液体薄层中的超声空化*

液体薄层中的超声空化,因其边界及所处空间的特殊性,而呈现出非常独特的空化结构和演化行为,在超声清洗、超声钎焊、表面处理、近场声悬浮、超声化学等领域都有所应用。该文梳理了近几年该课题组在液体薄层中的超声空化研究中的一些成果,力图揭示液体薄层内空泡、空化云、空化场的运动和分布规律,及其产生、发展和演化过程,以期对液体薄层中的超声空化行为有一个相对清晰和完整的认识。     

Cavitation in thin liquid layer: A review

This review tries to cover as many research fields and literatures associated with cavitation in thin liquid layer as possible. The intent was to summarize (i) list all the research fields related to cavitation in thin liquid layer that can be collected, (ii) advances in the investigation of cavitation in thin liquid layer, and (iii) draw attention to the relatively macroscopic cavitation behavior in thin liquid layer.     

Ultrasonic cavitation in thin liquid layers

The generation of ultrasonic cavitation in a thin liquid layer trapped between a large radiating surface and a hard reflector and bounded laterally by a gas–liquid interface is investigated. The theoretical analysis predicts that a large amplification of the acoustical pressure is obtained with this configuration. Experiments are conducted by driving the layer with horn-type transducers having a large emitting surface. Ultrasonic cavitation is obtained in a broad frequency range at low input intensity due to the amplification effect. Erosion tests on metallic foils demonstrate the existence of a region of intense cavitation activity which can be localised by controlling the input intensity.     

Ultrasonic cavitation at liquid/solid interface in a thin Ga–In liquid layer with free surface

Cavitation in thin layer of liquid metal has potential applications in chemical reaction, soldering, extraction, and therapeutic equipment. In this work, the cavitation characteristics and acoustic pressure of a thin liquid Ga–In alloy were studied by high speed photography, numerical simulation, and bubble dynamics calculation. A self-made ultrasonic system with a TC4 sonotrode, was operated at a frequency of 20 kHz and a max output power of 1000 W during the cavitation recording experiment. The pressure field characteristic inside the thin liquid layer and its influence on the intensity, types, dimensions, and life cycles of cavitation bubbles and on the cavitation evolution process against experimental parameters were systematically studied. The results showed that acoustic pressure inside the thin liquid layer presented alternating positive and negative characteristics within 1 acoustic period (T). Cavitation bubbles nucleated and grew during the negative-pressure stage and shrank and collapsed during the positive-pressure stage. A high bubble growth speed of 16.8 m/s was obtained and evidenced by bubble dynamics calculation. The maximum absolute pressure was obtained at the bottom of the thin liquid layer and resulted in the strongest cavitation. Cavitation was divided into violent and weak stages. The violent cavitation stage lasted several hundreds of acoustic periods and had higher bubble intensity than the weak cavitation stage. Cavitation cloud preferentially appeared during the violent cavitation stage and had a life of several acoustic periods. Tiny cavitation bubbles with life cycles shorter than 1 T dominated the cavitation field. High cavitation intensities were observed at high ultrasonication power and when Q235B alloy was used because such conditions lead to high amplitudes on the substrate and further high acoustic pressure inside the liquid.     

超声空化现象影响因素的实验研究

超声空化在许多不同的学科和工业生产中有着广泛的应用。超声空化的应用与声场的分布及空化的机理密切相关,精准地反映空化场和空化机理是超声空化技术实际应用的关键。该文通过分析采集的声信号和金属箔膜空蚀法对空化区域随液位发生变化的现象进行研究,并利用Matlab对金属箔膜空蚀程度量化。实验发现,超声波会在液面与实验箱体底部形成驻波场。在某一液体温度下,随着液位高度的变化,超声空化现象的出现具有周期性。并且,在同一液位下,当超声功率改变时,空化区域强度分布情况随之改变。小功率时各空化区域空化强度分布均匀,当功率增大到一定时,会出现空化屏蔽现象。该研究为超声清洗设备的改良提供了借鉴,对进一步认识和利用超声空化效应具有重要意义。     

Molecular dynamics simulation of effect of liquid layering around the nanoparticle on the enhanced thermal conductivity of nanofluids

The effect of the molecular layering at liquid–solid interface on the thermal conductivity of the nanofluid is investigated by an equilibrium molecular dynamics simulation. By tracking the position of the nanoparticle and the liquid atoms around the spherical nanoparticle, it was found that a thin layer of liquid is formed at the interface between the nanoparticle and liquid; this thin layer will move with the Brownian motion of the nanoparticle. Through the analysis of the density distribution of the liquid near the nanoparticle, it is found that more argon atoms are attracted to form the layer around the nanoparticle when the diameter of the nanoparticle is larger, and therefore lead to the more significant enhancement of the thermal conductivity of the nanofluid.     

High-speed imaging of ultrasound driven cavitation bubbles in blind and through holes

The interest in application of ultrasonic cavitation for cleaning and surface treatment processes has increased greatly in the last decades. However, not much is known about the behavior of cavitation bubbles inside the microstructural features of the solid substrates. Here we report on an experimental study on dynamics of acoustically driven (38.5 kHz) cavitation bubbles inside the blind and through holes of PMMA plates by using high-speed imaging. Various diameters of blind (150, 200, 250 and 1000 µm) and through holes (200 and 1000 µm) were investigated. Gas bubbles are usually trapped in the holes during substrate immersion in the liquid thus preventing their complete wetting. We demonstrate that trapped gas can be successfully removed from the holes under ultrasound agitation. Besides the primary Bjerknes force and acoustic streaming, the shape oscillations of the trapped gas bubble seem to be a driving force for bubble removal out of the holes. We further discuss the bubble dynamics inside microholes for water and Cu²⁺ salt solution. It is found that the hole diameter and partly the type of liquid media influences the number, size and dynamics of the cavitation bubbles. The experiments also showed that a large amount of the liquid volume inside the holes can be displaced within one acoustic cycle by the expansion of the cavitation bubbles. This confirmed that ultrasound is a very effective tool to intensify liquid exchange processes, and it might significantly improve micro mixing in small structures. The investigation of the effect of ultrasound power on the bubble density distribution revealed the possibility to control the cavitation bubble distribution inside the microholes. At a high ultrasound power (31.5 W) we observed the highest bubble density at the hole entrances, while reducing the ultrasound power by a factor of ten shifted the bubble locations to the inner end of the blind holes or to the middle of the through holes.     

Plasmon electro-optic effect in a subwavelength metallic nanograting with a nematic liquid crystal

The electro-optic effect in hybrid structures based on subwavelength metallic nanogratings in contact with a layer of a nematic liquid crystal has been experimentally studied. Metallic gratings are fabricated in the form of interdigitated electrodes, which makes it possible to use them not only as optical elements but also for the production of an electric field in a thin surface region of the layer of the liquid crystal. It has been shown that, owing to the electric-field-induced reorientation of molecules of the liquid crystal near the surface of the grating, it is possible to significantly control the spectral features of the transmission of light, which are caused by the excitation of surface plasmons. The electro-optic effect is superfast for liquid crystal devices because a change in the optical properties of the system requires the reorientation of molecules only in a very thin surface layer of the liquid crystal.     

超声清洗装置空化噪声谱分析法空化噪声级测量

空化强度是用以衡量液体介质中空化活动的剧烈程度,同时空化效应在超声清洗中起关键作用,因此,测量超声清洗槽中的空化强度便可了解其中空化活动的情况.当发生空化时,液体介质中会产生成分复杂空化噪声,对空化噪声谱进行分析和计算得到空化噪声级,据此可判断空化强度.实验测得结果表明:超声清洗装置内稳态空化分布广泛、均匀,瞬态空化分...     

An investigation of molecular layering at the liquid-solid interface in nanofluids by molecular dynamics simulation

The molecular layering at liquid-solid interface in a nanofluid is investigated by equilibrium molecular dynamics simulation. By tracking the positions of the nanoparticle and the liquid atoms around the spherical nanoparticle, it was found that an absorbed slip layer of liquid is formed at the interface between the nanoparticle and liquid; this thin layer will move with the Brownian motion of the nanoparticle. Through the analysis of the density distribution of the liquid near the nanoparticle it is found that the thickness of the layering is about 0.5 nm under the parameters used in the Letter.     

Influence of dissolved-air concentration on spatial distribution of bubbles for sonochemistry

The pulsation of ultrasonic cavitation bubbles at various dissolved-air concentration in a sonochemical reaction field of standing-wave type is investigated experimentally by laser-light scattering. When a thin light sheet, finer than half the wavelength of sound, is introduced into the cavitation bubbles at an antinode of sound pressure, the scattered light intensity oscillates. The peak-to-trough light intensity is correlated with the number of bubbles that contribute to the sonochemical reaction. It is shown that as the dissolved air concentration becomes higher, the weighted center of the spatial distribution of the peak-to-trough intensity tends to shift towards the liquid surface. At higher concentration of the dissolved air, a great deal of bubbles with size distribution generated due to coalescence between bubbles disturbs sound propagation to change the sound phase easily. A standing wave to trap tiny oscillating bubbles is established only at the side which is nearer to the liquid surface. Also at higher concentration, liquid flow induced by drag motion of bubbles by the action of radiation force becomes apparent and position-unstable region of bubble is enlarged from the side of sound source towards the liquid surface. Therefore, the position of oscillating bubbles active for sonochemical reaction is limited at the side which is nearer to the liquid surface at higher concentration of the dissolved air.     

Mechanism and characteristics of steam laser patterning

Steam laser patterning of thin films and/or solid surfaces has been studied by jetting a beam of steam, such as water vapor, onto a sample surface to form a thin liquid film on it and patterning the sample by laser etching along predetermined path. In steam laser patterning, bubbles are formed in a thin liquid film on a sample surface irradiated by a pulsed laser. When the collapsed shock wave generated at the moment of bubble collapse and the high-speed liquid jet formed during bubble collapse are strong enough, cavitation erosion of the sample surface takes place. Compared to dry laser patterning, the etching rate can be greatly enhanced and no shoulder-like structure is formed at the rim of the laser-irradiated spot in steam laser patterning due to this cavitation erosion effect. PACS 81.65.cf; 52.38.Mf; 79.20.Ds; 42.62.-b; 62.50.+p     

The effect of chloride ions on the corroded surface layer of 00Cr22Ni5Mo3N duplex stainless steel under cavitation

The effects of Cl⁻ on the corroded surface layer of 00Cr22Ni5Mo3N duplex stainless steel under cavitation in chloride solutions were investigated using nanoindentation in conjunction with XRD and XPS. The results demonstrate that Cl⁻ had a strong effect on the nano-mechanical properties of the corroded surface layer under cavitation, and there was a threshold Cl⁻ concentration. Furthermore, a close relationship between the nano-mechanical properties and the cavitation corrosion resistance of 00Cr22Ni5Mo3N duplex stainless steel was observed. The degradation of the nano-mechanical properties of the corroded surface layer was accelerated by the synergistic effect between cavitation erosion and corrosion. A key factor was the adsorption of Cl⁻, which caused a preferential dissolution of the ferrous oxides in the passive film layer on the corroded surface layer. Cavitation further promoted the preferential dissolution of the ferrous oxides in the passive film layer. Simultaneously, cavitation accelerated the erosion of the ferrite in the corroded surface layer, resulting in the degradation of the nano-mechanical properties of the corroded surface layer on 00Cr22Ni5Mo3N duplex stainless steel under cavitation.     

声场中水力空化泡的动力学特性

以水为工作介质,考虑了液体黏性、表面张力、可压缩性及湍流作用等情况,对文丘里管反应器中空化泡在声场作用下的动力学行为特性进行了数值研究.分析了超声波频率、声压及喉径比对空化泡运动特性以及空化泡崩溃时所形成泡温以及压力脉冲的影响.结果表明,超声将水力空化泡运动调制成稳态空化,有利于增强空化效果. 关键词： 超声波 水力空化 湍流 气泡动力学     

Streuung von 50 keV-Elektronen an flüssigem Helium

The behaviour of liquid helium in relation to electron-scattering has been investigated. Electrons of 50 keV have been scattered by thin targets of liquid helium with temperatures between 1.34 and 2.15 K. The thickness of targets was chosen between 0.047 and 0.62 mm. The angular distribution and the energy loss of the scattered electrons have been measured in dependence on the thickness of layer and on temperature. The extrapolated range of electrons in liquid helium has been determined.     

The effectiveness of thin layers as ultrasonic visualization media

A theoretical investigation is presented which examines the effect of introducing a thin layer into a previously established ultrasonic field, present within a liquid. The wave system induced within the layer is investigated and the distrubance to the field estimated.Various conclusions derived from the theory are discussed in relation to relevant ultrasonic visualization techniques, which ultilize thin layers.     

Study on the spatial distribution of the liquid temperature near a cavitation bubble wall

A simple new model of the spatial distribution of the liquid temperature near a cavitation bubble wall (T_li) is employed to numerically calculate T_li. The result shows that T_li is almost same with the ambient liquid temperature (T₀) during the bubble oscillations except at strong collapse. At strong collapse, T_li can increase to about 1510 K, the same order of magnitude with that of the maximum temperature inside the bubble, which means that the chemical reactions occur not only in gas-phase inside the collapsing bubble but also in liquid-phase just outside the collapsing bubble. Four factors (ultrasonic vibration amplitude, ultrasonic frequency, the surface tension and the viscosity) are considered to study their effects for the thin liquid layer. The results show that for the thin layer, the thickness and the temperature increase as the ultrasonic vibration amplitude rise; conversely, the thickness and the temperature decrease with the increase of the ultrasonic frequency, the surface tension or the viscosity.     

Peculiarities of evaporation of a thin water layer in the presence of a solvable surfactant

Evaporation of a thin layer of a polar liquid (water) having a free surface and located on a solid substrate is investigated. A solvable surfactant is placed on the free liquid-vapor interface. The surface tension is a linear function of the surface concentration of the surfactant. The surface energy of the solid-liquid contact line is a nonmonotonic function of the layer thickness and is the sum of the Van der Waals interaction and the specific interaction of the double electric layer on the interface. The effect of the solvable surfactant on the dynamics and stability of the propagation of the evaporation front in the thin liquid film is analyzed in the long-wave approximation in the system of Navier-Stokes equations.     

Investigation of the seeding-layer effect for a ferroelectric thin film with the transverse Ising model

The transition feature of a ferroelectric thin film with a seeding layer is studied based on the transverse Ising model. The influence of the seeding layer on the transition behavior of a ferroelectric thin film is investigated systemically, and the effect of the interaction parameters for the seeding layer on the phase diagram is also obtained. Meanwhile, the polarization and Curie temperature of the ferroelectric thin film are calculated for different seeding-layer structures. The results show that the polarization and Curie temperature of the film will be obviously modified on adding a seeding layer.     

Contactless Transfer of an Angular Momentum to a Liquid Layer Using a Scanning Laser Beam

The contactless transfer of an angular momentum to a liquid layer using a scanning laser beam has been experimentally investigated. This effect is observed for a glassy carbon target placed in water and irradiated by a pulsed laser beam, scanning the target along a closed trajectory. The target rotates in the same direction as the laser beam if the water layer is thin and in the opposite direction in the case of thick water layer. The possibility of transferring an angular momentum to liquid in the absence of target is demonstrated. The effect observed is interpreted as the result of joint action of convective flows and thermocapillary convection, which are induced in the liquid by laser heating.