be utilized to characterize different regimes of cavitation activity at different acoustic power densities. When cavitation activity sets on, the aforementioned signals correlate to fluctuations in the Schlieren contrast as well as the number of nucleated bubbles extracted from the Schlieren Images. This additionally proves that signals extracted from cavitation noise spectra truly represent a measure for cavitation activity. The cyclic behavior of cavitation activity is investigated and related to the evolution of the bubble populations in the ultrasonic tank. It is shown that cavitation activity is strongly linked to the occurrence of fast-moving bubbles. The origin of this “bubble streamers” is investigated and their role in the initialization and propagation of cavitation activity throughout the sonicated liquid is discussed. Finally, it is shown that bubble activity can be stabilized and enhanced by the use of pulsed ultrasound by conserving and recycling active bubbles between subsequent pulsing cycles.

Concepts: Acoustics, Ultrasound, Hertz, Sound, Sonar, Sonoluminescence, Sonochemistry, Cavitation

28

Acoustic cavitation plays an important role in sonochemical processes and the rate of sonochemical reaction is influenced by sonication parameters. There are several methods to evaluate cavitation activity such as chemical dosimetry. In this study, to comparison between iodide dosimetry and terephthalic acid dosimetry, efficacy of sonication parameters in reactive radical production has been considered by iodide and terephthalic acid dosimetries. For this purpose, efficacy of different exposure parameters on cavitations production by 1 MHz ultrasound has been studied. The absorbance of KI dosimeter was measured by spectrophotometer and the fluorescence of terephthalic acid dosimeter was measured using spectrofluorometer after sonication. The result of experiments related to sonication time and intensity showed that with increasing time of sonication or intensity, the absorbance is increased. It has been shown that the absorbance for continuous mode is remarkably higher than for pulsing mode (p-value < 0.05). Also results show that with increasing the duty cycles of pulsed field, the inertial cavitation activity is increased. With compensation of sonication time or intensity in different duty cycles, no significant absorbance difference were observed unless 20% duty cycle. A significant correlation between the absorbance and fluorescence intensities (count) at different intensity (R = 0.971), different sonication time (R = 0.999) and different duty cycle (R = 0.967) were observed (p-value < 0.05). It is concluded that the sonication parameters having important influences on reactive radical production. These results suggest that there is a correlation between iodide dosimetry and terephthalic acid dosimetry to examine the acoustic cavitation activity in ultrasound field.

Concepts: Spectroscopy, Light, Ultrasound, Hertz, Sound, Sonochemistry, Cavitation, Sonoporation

28

High intensity ultrasound can be used for the production of novel materials and provides an unusual route to known materials without bulk high temperatures, high pressures, or long reaction times. Several phenomena are responsible for sonochemistry and specifically the production or modification of nanomaterials during ultrasonic irradiation. The most notable effects are consequences of acoustic cavitation (the formation, growth, and implosive collapse of bubbles), and can be categorized as primary sonochemistry (gas-phase chemistry occurring inside collapsing bubbles), secondary sonochemistry (solution-phase chemistry occurring outside the bubbles), and physical modifications (caused by high-speed jets or shock waves derived from bubble collapse). This tutorial review provides examples of how the chemical and physical effects of high intensity ultrasound can be exploited for the preparation or modification of a wide range of nanostructured materials.

Concepts: Chemical reaction, Fluid dynamics, Chemistry, Acoustics, Ultrasound, Sonoluminescence, Sonochemistry, Cavitation

27

Experimental measurements and numerical analyses were performed to investigate the IC thresholds of two commercialized UCAs, albumin-shelled KangRun® and lipid-shelled SonoVue®. The IC thresholds of these two UCAs were measured at varied acoustic pulse lengths and bubble concentrations, according to the IC dose quantifications based on passive cavitation detection (PCD). Then, the shell properties of UCAs were estimated by fitting the measured acoustic attenuation data. Finally, the influences of acoustic pulse length and UCA shell properties on the microbubble nonlinear behaviors were discussed based on numerical simulations, which would give us better understanding of the dependence of microbubble IC threshold on the sonication condition and physical structure properties of the coating shells. The experimental results show that: (1) the IC threshold of UCAs is dependent on the acoustic driving conditions, the shell properties of UCAs and the bubble concentration; (2) for both the lipid- and albumin-shelled UCAs, the IC threshold generally decreases with the increasing UCA volume concentration; (3) IC threshold is observed higher for short-pulse excitation, then its value decreases as the acoustic pulse length increases from 5 to 20 cycles and finally tends to reach a steady state for even longer pulsed exposures.

Concepts: Concentration, Thermodynamics, Units of measurement, Numerical analysis, Height, Length, Sonochemistry, 1992 albums

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Through the introduction of multi-frequency sonication in High Intensity Focused Ultrasound (HIFU), enhancement of efficiency has been noted in several applications including thrombolysis, tissue ablation, sonochemistry, and sonoluminescence. One key experimental observation is that multi-frequency ultrasound can help lower the inertial cavitation threshold, thereby improving the power efficiency. However, this has not been well corroborated by the theory. In this paper, a numerical investigation on the inertial cavitation threshold of microbubbles (MBs) under multi-frequency ultrasound irradiation is conducted. The relationships between the cavitation threshold and MB size at various frequencies and in different media are investigated. The results of single-, dual and triple frequency sonication show reduced inertial cavitation thresholds by introducing additional frequencies which is consistent with previous experimental work. In addition, no significant difference is observed between dual frequency sonication with various frequency differences. This study, not only reaffirms the benefit of using multi-frequency ultrasound for various applications, but also provides a possible route for optimizing ultrasound excitations for initiating inertial cavitation.

Concepts: Acoustics, Ultrasound, Hertz, Sound, Sonoluminescence, Sonochemistry, Cavitation, Sonication

0

The results indicated that the ultrasonic sonochemistry which brings into play the acoustic cavitation phenomenon is more powerful and feasible in synthesizing the mixed oxides in contrast to the conventional solid-state approaches. The obtained results demonstrated that the sonochemical approach is able to obtain highly pure powder product at a much lower processing temperature of about 323K (50°C) in contrast to 1173K (900°C) which is essential for the synthesis by the mechanochemical approach. Sonochemical synthesis benefits from homogenous ordering the reactant ions (which have been dissolved in the solution mixture) into perovskite structure using ultrasonication. This indicates that the acoustic cavitation phenomenon is much more powerful and cost-effective than high energy ball milling in synthesizing nanopowders of the mixed oxide materials. Moreover, the sonochemical processing method is able to prepare the final powder products in a much shorter time by a one-step synthesis approach without the need for the successive calcination in contrast to the solid-state approach.

Concepts: Chemistry, Oxide, Ultrasound, Oxides, Sonoluminescence, Sonochemistry, Cavitation, Ultrasonics

0

In the accompanying article (Part I), a method is described to determine acoustic cavitation probabilities in tissue-mimicking materials (TMMs) using a high-intensity focused ultrasound (HIFU) transducer for both inducing and detecting the acoustic cavitation events, and its suitability for different sonication modes like continuous wave, single pulses (with pulse lengths from microseconds to milliseconds) and repeated burst signals is discussed. In Part II, the use of the method for a systematic study of the dependence of the acoustic cavitation thresholds in 3% (by weight) agar phantoms on the temporal sonication parameters is discussed. The values obtained at a frequency of 1.06 MHz, ranging from (0.58 ± 0.12) MPa for a 3-s continuous wave mode sonication to (5.2 ± 1.0) MPa for single shots with a length of 10 wave cycles, are discussed and interpreted on the basis of literature values and their self-consistency.

Concepts: Acoustics, Ultrasound, Frequency, Hertz, Sound, Sonar, Sonochemistry, Sonoporation

0

A method to determine acoustic cavitation probabilities in tissue-mimicking materials (TMMs) is described that uses a high-intensity focused ultrasound (HIFU) transducer for both inducing and detecting the acoustic cavitation events. The method was evaluated by studying acoustic cavitation probabilities in agar-based TMMs with and without scatterers and for different sonication modes like continuous wave, single pulses (microseconds to milliseconds) and repeated burst signals. Acoustic cavitation thresholds (defined here as the peak rarefactional in situ pressure at which the acoustic cavitation probability reaches 50%) at a frequency of 1.06 MHz were observed between 1.1 MPa (for 1 s of continuous wave sonication) and 4.6 MPa (for 1 s of a repeated burst signal with 25-cycle burst length and 10-ms burst period) in a 3% (by weight) agar phantom without scatterers. The method and its evaluation are described, and general terminology useful for standardizing the description of insonation conditions and comparing results is provided. In the accompanying second part, the presented method is used to systematically study the acoustic cavitation thresholds in the same material for a range of sonication modes.

Concepts: Wave, Acoustics, Ultrasound, Frequency, Hertz, Sound, Sonochemistry, Sonoporation

0

In this paper we report our most recent attempts to tackle a notorious problem across several scientific activities from the ultrasonics sonochemical perspective: reproducibility of results. We provide experimental results carried out in three different laboratories, using the same ingredients: ultrasound and a novel cavitation reactor bag. The main difference between the experiments is that they are aimed at different applications, KI liberation and MB degradation; and exfoliation of two nanomaterials: graphene and molybdenum disulfide. Iodine liberation rates and methylene blue degradation were higher for the cases where a cavitation intensification bag was used. Similarly, improved dispersion and more polydisperse exfoliated layers of nanomaterials were observed in the intensified bags compared to plain ones. The reproducibility of these new experiments is compared to previous experimental results under similar conditions. Our main conclusion is that despite knowing and understanding most physicochemical phenomena related to the origins and effects of cavitation, there is still a long path towards reproducibility, both in one laboratory, and compared across different laboratories. As emphasized in the sonochemical literature, the latter clearly illustrates the complexity of cavitation as nonlinear phenomenon, whose quantitative estimation represents a challenging aspect. We also provide a list of procedural steps that can help improving reproducibility and scale-up efforts.

Concepts: Scientific method, Chemistry, Experiment, Laboratory, Sonochemistry