The interaction of a laser-generated cavity with a solid boundary

In this paper new observations of a laser-generated cavitation bubble interacting with an inertial boundary are presented. Employing schlieren photography techniques and a thin film transducer placed on the surface of the boundary, the pressure stresses induced in the solid boundary and the surrounding fluid by collapsing bubbles, created very close to the solid surface, are experimentally measured. Liquid jet development, shock wave emission, and "splash" phenomena are identified. For different creation sites close to the boundary, the relevance of each of these phenomena with respect to potentially damaging pressure stresses in the boundary is speculated on.     

Designing and characterizing a multi-stepped ultrasonic horn for enhanced sonochemical performance

The commonly used ultrasonic horn generates localized cavitation below its converging tip resulting in a dense bubble cloud near the tip and limiting diffusion of reactive components into the bubble cloud or reactive radicals out of the bubble cloud. To improve contact between reactive components, a novel ultrasonic horn design was developed based on the principles of the dynamic wave equation. The horn, driven at 20 kHz, has a multi-stepped design with a cone-shaped tip increasing the energy-emitting surface areas and creating multiple reactive zones. Through different physical and chemical experiments, performance of the horn was compared to a typical horn driven at 20 kHz. Hydrophone measurements showed high acoustic pressure areas around the horn neck and tip. Sonochemiluminescence experiments verified multiple cavitation zones consistent with hydrophone readings. Calorimetry and dosimetry results demonstrated a higher energy efficiency (31.3%) and a larger hydroxyl radical formation rate constant (0.36 μM min⁻¹) compared to typical horns. In addition, the new horn degraded naphthalene faster than the typical horn tested. The characterization results demonstrate that the multi-stepped horn configuration has the potential to improve the performance of ultrasound as an advanced oxidation technology by increasing the cavitation zone in the solution.     

Effect of polymer surface activity on cavitation nuclei stability against dissolution

The persistence of acoustic cavitation in a pulsed wave ultrasound regime depends upon the ability of cavitation nuclei, i.e., bubbles, to survive the off time between pulses. Due to the dependence of bubble dissolution on surface tension, surface-active agents may affect the stability of bubbles against dissolution. In this study, measurements of bubble dissolution rates in solutions of the surface-active polymer poly(propyl acrylic acid) (PPAA) were conducted to test this premise. The surface activity of PPAA varies with solution pH and concentration of dissolved polymer molecules. The surface tension of PPAA solutions (55-72 dynes/cm) that associated with the polymer surface activity was measured using the Wilhelmy plate technique. Samples of these polymer solutions then were exposed to 1.1 MHz high intensity focused ultrasound, and the dissolution of bubbles created by inertial cavitation was monitored using an active cavitation detection scheme. Analysis of the pulse echo data demonstrated that bubble dissolution time was inversely proportional to the surface tension of the solution. Finally, comparison of the experimental results with dissolution times computed from the Epstein-Plesset equation suggests that the radii of residual bubbles from inertial cavitation increase as the surface tension decreases.     

On the physical origin of conical bubble structure under an ultrasonic horn

The cavitation field generated by an ultrasonic horn at low frequency and high power is known to self-organize into a conical bubble structure. The physical mechanism at the origin of this bubble structure is investigated using numerical simulations and acoustic pressure measurements. The thin bubbly layer lying at horn surface is shown to act as a nonlinear thickness resonator that amplifies acoustic pressure and distorts acoustic waveform. This mechanism explains the self-stabilization of the conical bubble structure as well as the generation of shock wave and the focusing at very short distance.     

Ultrasonic polymerization of CuO@PNIPAM and its temperature tuning glucose sensing behavior

The extraordinary high pressure and temperature produced during cavitation is crucial for ultrasonic sonochemistry. However, the cavitation effect is usually confined to a small zone nearby the ultrasonic horn, outside of which ultrasound produces much less effects on chemical reaction. In present work, in order to expand the range of effective zone and intensify the cavitation effect, N₂ aeration was introduced to an ultrasonic polymerization process of CuO@PNIPAM in aqueous solution. By increasing the number of bubble nucleus gathered on the CuO surface and lowering the surface tension of the aqueous solution, the cavitation effect is intensified on the CuO surface within the whole reaction vessel, which benefits the covalently bonding between PNIPAM and CuO to a large degree and results in the formation of CuO@PNIPAM hybrid composite with excellent interfacial bonding. It is promising that the hybrid composite can be applied as temperature responsive glucose sensing platform with ON and OFF states due to the wettability change of PNIPAM versus temperature.     

In situ measurement of cavitation damage from single bubble collapse using high-speed chronoamperometry

We quantitatively study cavitation damage non-invasively, in-place and time-resolved at microsecond resolution. A single, laser-induced bubble is generated in an aqueous NaCl solution close to the surface of an aluminum sample. High-speed chronoamperometry is used to record the corrosion current flowing between the sample and an identical aluminum electrode immersed in the same solution. This configuration makes it possible to measure the cavitation damage in the nanometer thin passive layer of the aluminum surface via the corrosion current from the repassivation. Synchronously with the corrosion current, the bubble dynamics is recorded via high-speed imaging. Correlation between the two measurements allows contributing cavitation damage to the respective stages of the bubble dynamics. The largest cavitation-induced currents were observed for the smallest initial bubble-to-surface stand-off distances. As the bubble re-expands and collapses again in several stages, further current peaks were detected implying a sequence of smaller damage. At intermediate stand-offs, the bubble was not damaging and at large stand-off distances, the bubble was only damaging during the second collapse which again occurs at the solid surface.     

Investigation of cavitation bubble collapse in hydrophobic concave using the pseudopotential multi-relaxation-time lattice Boltzmann method

The interaction between cavitation bubble and solid surface is a fundamental topic which is deeply concerned for the utilization or avoidance of cavitation effect.The complexity of this topic is that the cavitation bubble collapse includes many extreme physical phenomena and variability of different solid surface properties.In the present work,the cavitation bubble collapse in hydrophobic concave is studied using the pseudopotential multi-relaxation-time lattice Boltzmann model(MRT-LB).The model is modified by involving the piecewise linear equation of state and improved forcing scheme.The fluid-solid interaction in the model is employed to adjust the wettability of solid surface.Moreover,the validity of the model is verified by comparison with experimental results and grid-independence verification.Finally,the cavitation bubble collapse in a hydrophobic concave is studied by investigating density field,pressure field,collapse time,and jet velocity.The superimposed effect of the surface hydrophobicity and concave geometry is analyzed and explained in the framework of the pseudopotential LBM.The study shows that the hydrophobic concave can enhance cavitation effect by decreasing cavitation threshold,accelerating collapse and increasing jet velocity.     

The effect of static pressure on the inertial cavitation threshold

The amplitude of the acoustic pressure required to nucleate a gas or vapor bubble in a fluid, and to have that bubble undergo an inertial collapse, is termed the inertial cavitation threshold. The magnitude of the inertial cavitation threshold is typically limited by mechanisms other than homogeneous nucleation such that the theoretical maximum is never achieved. However, the onset of inertial cavitation can be suppressed by increasing the static pressure of the fluid. The inertial cavitation threshold was measured in ultrapure water at static pressures up to 30?MPa (300 bars) by exciting a radially symmetric standing wave field in a spherical resonator driven at a resonant frequency of 25.5 kHz. The threshold was found to increase linearly with the static pressure; an exponentially decaying temperature dependence was also found. The nature and properties of the nucleating mechanisms were investigated by comparing the measured thresholds to an independent analysis of the particulate content and available models for nucleation.     

The Rayleigh-like collapse of a conical bubble

Key to the dynamics of the type of bubble collapse which is associated with such phenomena as sonoluminescence and the emission of strong rebound pressures into the liquid is the role of the liquid inertia. Following the initial formulation of the collapse of an empty spherical cavity, such collapses have been termed "Rayleigh-like." Today this type of cavitation is termed "inertial," reflecting the dominant role of the liquid inertia in the early stages of the collapse. While the inertia in models of spherical bubble collapses depends primarily on the liquid, experimental control of the liquid inertia has not readily been achievable without changing the liquid density and, consequently, changing other liquid properties. In this paper, novel experimental apparatus is described whereby the inertia at the early stages of the collapse of a conical bubble can easily be controlled. The collapse is capable of producing luminescence. The similarity between the collapses of spherical and conical bubbles is investigated analytically, and compared with experimental measurements of the gas pressures generated by the collapse, the bubble wall speeds, and the collapse times.     

碟形超声变幅杆的设计

优化超声变幅杆的形状结构可有效地提高水域声场分布和空化区域,提升对水域超声空化效果。通过模拟分析发现传统超声变幅杆在水域中具有声场分布均匀性差、变幅杆端部声压高等特征,不利于声波在水域中传播。基于此,提出并优化设计了一种具有碟形结构的变幅杆,位于变幅杆的最大振幅处的碟形结构,有更大的振动位移;模拟表明其水域声场和声压均衡度显著优于传统变幅杆,铝箔空化腐蚀实验进一步证实了其水域中的声压分布均匀性。同时,实验通过铝箔的空化腐蚀、KI剂量测定及工件表面油渍去除对比了传统变幅杆和碟形变幅杆,分析表明碟形变幅杆所在水域中有较大的空化腐蚀区域,腐蚀速率明显提升,声化学反应速率提高,油渍去除程度增强,说明了设计的碟形变幅杆能够促进空化泡的产生,增加水域空化区域。     

曲面固壁附近空化泡溃灭动力学的 流体体积数值研究*

流体体积法(VOF)可以便捷、高效地实现对多相流界面的捕捉和追踪。本文基于VOF方法,对单个空化泡在曲面固壁附近的运动进行了数值模拟,从实验对比、压力场、速度场、温度场演化、溃灭时间、射流速度、固壁温度等方面分析了空化泡溃灭过程的热动力学影响。结果表明,数值模拟得到的空化泡形态演化与实验观测到的现象一致,随着位置参数、泡内外压差及曲面固壁尺寸的改变,空化泡热动力学行为也将发生变化,受到流体运动及射流冲击的影响,溃灭瞬间产生的高温高压使得曲面固壁温度升高。本文研究的曲面固壁附近空化泡溃灭效应,揭示了空化泡与曲面固壁间的相互作用规律,对学术研究及工程应用都具有重要意义。     

Numerical modelling of ultrasonic waves in a bubbly Newtonian liquid using a high-order acoustic cavitation model

To address difficulties in treating large volumes of liquid metal with ultrasound, a fundamental study of acoustic cavitation in liquid aluminium, expressed in an experimentally validated numerical model, is presented in this paper. To improve the understanding of the cavitation process, a non-linear acoustic model is validated against reference water pressure measurements from acoustic waves produced by an immersed horn. A high-order method is used to discretize the wave equation in both space and time. These discretized equations are coupled to the Rayleigh-Plesset equation using two different time scales to couple the bubble and flow scales, resulting in a stable, fast, and reasonably accurate method for the prediction of acoustic pressures in cavitating liquids. This method is then applied to the context of treatment of liquid aluminium, where it predicts that the most intense cavitation activity is localised below the vibrating horn and estimates the acoustic decay below the sonotrode with reasonable qualitative agreement with experimental data.     

Electrical detection of the ultrasonic cavitation onset

We propose a new technique for the study of ultrasonic cavitation. This method is based on the quantification of the electrical admittance variations of the emitter in a range around the resonance frequency at different excitation levels. As the cavitation threshold is reached, the state of the fluid is changing; we evaluate these changes. The high-power piezoelectric transducer is modelled through an analytical model, which is used to relate the characteristics of the fluid domain (bubble density, extent) to the electrical admittance (peak value, resonance frequency, and bandwidth). Thus, the admittance we measure allows us to determine the characteristics of the bubbly liquid. The procedure is applied to the inertial cavitation field generated at 24kHz at very high amplitudes. The results obtained show that a very high bubble density layer is formed at the surface of the sonotrode.     

Non-lorentz transformations of reference systems

The structure of transformations is investigated which describe the transition from an inertial reference system to a noninertial one and conversely. As a result of such transition the curvature of the space of events is modified, and one is thus able to discuss its observational character. This leads to a new interpretation of the general relativity theory in which the space curvature is now related only to the noninertial motion of the observer. Transition from a noninertial reference system corresponding to the Schwarzschild space to an inertial reference system (plane space) is considered as an example.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 104–112, May, 1973.     

惯性定律不存在循环论证问题--从爱因斯坦对惯性定律和惯性系的分析谈起

惯性原理本身并不存在循环论证问题,所谓的循环论证来自利用惯性定律去寻找和确定惯性系,实际应用的“惯性系”本质上都是非惯性系,寻找和确定严格惯性系不是必要的．     

Implementation of the Laser Diffraction Technique for Acoustic Cavitation Bubble Investigations

Knowledge of the acoustic cavitation cloud would be useful for improving ultrasound reactor design. Among the characterisation techniques, few are adapted to bubble investigations in an intense ultrasound field. Some problems raised by these measurements result from interactions between the acoustic pressure wave and the measuring light wave. This paper reports the implementation of the laser diffraction technique to determine the size and volume concentration of bubbles generated by a dipping horn operating at 20 kHz. Measurements were performed with a Malvern 2600 instrument. The size distribution, deduced from the diffraction pattern scattered by the bubble cloud crossed by a laser beam, is disturbed by the acoustic pressure wave involving deviation of a light beam at low diffusion angles (acousto‐optic effect). A bubble size correction procedure based on the subtraction of the light energy due to the ultrasound wave is described. The size measurements, and thus the correction procedure, were validated by a second laser technique based on a different measuring principle: phase Doppler interferometry. The measurement reliability was further confirmed by an original application of laser diffraction based on measurements performed just after sonication. These three methods lead to a mean bubble size (Sauter mean diameter) of about 10 μm at a high ultrasound power input. Concerning the void fraction, only measurements achieved after sonication and by laser diffraction predict a correct estimation of this parameter.     

Control of the dynamics of a boiling vapour bubble using pressure-modulated high intensity focused ultrasound without the shock scattering effect: A first proof-of-concept study

Boiling histotripsy is a promising High-Intensity Focused Ultrasound (HIFU) technique that can be used to induce mechanical tissue fractionation at the HIFU focus via cavitation. Two different types of cavitation produced during boiling histotripsy exposure can contribute towards mechanical tissue destruction: (1) a boiling vapour bubble at the HIFU focus and (2) cavitation clouds in between the boiling bubble and the HIFU source. Control of the extent and degree of mechanical damage produced by boiling histotripsy is necessary when treating a solid tumour adjacent to normal tissue or major blood vessels. This is, however, difficult to achieve with boiling histotripsy due to the stochastic formation of the shock scattering-induced inertial cavitation clouds. In the present study, a new histotripsy method termed pressure-modulated shockwave histotripsy is proposed as an alternative to or in addition to boiling histotripsy without inducing the shock scattering effect. The proposed concept is (a) to generate a boiling vapour bubble via localised shockwave heating and (b) subsequently control its extent and lifetime through manipulating peak pressure magnitudes and a HIFU pulse length. To demonstrate the feasibility of the proposed method, bubble dynamics induced at the HIFU focus in an optically transparent liver tissue phantom were investigated using a high speed camera and a passive cavitation detection systems under a single 10, 50 or 100 ms-long 2, 3.5 or 5 MHz pressure-modulated HIFU pulse with varying peak positive and negative pressure amplitudes from 5 to 89 MPa and −3.7 to −14.6 MPa at the focus. Furthermore, a numerical simulation of 2D nonlinear wave propagation with the presence of a boiling bubble at the focus of a HIFU field was conducted by numerically solving the generalised Westervelt equation. The high speed camera experimental results showed that, with the proposed pressure-modulated shockwave histotripsy, boiling bubbles generated by shockwave heating merged together, forming a larger bubble (of the order of a few hundred micron) at the HIFU focus. This coalesced boiling bubble then persisted and maintained within the HIFU focal zone until the end of the exposure (10, 50, or 100 ms). Furthermore, and most importantly, no violent cavitation clouds which typically appear in boiling histotripsy occurred during the proposed histotripsy excitation (i.e. no shock scattering effect). This was likely because that the peak negative pressure magnitude of the backscattered acoustic field by the boiling bubble was below the cavitation cloud intrinsic threshold. The size of the coalesced boiling bubble gradually increased with the peak pressure magnitudes. In addition, with the proposed method, an oval shaped lesion with a length of 0.6 mm and a width of 0.1 mm appeared at the HIFU focus in the tissue phantom, whereas a larger lesion in the form of a tadpole (length: 2.7 mm, width: 0.3 mm) was produced by boiling histotripsy. Taken together, these results suggest that the proposed pressure-modulated shockwave histotripsy could potentially be used to induce a more spatially localised tissue destruction with a desired degree of mechanical damage through controlling the size and lifetime of a boiling bubble without the shock scattering effect.     

Use of the Lorentz Transformations in Noninertial Reference Frames

The Lorentz transformations are used within the model of a noninertial reference frame without infinitely high accelerations arising at instantaneous jumps of an accelerated observer between different inertial reference frames. It is demonstrated that the twin paradox can be explained within this model with the help of the Lorentz transformations. Based on the model of a noninertial reference frame, the acceleration a measured in the noninertial reference frame is related to the acceleration a measured in an inertial reference frame.     

Numerical investigation of the inertial cavitation threshold by dual-frequency excitation in the fluid and tissue

Inertial cavitation thresholds, which are defined as bubble growth by 2-fold from the equilibrium radius, by two types of ultrasonic excitation (at the classical single-frequency mode and dual-frequency mode) were calculated. The effect of the dual-frequency excitation on the inertial cavitation threshold in the different surrounding media (fluid and tissue) was studied, and the paramount parameters (driving frequency, amplitude ratio, phase difference, and frequency ratio) were also optimized to maximize the inertial cavitation. The numerical prediction confirms the previous experimental results that the dual-frequency excitation is capable of reducing the inertial cavitation threshold in comparison to the single-frequency one at the same output power. The dual-frequency excitation at the high frequency (i.e., 3.1 + 3.5 MHz vs. 1.1 + 1.3 MHz) is preferred in this study. The simulation results suggest that the same amplitudes of individual components, zero phase difference, and large frequency difference are beneficial for enhancing the bubble cavitation. Overall, this work may provide a theoretical model for further investigation of dual-frequency excitation and guidance of its applications for a better outcome.     

近复杂几何固壁空泡溃灭的伪势格子Boltzmann建模*

格子Boltzmann方法伪势多相模型具有高效性和复杂几何边界实施的简易性。该文采用改进作用力的伪势多相模型,通过优化参数实现最大程度的热力学一致性,进而提高模型的密度比和稳定性。分别从伪速度、网格不变性、Young-Laplace验证等方面研究了改进模型的性能。通过改进的模型模拟了复杂几何固壁附近空泡溃灭过程。分析了空化泡溃灭阶段的密度场、压力场和速度场演化过程,以及复杂几何固壁附近的空泡动力学特性。结果表明伪势格子Boltzmann方法在探索空泡溃灭和复杂几何固壁间的相互作用规律研究中是一种有效的工具。