Weak-focused acoustic vortex generated by a focused ring array of planar transducers and its application in large-scale rotational object manipulation

Contactless manipulation of multi-scale objects using the acoustic vortex(AV) tweezers offers tremendous perspectives in biomedical applications.However,it is still hindered by the weak acoustic radiation force(ARF) and torque(ART)around the vortex center.By introducing the elevation angle to the planar transducers of an N-element ring array,the weakfocused acoustic vortex(WFAV) composed of a main-AV and N paraxial-AVs is constructed to conduct a large-scale object manipulation.Different from the traditional focused AV(FAV) generated by a ring array of concave spherical transducers,a much larger focal region of the WFAV is generated by the main lobes of the planar transducers with the size inversely associated with the elevation angle.With the pressure simulation of the acoustic field,the capability of the rotational object driving in the focal plane for the WFAV is analyzed using the ARF and the ART exerted on an elastic ball based on acoustic scattering.With the experimental system built in water,the generation of the WFAV is verified by the scanning measurements of the acoustic field and the capability of object manipulation is also analyzed by the rotational trapping of floating particles in the focal plane.The favorable results demonstrate the feasibility of large-scale rotational manipulation of objects with a strengthened ART and a reduced acousto-thermal damage to biological tissues,showing a promising prospect for potential applications in clinical practice.     

Ultrasonic properties of transducer backings

For non-destructive testing applications it is often necessary to limit the number of cycles in a pulse produced by ultrasonic transducers. Usually a backing with a high ultrasonic attenuation and with an acoustic impedance chosen to match that of the piezoelectric active element of the transducer is employed. The acoustic properties of the commonly used tungsten/Araldite backings are compared theoretically with those of composites made by pressing tungsten powder with a powder of a plastic metal. The change in ultrasonic velocity and attenuation due to scattering by the tungsten particles is investigated and the dependence on frequency is determined.     

Anisotropy in high-frequency broadband acoustic backscattering in the presence of turbulent microstructure and zooplankton

High-frequency broadband (120-600 kHz) acoustic backscattering measurements have been made in the vicinity of energetic internal waves. The transducers on the backscattering system could be adjusted so as to insonify the water-column either vertically or horizontally. The broadband capabilities of the system allowed spectral classification of the backscattering. The distribution of spectral shapes is significantly different for scattering measurements made with the transducers oriented horizontally versus vertically, indicating that scattering anisotropy is present. However, the scattering anisotropy could not be unequivocally explained by either turbulent microstructure or zooplankton, the two primary sources of scattering expected in internal waves. Daytime net samples indicate a predominance of short-aspect-ratio zooplankton. Using zooplankton acoustic scattering models, a preferential orientation of the observed zooplankton cannot explain the measured anisotropy. Yet model predictions of scattering from anisotropic turbulent microstructure, with inputs from coincident microstructure measurements, were not consistent with the observations. Possible explanations include bandwidth limitations that result in many spectra that cannot be unambiguously attributed to turbulence or zooplankton based on spectral shape. Extending the acoustic bandwidth to cover the range from 50?kHz to 2?MHz could help improve identification of the dominant sources of backscattering anisotropy.     

The-effective directivity characteristic of a pulsed ultrasound transducer and its measurement by semi-automatic means

A simple electronic apparatus for plotting the effective directivity characteristics of pulsed, or continuously excited, ultrasound (1–10 MHz) transducers has been built. Beam profiles are plotted by recording a dc signal, derived from the peak amplitude of either the echo from a steel ball or the signal from a small hydrophone, as a function of position in the acoustic field. An additional capability of direct plotting of isoamplitude contours provides a particularly easy and informative means of recording and displaying the acoustic field.The apparatus has been used to investigate the relationship, in grey scale imaging systems, between dynamic range compression characteristics and effective display resolution. It is shown that, so long as the gain characteristics of the system remain constant, the display resolution capability is inversely related to the strength of the scatterers to be visualized.     

Functional-analytical solution to an acoustic tomography problem, based on data from point transducers

A two-dimensional version of the functional-analytical algorithm for solving the inverse scattering problem is considered and modeled. Acoustic fields, transmitted and received by quasi-point transducers, are used as experimental data. The algorithm is mathematically rigorous and does not require either linearization of the models or iterations, making it attractive for a variety of applications in acoustic tomography.     

Parameters of microparticles responsible for the monopole sound scattering in a liquid

A monopole scattering of sound by microparticles in a liquid is considered for microparticles of two types: spherically symmetric particles similar to gas bubbles in a liquid and disk-shaped gas-filled particles whose bases may oscillate in antiphase. A transformation of the scattering amplitude to the function that, in particular, describes the oscillations of a particle removed from the liquid is proposed. This function extends the possibilities of the acoustic diagnostics of such particles. Estimates of the sound velocity in water containing disk-shaped particles suggest that the sound velocity increase observed in sea water with plankton can be explained by the monopole scattering of sound from a single plankton particle modeled as a rigid gas-filled disk with oscillating parts of its bases.     

Realization of a multipath ultrasonic gas flowmeter based on transit-time technique

A microcomputer-based ultrasonic gas flowmeter with transit-time method is presented. Modules of the flowmeter are designed systematically, including the acoustic path arrangement, ultrasound emission and reception module, transit-time measurement module, the software and so on. Four 200 kHz transducers forming two acoustic paths are used to send and receive ultrasound simultaneously. The synchronization of the transducers can eliminate the influence caused by the inherent switch time in simple chord flowmeter. The distribution of the acoustic paths on the mechanical apparatus follows the Tailored integration, which could reduce the inherent error by 2–3% compared with the Gaussian integration commonly used in the ultrasonic flowmeter now. This work also develops timing modules to determine the flight time of the acoustic signal. The timing mechanism is different from the traditional method. The timing circuit here adopts high capability chip TDC-GP2, with the typical resolution of 50 ps. The software of Labview is used to receive data from the circuit and calculate the gas flow value. Finally, the two paths flowmeter has been calibrated and validated on the test facilities for air flow in Shaanxi Institute of Measurement & Testing.     

Pulling force of acoustic-vortex beams on centered elastic spheres based on the annular transducer model

To solve the difficulty of generating an ideal Bessel beam, an simplified annular transducer model is proposed to study the axial acoustic radiation force(ARF) and the corresponding negative ARF(pulling force) exerted on centered elastic spheres for acoustic-vortex(AV) beams of arbitrary orders. Based on the theory of acoustic scattering, the axial distributions of the velocity potential and the ARF for AV beams of different orders generated by the annular transducers with different physical sizes are simulated. It is proved that the pulling force can be generated by AV beams of arbitrary orders with multiple axial regions. The pulling force is more likely to exert on the sphere with a smaller k_0a(product of the wave number and the radius) for the AV beam with a bigger topological charge due to the strengthened off-axis acoustic scattering. The pulling force decreases with the increase of the axial distance for the sphere with a bigger k_0a.More pulling force areas with wider axial regions can be formed by AV beams using a bigger-sized annular transducer.The theoretical results demonstrate the feasibility of generating the pulling force along the axes of AV beams using the experimentally applicable circular array of planar transducers, and suggest application potentials for multi-position stable object manipulations in biomedical engineering.     

Scattering measurements from a dissolving bubble

A laboratory-scale study on acoustic scattering from a single bubble undergoing dissolution in undersaturated fresh water is presented. Several experiments are performed with the acoustic source driven with five-cycle tone bursts, center frequency of 120 kHz, to insonify a single bubble located on axis of the combined beam of the set of transducers. The bubble is placed on a fine nylon thread located in the far field of the transducer set, arranged in bistatic configuration, in a tank filled with undersaturated water. Backscattered waveforms from the bubble target are acquired every few seconds for several hours until the bubble has completely dissolved, and detailed dissolution curves are produced from the acoustic data. The rate of bubble dissolution is calculated using the solution developed by Epstein and Plesset [J. Chem. Phys. 18, 1505-1509 (1950)]. The results of the experiments performed are in agreement with the calculations.     

Laboratory-scale measurement of scattering from modelled Arctic ice ridges

An experiment is performed to measure acoustic scattering from scale modelled ice ridges in both specular (forward) and non-specular (backward) directions, for comparison with predictions from theoretical models. The experiment uses a 100 kHz transmitter emitting sinusoidal bursts. An array of miniature transducers is used to measure the scattered field as a function of scattering angle. Experimental results are obtained for scattering from different types of rods simulating ice ridges, and also the reflection coefficient from an acrylic block. The results show good agreement with the Twersky model predictions. This experiment establishes an effective technique, using scale models in the laboratory, to compare theoretical predictions and field experimental data.     

Investigation of the influence of humidity on the ultrasonic agglomeration of submicron particles in diesel exhausts

Removing very fine particles in the 0.01-1 micro m range generated in diesel combustion is important for air pollution abatement because of the impact such particles have on the environment. By forming larger particles, acoustic agglomeration of submicron particles is presented as a promising process for enhancing the efficiency of the current filtration systems for particle removal. Nevertheless, some authors have pointed out that acoustic agglomeration is much more efficient for larger particles than for smaller particles. This paper studies the effect of humidity on the acoustic agglomeration of diesel exhausts particles in the nanometer size range at 21 kHz. For the agglomeration tests, the experimental facility basically consists of a pilot scale plant with a diesel engine, an ultrasonic agglomeration chamber a dilution system, a nozzle atomizer, and an aerosol sampling and measuring station. The effect of the ultrasonic treatment, generated by a linear array of four high-power stepped-plate transducers on fumes at flow rates of 900 Nm(3)/h, was a small reduction in the number concentration of particles at the outlet of the chamber. However, the presence of humidity raised the agglomeration rate by decreasing the number particle concentration by up to 56%. A numerical study of the agglomeration process as a linear combination of the orthokinetic and hydrodynamic agglomeration coefficients resulting from mutual radiation pressure also found that acoustic agglomeration was enhanced by humidity. Both results confirm the benefit of using high-power ultrasound together with humidity to enhance the agglomeration of particles much smaller than 1 micro m.     

炸药爆炸作用下液体破碎后颗粒尺寸分布的研究

 对液体抛撒的液滴尺寸进行研究在军事和民用上是很重要的,国内刚开始使用激光散射仪开展此项研究工作。利用R. A. Dobbins等人的液体颗粒测量技术,研制了一套既简单又实用的测量液体抛撒过程中液滴尺寸的实验装置——激光散射仪。对于激光与液体微粒的相互作用,当微粒的反射与折射和吸收效应可被忽略时,可导出液体微粒对激光散射的光强公式。只要测量激光被微粒散射的光强,就可推算出微粒的Sauter平均直径。在使用激光散射仪测量液体抛撒液滴尺寸的实验中,用水代替爆炸抛撒液体,测量结果表明：液体抛撒二次破碎中,在固定位置测量到的云雾区液滴Sauter平均直径随测量时间的增加呈现出减小的趋势;而云雾区的宽度则随着与抛撒中心距离的增大而呈现出增加的趋势;云雾区前沿的液滴Sauter平均直径随着与抛撒中心距离的增加而呈现出先逐渐增大然后迅速减小的趋势。为便于比较,对燃料抛撒二次破碎进行了回收法测量和数值模拟计算,其测量与计算结果与用激光散射仪测量的结果有较好的一致性。     

Effects of interaction between two bubble scatterers

The backscattering of sound from two regularly arranged bubbles is studied theoretically and experimentally. In well-controlled laboratory experiments a bistatic acoustic system is used to interrogate the scatterers, which are placed on a very fine thread at the same distance d from the combined beam axis of the set of transmitting and receiving transducers. The radius of each bubble is 585 microm. The frequency range is 80-140 kHz, and d is varied so that the variable kd spans the range 0.2-21, where k is the acoustic wave number. Scattering calculations are carried out using an exact, closed-form solution derived from the multiple scattering series. Several experiments are performed, and the results are in close agreement with the calculations. It is verified that multiple scattering induces an oscillatory behavior about the exact coherent scattering level, with decreasing amplitude for increasing kd. For interbubble distance 2d approximately lambda/2 the backscattered radiation is maximized, while for 2d相似文献   

Agglomeration of particles by a converging ultrasound field and their quantitative assessments

The acoustic radiation force resulting from acoustic waves have been extensively studied for the contact-free generation of organized patterning arrays. The precise arrangement of microscopic objects clustered at the pressure nodes is critical to the development of functional structures and patterned surfaces. However, the size of the clusters is restricted by the saturation limit of the acoustic nodes. Here, we present a bulk acoustic wave (BAW) platform, which employs a two-dimensional acoustic wave to propel particles of various sizes. Experimentally, when particles are large, significant acoustic energy is scattered and partly absorbed by the matched layers in front of the sensors. The acoustic radiation force from a convergent acoustic pressure field agglomerates the large polystyrene (PS) particles towards the central region instead of the pressure nodes. The parametric analysis has been performed to assess the transition in the particles from clustering at the organized nodal arrays to agglomerating in the central region, which is a function of particle size, particle concentration, and load voltage. Statistically, the particles can agglomerate with a cluster ratio greater than 70%, and this ratio can be improved by increasing the load power/voltage supplied to the transducers. With its ability to perform biocompatible, label-free, and contact-free self-assembly, this concept offers a new possibility in the fabrication of colloidal layers, the recreation of tissue microstructure, the development of organoid spheroid cultures, the migration of microorganisms, and the assembly of bioprinting materials.     

近岸4种鱼的声散射特征提取及融合实验研究*

为了有效地提取表征鱼类间差异的声散射特征参数,该文通过绳系法实验研究了近岸4种经济鱼类的声散射信号特征提取及融合方法。首先,通过自研双频鱼探仪采集花鲈、许氏平鲉、黑鲷和斑石鲷的个体鱼声散射信号;然后,分别测定200 kHz和450 kHz换能器下鱼体的目标强度,同时提取鱼声散射信号的时频域统计特征;最后,将降维后的时频特征与频差特征融合组成新的特征向量。该文通过实验验证了该方法的有效性,基于组合特征的支持向量机识别准确率达93%。结果表明,鱼的频率响应特性和鱼声散射信号的时频域统计特征能一定程度上反映鱼的固有属性,有效地增加判别依据能显著提高以上4种鱼类的识别准确率。     

Relation of the insertion loss and the triple transit echo in surface acoustic wave unidirectional transducers

The scattering matrix of surface acoustic wave unidirectional transducers is calculated on the assumption that it is passive, reversible and lossless. Then the insertion loss and the triple transit echo are obtained as a function of the normalized radiation conductance. The propriety of the theoretical results is verified by experiments.     

Mechanical characterization of microparticles by scattered ultrasound

A technique for determining the compressibility and density of individual microparticles in suspension is described. The particles have diameters on the order of 10 microns Ultrasonic tone bursts of 2-microseconds duration and 30-MHz center frequency scatter from individual particles as they traverse the confocal zone of two transducers. The resulting scattered tone bursts are detected at 90 degrees and 180 degrees (backscattering). The received rf signals are demodulated, peak detected, digitized, and stored in computer memory. Using Rayleigh scattering theory, the compressibility and density of a particle can be computed given knowledge of the particle size and host fluid properties. Results of experiments with latex microspheres are presented and compared with calculations based on long-wavelength (Rayleigh) and elastic scattering theory.     

Microparticle column geometry in acoustic stationary fields

Particles suspended in a fluid will experience forces from stationary acoustic fields. The magnitude of the force depends on the time-averaged energy density of the field and the material properties of the particles and fluid. Forces acting on known particles smaller than 20 microm were studied. Within a 500 kHz acoustic beam generated by a plane-piston circular source, observations were made of the geometry of the particle column that is formed. Varying the acoustic energy altered the column width in a manner predicted by equations for the primary acoustic radiation force from scattering of particles in the long-wavelength limit. The minimum pressures required to trap gas, solid, and liquid particles in a water medium at room temperature were also estimated to within 12%. These results highlight the ability of stationary acoustic fields from a plane-piston radiator to impose nano-Newton-scale forces onto fluid particles with properties similar to biological cells, and suggest that it is possible to accurately quantify these forces.     

DLS‐Based Fractal Analysis in Early Stage Gelation

The fractal dimension of colloidal particles emerging from geling resorcinol/formaldehyde solutions is determined using a commercial apparatus for dynamic light scattering measurements. The method is considered particularly suited to quality control, e.g. in the production of colloid particles from sol‐gel processes.     

Angular scatter ultrasound imaging of wavelength scale targets

A bistatic ultrasound imaging system is demonstrated that uses two 32-element linear phased array transducers oriented at an angle of 40 degrees to one another. The system simultaneously acquires and displays in real time one conventional backscatter image and one "angular scatter" image formed using side-scattered echoes from the same B-mode sector region. Experiments are presented that show differences in the magnitudes of backscatter and angular scatter signals acquired from three nylon monofilaments with diameters less than one wavelength and from soft tissue structures in vivo. The relative magnitudes of angular scatter signals from the monofilaments are qualitatively consistent with a theoretical analysis of acoustic scattering from elastic cylinders. Larger tissue features are more clearly defined in angular scatter images. This result is attributed to the orientation of specularly reflecting surfaces and the expected influence of scattering angle on the system's sensitivity to different scatterer spacings.