零阶Bessel驻波场中任意粒子声辐射力和力矩的Born近似

声辐射力和声辐射力矩的计算是实现粒子精准操控的重要基础.基于经典声散射理论的偏波级数展开法较难直接用于复杂模型的研究,而纯数值的方法则不利于进行系统的参数化分析.基于Born近似的基本原理,推导了低频情况下零阶Bessel驻波场中心任意粒子的声辐射力和力矩表达式.在此基础上,以球形粒子、椭球形粒子和柱形粒子为例进行详细地计算,并考虑声参数的非均匀性对声辐射力和力矩的影响.仿真结果表明,在低频范围内Born近似具有很高的精度,随着频率的增加和粒子与流体的阻抗匹配变差,Born近似的精度逐渐下降.对于倾斜放置于零阶Bessel驻波场中的椭球形粒子和柱形粒子,非对称性会导致其受到声辐射力矩的作用.在粒子尺寸远小于波长的情况下,声辐射力特性与粒子的具体形状几乎无关,但声辐射力矩不然.最后,引入周围流体的黏滞效应并对声辐射力的表达式进行了修正.该研究预期可以为生物医学、材料科学等领域利用驻波场声镊子实现微小粒子的精准操控提供一定的理论指导.     

基于一维声栅共振场的大规模微粒并行排列的实验研究

声操控微粒技术可以非接触无损伤地控制声场中的物体运动,其在精密制造、材料工程、体外诊断等领域具有广阔的应用前景.传统声操控微粒技术一般采用自由声场,如利用单个换能器或阵列换能器产生的聚焦声场、行波场或驻波场等.然而,一般单个换能器产生的声场仅能操控单个微粒;而阵列换能器的驱动系统复杂,导致操控器件成本高昂且难以微型化;因此,亟需研究新的声场形态实现多样性微粒操控.本工作中,采用单个换能器产生的平面波激发一维声栅的共振声场,实验实现了大规模泡沫微球的周期排列操控.其操控机制是由于声栅狭缝中法布里-珀罗谐振声场与声栅表面周期衍射场共振耦合,在声栅表面形成周期分布的局域梯度声场,导致微粒在平行于声栅表面受到声捕获力,在垂直于声栅表面受到指向表面的声吸引力,实现了微粒周期排列在声栅表面上.该工作为利用超声在空气中大规模排列微粒提供了理论基础和技术支持.     

基于亚波长管道增强的漩涡声场悬浮操控微粒和液滴的实验研究

声悬浮技术可以在无接触的情况下操控微粒和液滴,因此已被广泛应用于化学分析、液滴动力学和生物反应器等领域.目前声悬浮技术的主要工作是在开放环境中进行悬浮等操控.本文提出了亚波长管道增强型空气声镊的概念,利用亚波长声波导管进行声场操控及微粒和液滴悬浮.通过4个小型换能器激发有限长度亚波长圆波导管的单一低阶声学模态,可以在有限长度的波导管内产生漩涡声场.实验发现由于亚波长结构对声场的增强作用,亚波长管道增强的漩涡声场在径向和轴向悬浮力大小上均有较大提升,因此可对发泡聚苯乙烯颗粒和水滴实施悬浮和自转等操控.这项工作将亚波长声波导管的概念引入声场操控中,有望加深对声场和物质相互作用的物理理解,开发新型小型化悬浮微粒和液滴的声学操纵器件.     

椭球粒子声辐射力计算及分析

针对水下椭球粒子,以声散射理论为基础,采用分波序列的方法,建立了椭球粒子声辐射力的理论计算模型。进而根据声辐射力计算公式,以刚性椭球粒子和液体椭球粒子为例,计算并分析不同Bessel波束作用下椭球粒子的轴向声辐射力函数特征。数值仿真计算结果表明,对于刚性椭球粒子,扁平椭球粒子相对于细长椭球粒子更有助于激发负声辐射力;对于液体椭球粒子,细长椭球粒子相对于扁平椭球粒子更加容易产生负声辐射力;对于不同介质的椭球粒子,不同的入射波束激发的负声辐射力的效果也存在明显的差异。该结果为复杂的尺寸和介质粒子声操控技术提供了理论的可行性。     

Gauss声束对离轴椭圆柱的声辐射力矩

利用部分波展开法求解得到了Gauss声束入射下刚性和非刚性椭圆柱的声散射系数,推导了一般情况下的声辐射力矩表达式.在此基础上,通过一系列数值仿真详细分析了离轴距离、入射角度和束腰半径对声辐射力矩的影响.结果表明:正向与负向声辐射力矩均可以在一定条件下存在;低频情况下刚性椭圆柱比非刚性椭圆柱更容易产生较强的声辐射力矩;特定频率的入射声场可以激发出非刚性椭圆柱不同阶的共振散射模式,因而非刚性椭圆柱的声辐射力矩峰值与频率的关系更密切;增加束腰半径有利于扩大散射截面,进而增加椭圆柱的声辐射力矩.该研究结果预期可以为利用声辐射力矩实现粒子的可控旋转和流体黏度的反演提供一定的理论指导.     

利用声辐射模态重构任意目标的散射声场

水下目标散射声场的重构可以作为水下目标散射特性的研究基础。本文主要利用声辐射模态对水下目标进行散射声场重构研究。首先,在借助声传递矩阵给出的任意结构声辐射模态的流体域求解方法基础上,通过理论证明了目标的散射声压与声辐射模态具有函数关系。其次,借助声场分布模态的概念,同时考虑到声场分布模态病态及声压测量易受噪声污染,提出基于声辐射模态的正则化散射声场重构算法。仿真结果表明,波数越低,重构所需声辐射模态阶数越少,在较高波数时仅需总模态数的大约20%即可对声场进行重构。与基于边界元的声场重构算法相比,计算量减小了至少80%,且克服了赫姆霍兹积分方程最小二乘法仅对球壳结构的重构效果较好而不适用于长条形结构重构的缺陷。     

平面行波场中多个粒子受到的声辐射力*

从粒子在行波场中的声散射出发,研究当声场中存在稀疏分布的多个粒子时粒子受到的声辐射力,并且给出了适用于声场中任意位置的粒子声辐射力计算公式。由于声辐射力为非线性力,当声场中存在多个粒子时,直接计算粒子受到的声辐射力非常复杂。结果表明,当声场中存在多个稀疏分布的粒子时,这一多粒子系统可以视为多个单独的双粒子系统的叠加,只需要分别计算各个双粒子系统的声辐射力就可以通过叠加得到声场中任意粒子的声辐射力。这一结果有助于利用声辐射力对微小粒子进行精细操控。     

声驻波场中空化泡的动力学特性

以水为工作介质, 考虑了液体的可压缩性, 研究了驻波声场中空化泡的运动特性, 模拟了驻波场中各位置处空化泡的运动状态以及相关参数对各位置处空化泡在主Bjerknes力作用下运动方向的影响. 结果表明: 驻波声场中, 空化泡的运动状态分为三个区域, 即在声压波腹附近空化泡做稳态空化, 在偏离波腹处空化泡做瞬态空化, 在声压波节附近, 空化泡在主Bjerknes 力作用下, 一直向声压波节处移动, 显示不发生空化现象; 驻波场中声压幅值增加有利于空化的发生, 但声压幅值增加到一定上限时, 压力波腹区域将排斥空化泡, 并驱赶空化泡向压力波节移动, 不利于空化现象的发生; 当声频率小于初始空化泡的共振频率时, 声频率越高, 由于主Bjerknes 力的作用将有更多的空化泡向声压波节移动, 不利于空化的发生, 尤其是驻波场液面的高度不应是声波波长的1/4; 当声频率一定时, 空化泡初始半径越大越有利于空化现象的发生, 但当空化泡的初始半径超过声频率的共振半径时, 由于主Bjerknes力的作用将有更多的空化泡向声压波节移动, 不利于空化的发生.     

基于联合波叠加法的浅海信道下圆柱壳声辐射研究

针对浅海信道下弹性结构声辐射预报尚无高效可靠的研究方法,提出了一种浅海信道下弹性结构声辐射快速预报的联合波叠加法.该方法结合了浅海信道传输函数、多物理场耦合数值计算法和波叠加法理论,运用该方法可对浅海信道下弹性结构辐射声场进行快速预报.经数值法和解析解法验证后,从信道下辐射源、环境影响和辐射声场测量的角度研究分析了浅海信道下弹性圆柱壳的声辐射特性,阐释了进行浅海信道下结构声辐射研究的必要性.研究结果表明,仅在低频浅海信道下弹性结构可近似等效为点源,信道上下边界对声场产生显著的耦合影响,高频段的空间声场指向性分布尤为明显,垂直线列阵进行信道下结构辐射声功率测量时,测量结果受到信道环境边界和潜深的影响较大.     

声操控微粒研究进展*

声操控微粒是利用声波与微粒之间动量和能量交换产生的声辐射力操纵微粒的运动,具有非接触、生物兼容性好、无需对微粒进行化学生物标记、装置简单易集成等优点,在精密制造、精准医疗等领域具有广阔的应用前景,是当前操控领域的研究热点。该文主要综述最近十年声辐射力理论研究、声场调控方法以及微粒操控形式等方面的研究工作,并对声操控的未来发展方向进行了展望。     

高斯波束对界面附近离轴球形粒子的轴向声辐射力

从声波的散射理论出发,利用级数展开法得到高斯波束的波束因子,推导其对阻抗边界下离轴球形粒子声辐射力.针对刚性球与液体球两种球形粒子进行数值模拟,与自由空间的情况进行比较.讨论边界反射系数、粒子与边界距离、束腰半径以及离轴角度与距离等对声辐射力的影响.仿真结果表明：边界反射系数的增大会引起声辐射力的增加,但不改变峰值的位置;在合适的频率处,可以产生负向声辐射力;声辐射力随粒子与边界距离呈周期性变化;束腰半径的影响主要体现在中高频;随着粒子偏离传播轴的距离和角度增大,声辐射力明显衰减.该研究为利用高斯波束实现对粒子的操纵提供理论基础.     

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.     

Interaction of a high-order Bessel beam with a submerged spherical ultrasound contrast agent shell - Scattering theory

Background and objective

Acoustic scattering properties of ultrasound contrast agents are useful in extending existing or developing new techniques for biomedical imaging applications. A useful first step in this direction is to investigate the acoustic scattering of a new class of acoustic beams, known as helicoidal high-order Bessel beams, to improve the understanding of their scattering characteristics by an ultrasound contrast agent, which at present is very limited.

Method

The transverse acoustic scattering of a commercially available albuminoidal ultrasound contrast agent shell filled with air or a denser gas such as perfluoropropane and placed in a helicoidal Bessel beam of any order is examined numerically. The shell is assumed to possess an outer radius a = 3.5 microns and a thickness of ∼105 nm. Moduli of the total and resonance transverse acoustic scattering form functions are numerically evaluated in the bandwidth 0 < ka? 3, which corresponds to a frequency bandwidth of 0-205 MHz that covers a wide range of applications for imaging with contrast agents. Particular attention is paid to the shell’s material, the content of its interior hollow region and the fluid surrounding its exterior. The contrast agent shell is assumed to be immersed in an ideal compressible fluid so the viscous corrections are not considered. Analytical equations are derived and numerical calculations of the total and resonance form functions are performed with particular emphasis on the effect of varying the half-cone angle, the order of the helicoidal Bessel beam as well as the fluid that fills the interior hollow space.

Results and conclusion

It is shown that shell wave resonance modes can be excited on an encapsulated micro-bubble. The forward and backscattering vanish for a helicoidal high-order Bessel beam. Additionally, the fluid filling the inner core affects the shell’s response significantly. Moreover, there is no monopole contribution to the axial scattering of a helicoidal Bessel beam of order m ? 1 so that the dynamics of contrast agents would be significantly altered. The main finding of the present theory is the suppression or enhancement for a particular resonance that may be used to advantage in imaging with ultrasound contrast agents for clinical applications.     

Axial acoustic radiation force on an elastic spherical shell near an impedance boundary for zero-order quasi-Bessel–Gauss beam

Shell structures have increasingly widespread applications in biomedical ultrasound fields such as contrast agents and drug delivery,which requires the precise prediction of the acoustic radiation force under various circumstances to improve the system efficiency.The acoustic radiation force exerted by a zero-order quasi-Bessel-Gauss beam on an elastic spherical shell near an impedance boundary is theoretically and numerically studied in this study.By means of the finite series method and the image theory,a zero-order quasi-Bessel-Gauss beam is expanded in terms of spherical harmonic functions,and the exact solution of the acoustic radiation force is derived based on the acoustic scattering theory.The acoustic radiation force function,which represents the radiation force per unit energy density and per unit cross-sectional surface,is especially investigated.Some simulated results for a polymethyl methacrylate shell and an aluminum shell are provided to illustrate the behavior of acoustic radiation force in this case.The simulated results show the oscillatory property and the negative radiation force caused by the impedance boundary.An appropriate relative thickness of the shell can generate sharp peaks for a polymethyl methacrylate shell.Strong radiation force can be obtained at small half-cone angles and the beam waist only affects the results at high frequencies.Considering that the quasi-Bessel-Gauss beam possesses both the energy focusing property and the non-diffracting advantage,this study is expected to be useful in the development of acoustic tweezers,contrast agent micro-shells,and drug delivery applications.     

Radiation force and torque on a two-dimensional circular cross-section of a non-viscous eccentric layered compressible cylinder in acoustical standing waves

The purpose of this study is to develop an analytical formalism and derive series expansions for the time-averaged force and torque exerted on a compound coated compressible liquid-like cylinder,insonified by acoustic standing waves having an arbitrary angle of incidence in the polar(transverse)plane.The host medium of wave propagation and the eccentric liquid-like cylinder are non-viscous.Numerical computations illustrate the theoretical analysis with particular emphases on the eccentricity of the cylinder,the angle of incidence and the dimensionless size parameters of the inner and coating cylindrical fluid materials.The method to derive the acoustical scattering,and radiation force and torque components conjointly uses modal matching with the addition theorem,which adequately account for the multiple wave interaction effects between the layer and core fluid materials.The results demonstrate that longitudinal and lateral radiation force components arise.Moreover,an axial radiation torque component is quantified and computed for the non-absorptive compound cylinder,arising from geometrical asymmetry considerations as the eccentricity increases.The computational results reveal the emergence of neutral,positive,and negative radiation force and torque depending on the size parameter of the cylinder,the eccentricity,and the angle of incidence of the insonifying field.Moreover,based on the law of energy conservation applied to scattering,numerical verification is accomplished by computing the extinction/scattering energy efficiency.The results may find some related applications in fluid dynamics,particle trapping,mixing and manipulation using acoustical standing waves.     

Ultrasonic surface acoustic wave-assisted separation of microscale droplets with varying acoustic impedance

In droplet-based microfluidic platforms, precise separation of microscale droplets of different chemical composition is increasingly necessary for high-throughput combinatorial chemistry in drug discovery and screening assays. A variety of droplet sorting methods have been proposed, in which droplets of the same kind are translocated. However, there has been relatively less effort in developing techniques to separate the uniform-sized droplets of different chemical composition. Most of the previous droplet sorting or separation techniques either rely on the droplet size for the separation marker or adopt on-demand application of a force field for the droplet sorting or separation. The existing droplet microfluidic separation techniques based on the in-droplet chemical composition are still in infancy because of the technical difficulties. In this study, we propose an acoustofluidic method to simultaneously separate microscale droplets of the same volume and dissimilar acoustic impedance using ultrasonic surface acoustic wave (SAW)-induced acoustic radiation force (ARF). For extensive investigation on the SAW-induced ARF acting on both cylindrical and spherical droplets, we first performed a set of the droplet sorting experiments under varying conditions of acoustic impedance of the dispersed phase fluid, droplet velocity, and wave amplitude. Moreover, for elucidation of the underlying physics, a new dimensionless number AR_D was introduced, which was defined as the ratio of the ARF to the drag force acting on the droplets. The experimental results were comparatively analyzed by using a ray acoustics approach and found to be in good agreement with the theoretical estimation. Based on the findings, we successfully demonstrated the simultaneous separation of uniform-sized droplets of the different acoustic impedance under continuous application of the acoustic field in a label-free and detection-free manner. Insomuch as on-chip, precise separation of multiple kinds of droplets is critical in many droplet microfluidic applications, the proposed acoustofluidic approach will provide new prospects for microscale droplet separation.     

On acoustic radiation accompanying vibrations of a charged droplet

The dispersion equation relation for the spectrum of capillary oscillations of a charged droplet in a compressible ambient has been derived. It has been shown that such oscillations in the case of droplets, whose sizes and charges are typical of drops in clouds, fogs, or raindrops, cause the generation of sonic and ultrasonic waves in the ambient. An expression for the total intensity of the acoustic radiation generated by a single droplet has been obtained.     

自由空间中球形粒子的负向声辐射力

针对一定声场作用下自由空间中的球形粒子,首先分析了声散射过程中的吸收声功率、散射声功率和损失声功率以及三者之间的关系,并通过计算发现了由于参数选取不当导致的负吸收现象。接着从动量守恒定律出发推导了声辐射力的一般表达式,阐释了声辐射力与声能流之间的关系,并从理论和计算两方面验证了负向声辐射力的存在。当负向声辐射力产生时,声波的背向散射被抑制。在此基础上,进一步研究了粒子的偏心特性和流体的黏度这两种常见因素对负向声辐射力的影响。利用球函数的加法公式推导了偏心球的散射系数和声辐射力公式,结果显示偏心距离、粒子的材料等都会显著改变负向声辐射力的产生条件。在低频近似下,由于流体黏度附加的正向声辐射力是否能完全抵消原来的负向声辐射力将决定最终的声辐射力方向。该结果对利用负向声辐射力制成单行波声学镊子来实现对特定粒子的操控有着理论指导意义。      

计算水下凹面目标散射声场的声束弹跳法

为了解决含有多次散射时水下目标声散射场的计算问题,提出了一种声束弹跳方法。把入射声波划分为若干声束,根据几何声学方法计算每条声束在目标表面的反射方向和能量损失,利用物理声学方法计算最后一次反射的声束所对应的面元的散射场,通过计算所有声束产生的散射场的叠加得到整个目标的散射场。计算了直角凹面圆锥体的散射声场,并对具体模型进行了水池测量实验,理论计算和实验测量结果一致。表明该方法作为一种高频近似的数值计算方法,可以计算存在多次散射时水下目标的散射声场。