Theoretical and experimental examination of near-field acoustic levitation

A planar object can be levitated stably close to a piston sound source by making use of acoustic radiation pressure. This phenomenon is called near-field acoustic levitation [Y. Hashimoto et al., J. Acoust. Soc. Am. 100, 2057-2061 (1996)]. In the present article, the levitation distance is predicted theoretically by numerically solving basic equations in a compressible viscous fluid subject to the appropriate initial and boundary conditions. Additionally, experiments are carried out using a 19.5-kHz piston source with a 40-mm aperture and various aluminum disks of different sizes. The measured levitation distance agrees well with the theory, which is different from a conventional theory, and the levitation distance is not inversely proportional to the square root of the surface density of the levitated disk in a strict sense.     

声悬浮过程的格子Boltzmann方法研究

采用轴对称多弛豫时间格子Boltzmann(LB)方法,研究了圆柱形封闭谐振腔中圆盘形样品的声悬浮过程.模拟结果表明,(001)模式下谐振腔的共振长度L=0.499λ,在谐振腔中心引入样品后共振漂移量δL≈-0.9,这与线性声学理论计算结果基本相符.声悬浮力的LB模拟过程包含了黏滞性效应和共振漂移效应,所获得的模拟结果与理论公式计算值在量值上一致,而且其在细节上更符合实验现象.此外,LB模拟还揭示出了声悬浮过程中的声压波形畸变、声流和声辐射压等非线性声学效应.     

Particle separation in microfluidics using different modal ultrasonic standing waves

Microfluidic technology has great advantages in the precise manipulation of micro and nano particles, and the separation of micro and nano particles based on ultrasonic standing waves has attracted much attention for its high efficiency and simplicity of structure. This paper proposes a device that uses three modes of ultrasonic standing waves to continuously separate particles with positive acoustic contrast factor in microfluidics. Three modes of acoustic standing waves are used simultaneously in different parts of the microchannel. According to the different acoustic radiation force received by the particles, the particles are finally separated to the pressure node lines on both sides and the center of the microchannel. In this separation method, initial hydrodynamic focusing and satisfying various equilibrium constraints during the separation process are the key. Through numerical simulation, the resonance frequency of the interdigital transducer, the distribution of sound pressure in the liquid, and the relationship between the interdigital electrode voltage and the output sound pressure are obtained. Finally, the entire separation process in the microchannel was simulated, and the separation of the two particles was successfully achieved. This work has laid a certain theoretical foundation for the rapid diagnosis of diseases in practical applications.     

Non-contact transportation using near-field acoustic levitation

Near-field acoustic levitation, where planar objects 10 kg in weight can levitate stably near the vibrating plate, is successfully applied both to non-contact transportation of objects and to a non-contact ultrasonic motor. Transporting apparatuses and an ultrasonic motor have been fabricated and their characteristics measured. The theory of near-field acoustic levitation both for a piston-like sound source and a flexural vibration source is also briefly described.     

双层周期加肋有限长圆柱壳声散射精细特征研究

研究了双层周期性加肋有限长圆柱壳在水中的声散射特性. 壳体振动用薄壳理论的Donnell 方程描述,环肋振动用相互独立的薄板纯弯曲振动和平面应力状态下的振动方程描述,忽略弦间流体对环肋轴向力的作用. 数值计算给出远场收发合置情况下的周向目标强度和角度-频率谱图,并据此进行机理分析. 计算结果表明远场散射声场中除壳体弹性贡献外,弦间流体以及环肋与内外壳的相互作用对散射声场的贡献也是很重要的,并且在角度-频率谱中出现了舷间流体引起的流体附加波以及周期环肋引起的Bragg散射等回波精细特征,其中流体附加波是双层加肋圆柱壳声散射最重要的散射精细特征,是以往单层圆柱壳声散射所不具有的现象. 最后通过实验对理论推导进行了验证,实验与理论基本符合. 关键词： 声散射 圆柱壳 环肋 流体附加波     

Filamentary coagulation of heavy particles suspended in water in an ultrasonic field

A stable formation of long thin filaments involving heavy particles (heavier than water) suspended in water in an acoustic resonator was observed. The filaments consisted of regularly spaced thin disks formed by coalescent particles. The particles in the disks were tightly packed forming almost solid structures. The disks were arranged perpendicularly to the filament axis and were spaced at half-sound-wavelength intervals. The diameter of the disks was about half-wavelength. The length of the filaments was about hundreds of the sound wavelength. Experiments were performed with particles of different natures and sizes. The filaments were not destroyed under wide variations of sound frequency, acoustic power, length of resonator, size and configuration of the mirror, or its angle of rotation. The particles suspended in water not only visualized the acoustic field but also affected its configuration. The formation of filaments was explained by a nonthermal self-focusing of sound in the resonator, which was caused by the decrease in sound velocity in the region of maximum concentration of particles due to coagulation in the inhomogeneous sound field.     

Acoustic nonlinearity parameter tomography for biological tissues via parametric array from a circular piston source--theoretical analysis and computer simulations

The acoustic nonlinearity parameter B/A describes the nonlinear features of a medium and may become a novel parameter for ultrasonic tissue characterization. This paper presents a theoretical analysis for acoustic nonlinear parameter tomography via a parametric array. As two primary waves of different frequencies are radiated simultaneously from a circular piston source, a secondary wave at the difference frequency is generated due to the nonlinear interaction of the primary waves. The axial and radial distributions of sound pressure amplitude for the generated difference frequency wave in the near field are calculated by a superposition of Gaussian beams. The calculated results indicated that the difference frequency component of the parametric array grows linearly with distance from the piston source. It therefore provides a better source to do the acoustic nonlinearity parameter tomography because the fundamental and second harmonic signals both have a near field that goes through many oscillations due to diffraction. By using a finite-amplitude insert substitution method and a filtered convolution algorithm, a computer simulation for B/A tomography from the calculated sound pressure of the difference frequency wave is studied. For biological tissues, the sound attenuation is considered and compensated in the image reconstruction. Nonlinear parameter computed tomography (CT) images for several biological sample models are obtained with quite good quality in this study.     

Acoustic levitation and its application in estimation of high power sound field

Acoustic levitation has been studied using airborne radiation from a high power magnetostrictive ferrite transducer of frequency 18 kHz. Spherical solid objects as well as liquid drops of varying weight and radii have been levitated under I G at the velocity antinodes of the standing waves formed with the help of a reflector. The dependence of the maximum weight of a spherical object that can be levitated on the sound pressure level and the radius of the obstacle has been studied. An attempt has been made to estimate the radiated high power sound intensity from the maximum weight of the spherical object that can be levitated.     

气体物理性质对近场超声悬浮特性的影响研究

在利用近场超声悬浮技术搬运晶圆的过程中,为提高近场超声悬浮力,该文研究了气体物理性质与近场超声悬浮力之间的关系。利用声辐射压理论与流体力学理论对不同气体介质下的悬浮力进行建模与求解。搭建了可测量不同气体环境中悬浮力的实验平台。实验发现氩气环境下超声悬浮力平均值相对空气环境下提升24.8%。结合实验与理论计算分析了气体密度、比热容比、声速与动力黏度对悬浮力的影响,推断气体的比热容比为影响近场超声悬浮力的主要参数。该方法为晶圆搬运的环境适用性提供了参考价值。     

用于超声悬浮场可视化的光学全息相位倍增方法研究

基于二次曝光全息干涉测量原理,采用四倍相位倍增光路对声悬浮声压场分布进行了可视化研究.拍摄了给定模式的超声悬浮场在不同超声换能器激励电流时的光学全息图并进行光学再现,由再现光波的全息干涉条纹图样得到了反映声压场分布的光波波前相位变化,并进行了对比分析.结果表明,与无相位倍增光路相比,采用四倍相位倍增光路可使同一声压场的全息干涉条纹数目大大增加,测量灵敏度明显提高,为进一步研究声悬浮场声压分布提供了一种有效方法.     

水下复杂壳体结构在多源激励下的振动及声辐射特性研究

为了综合评估水下复杂壳体结构在多源激励下的振动与声辐射特性,专门设计了一个多舱段复杂壳体模型,分别进行了单台设备激励、多台设备激励的壳体振动与声辐射试验。试验结果表明,多源激励下的结构响应和辐射声场可以近似为单源激励下结构响应和辐射声场的非相干叠加。通过对激励源与结构响应、辐射声场之间传递关系的理论分析,讨论了以单源激励响应非相干叠加合成得到多源激励响应的合理性和误差。研究结果对复杂壳体结构在实际激励下的振动与噪声预报具有重要价值。      

功率超声珩磨磨削区空化声场的建模与仿真研究

通过建立Φ47功率超声珩磨磨削区流体-结构耦合声场的理论模型,采用ANSYS有限元分析软件对磨削区的空化声场进行了仿真分析,得出磨削区二维、三维空化声场的分布情况和声压幅值,并利用超声波功率测量仪对磨削区的空化声场进行模拟实验测量。结果表明:在谐振频率18.6 kHz下,声场主要集中在油石的中部,二维和三维声场的声压最大值分别为1.39 MPa和1.47 MPa,与实验测得声压最大值1.3 MPa在同一个数量级,为进一步研究功率超声珩磨空化效应提供理论和实验的基础。      

复频聚焦超声换能器水中焦区商场特性的研究

本文对新研制的一种复频聚焦超声换能器的辐射声场进行了实验研究,测定了辐射压在轴向上的分布曲线,进而确定了焦区位置,这与理论所得结果符合较好,测定了辐射声压在焦平面上的分布曲线,并对焦区声场进行了定位,最后对换能器在水中焦区辐射声的频谱进行了研究,不仅观测到了两个源波,而且还观测到了和频波,差频波以及倍频波,证实了声散射声效应的存在。     

A new type of high power composite ultrasonic transducer

A new type of high power composite ultrasonic transducer was proposed and studied. The composite transducer consists of a sandwich longitudinal piezoelectric transducer, an isotropic metal hollow cylinder with large radial dimension, and the front and back metal radiation mass. By means of its special structure and Poisson’s effect, the composite transducer can produce vibrations both in its longitudinal and radial directions, and therefore, it can radiate sound waves in three-dimensional space. The electro-mechanical equivalent circuit of the composite transducer was derived and the resonance frequency equation was obtained analytically. Numerical methods were used to simulate the vibration of the composite transducer, and the vibrational displacement distribution, the resonance frequency and the radiation sound field are given. Some composite transducers are designed and manufactured; their resonance frequencies and the radiation acoustic field are measured and compared with the analytical and numerical results. It can be seen that the measured frequencies and acoustic field contour are in good agreement with the analytical and numerical results. It is expected that this kind of composite ultrasonic transducer can be used in more and more power ultrasonic applications, such as ultrasonic cleaning, ultrasonic extraction, ultrasonic sonochemistry and other ultrasonic liquid processing, where high ultrasonic power and large ultrasonic processing space are needed increasingly.     

有限振幅声波间的非线性相互作用对声源远场指向性的影响

根据Fenlon理论推导得到了多频声源的辐射声压.基于单频声源谐波指向性的求解方法,得到了二阶近似下的双频声源辐射出的声波相互作用时的远场指向性.分别研究和讨论了在初始辐射声压和频率不同情况下,两列波的相互作用对其中一列波的一阶波和二阶波远场指向性的影响.结果表明,声波间的相互作用对声源远场指向性的影响根据各波之间的相对初始辐射声压和相对频率的不同而有所变化. 关键词： 有限振幅声波 非线性相互作用 远场指向性     

变厚度聚焦换能器对声焦域轴向长度影响的研究*

针对不同厚度的病变组织,改变声焦域轴向长度能提高高强度聚焦超声在临床治疗过程中的安全性和有效性。基于多频超声波叠加原理,该文提出了变厚度(多频)聚焦换能器,并设计了两种类型变厚度聚焦换能器。根据瑞利积分法推导了变厚度聚焦换能器声场,计算和分析了变厚度聚焦换能器的声焦域轴向长度,并与等厚度(单频)聚焦换能器声焦域轴向长度进行对比。结果显示,变厚度聚焦换能器中心到边缘的厚度变化趋势与声焦域轴向长度变化相关,中间薄两边厚换能器声焦域轴向长度缩短,中间厚两边薄换能器声焦域轴向长度变长,且实验验证了理论的正确性。研究结果可为变厚度聚焦换能器声场研究和高强度聚焦超声的临床治疗提供参考。     

A Helmholtz resonator buried in the ground as a source of seismic waves and low-frequency sound in the atmosphere

The procedure is given for calculating the total power of low-frequency sound and seismic waves produced by a Helmholtz resonator in the form of an air-filled spherical cavity buried in the ground and supplied with a hole through which it is connected with the atmosphere. The sound is generated by air oscillations in the resonator’s neck section that is open to the atmosphere, while the compression and shear elastic waves are generated in the bulk of the ground by cyclic pressure fluctuations that act on the spherical walls of the cavity. Calculations show that the coincidence of the resonance frequencies (within approximately ten to hundred hertz), at which both the sound radiation to the atmosphere and the elastic seismic radiation in the form of longitudinal and transverse bulk waves are maximum, can occur only when the resonator is placed in a loose ground characterized by reduced elastic characteristics. In these conditions, the power of transverse waves exceeds the sound power by a factor of two and the power of longitudinal waves is smaller than the sound power by a factor of several tens.     

Acoustic rotational manipulation using orbital angular momentum transfer

We report on the first quantitative test of acoustic orbital angular momentum transfer to a sound absorbing object immersed in a viscous liquid. This is done by realizing an original experiment that is to spin a millimeter-size target disk using an ultrasonic vortex beam. We demonstrate the balance between the acoustic radiation torque calculated from the Brillouin stress tensor and the viscous torque evaluated from the steady state spinning frequency. Moreover, we unveil a rotational acoustic streaming phenomenon that results from the acoustic angular momentum transfer to the host fluid. We show that it lowers the viscous torque, thereby restoring the torque balance.     

Application of refraction and diffraction of light to the measurement of the liquid surface shape deformed by ultrasonic radiation pressure

A method of measuring the liquid surface (hump) deformed by ultrasonic radiation pressure is proposed. The method is based on the usage of a hump as a liquid lens with an interference structure formed by sound waves falling on the surface. The hump shape is evaluated by means of shadowgrams obtained with laser illumination. The possibility of using the hump as a sensor of processes occuring in the liquid through which sound waves are passing is shown.     

Contactless transport of matter in the first five resonance modes of a line-focused acoustic manipulator

The first five resonance modes for transport of matter in a line-focused acoustic levitation system are investigated. Contactless transport was achieved by varying the height between the radiating plate and the reflector. Transport and levitation of droplets in particular involve two limits of the acoustic forces. The lower limit corresponds to the minimum force required to overcome the gravitational force. The upper limit corresponds to the maximum acoustic pressure beyond which atomization of the droplet occurs. As the droplet size increases, the lower limit increases and the upper limit decreases. Therefore to have large droplets levitated, relatively flat radiation pressure amplitude during the translation is needed. In this study, using a finite element model, the Gor'kov potential was calculated for different heights between the reflector and the radiating plate. The application of the Gor'kov potential was extended to study the range of droplet sizes for which the droplets can be levitated and transported without atomization. It was found that the third resonant mode (H(3)-mode) represents the best compromise between high levitation force and smooth pattern transition, and water droplets of millimeter radius can be levitated and transported. The H(3)-mode also allows for three translation lines in parallel.