Spatial confinement of ultrasonic force fields in microfluidic channels

We demonstrate and investigate multiple localized ultrasonic manipulation functions in series in microfluidic chips. The manipulation functions are based on spatially separated and confined ultrasonic primary radiation force fields, obtained by local matching of the resonance condition of the microfluidic channel. The channel segments are remotely actuated by the use of frequency-specific external transducers with refracting wedges placed on top of the chips. The force field in each channel segment is characterized by the use of micrometer-resolution particle image velocimetry (micro-PIV). The confinement of the ultrasonic fields during single- or dual-segment actuation, as well as the cross-talk between two adjacent fields, is characterized and quantified. Our results show that the field confinement typically scales with the acoustic wavelength, and that the cross-talk is insignificant between adjacent fields. The goal is to define design strategies for implementing several spatially separated ultrasonic manipulation functions in series for use in advanced particle or cell handling and processing applications. One such proof-of-concept application is demonstrated, where flow-through-mode operation of a chip with flow splitting elements is used for two-dimensional pre-alignment and addressable merging of particle tracks.     

A three-dimensional model for T-shaped acoustic resonators with sound absorption materials

Recent development in noise control using T-shaped acoustic resonators calls for the development of more reliable and accurate models to predict their acoustic characteristics, which is unfortunately lacking in the literature. This paper attempts to establish such a model based on three-dimensional theory for T-shaped acoustic resonators containing sound absorption materials. The model is validated by experiments using various configurations. Predictions on fundamental and high-order resonance frequencies are compared with those obtained from the one-dimensional model and finite element analyses, and the effects of the physical and geometric parameters of the absorption materials on the resonance frequencies and Q-factor are also investigated numerically and experimentally. Limitations and applicability of existing one-dimensional models are assessed. The proposed general three-dimensional model proved to be able to provide an accurate and reliable prediction on the resonance frequencies for T-shaped acoustic resonators with or without absorption materials. This can eventually meet the requirement for resonator array design in terms of accuracy.     

Noise control in enclosures: modeling and experiments with T-shaped acoustic resonators

This paper presents a theoretical and experimental study of noise control in enclosures using a T-shaped acoustic resonator array. A general model with multiple resonators is developed to predict the acoustic performance of small resonators placed in an acoustic enclosure. Analytical solutions for the sound pressure inside the enclosure and the volume velocity source strength out of the resonator aperture are derived when a single resonator is installed, which provides insight into the physics of acoustic interaction between the enclosure and the resonator. Based on the understanding of the coupling between the individual resonators and enclosure modes, both targeted and nontargeted, a sequential design methodology is proposed for noise control in the enclosure using an array of acoustic resonators. Design examples are given to illustrate the control performance at a specific or at several resonance peaks within a frequency band of interest. Experiments are conducted to systematically validate the theory and the design method. The agreement between the theoretical and experimental results shows that, with the help of the presented theory and design methodology, either single or multiple resonance peaks of the enclosure can be successfully controlled using an optimally located acoustic resonator array.     

The design of Helmholtz resonator based acoustic lenses by using the symmetric Multi-Level Wave Based Method and genetic algorithms

Sonic crystals can be used as acoustic lenses in certain frequencies and the design of such systems by creating vacancies and using genetic algorithms has been proven to be an effective method. So far, rigid cylinders have been used to create such acoustic lens designs. On the other hand, it has been proven that Helmholtz resonators can be used to construct acoustic lenses with higher refraction index as compared to rigid cylinders, especially in low frequencies by utilizing their local resonances. In this paper, these two concepts are combined to design acoustic lenses that are based on Helmholtz resonators. The Multi-Level Wave Based Method is used as the prediction method. The benefits of the method in the context of design procedure are demonstrated. In addition, symmetric boundary conditions are derived for more efficient calculations. The acoustic lens designs that use Helmholtz resonators are compared with the acoustic lens designs that use rigid cylinders. It is shown that using Helmholtz resonator based sonic crystals leads to better acoustic lens designs, especially at the low frequencies where the local resonances are pronounced.     

Mode-switching: A new technique for electronically varying the agglomeration position in an acoustic particle manipulator

Acoustic radiation forces offer a means of manipulating particles within a fluid. Much interest in recent years has focussed on the use of radiation forces in microfluidic (or “lab on a chip”) devices. Such devices are well matched to the use of ultrasonic standing waves in which the resonant dimensions of the chamber are smaller than the ultrasonic wavelength in use. However, such devices have typically been limited to moving particles to one or two predetermined planes, whose positions are determined by acoustic pressure nodes/anti-nodes set up in the ultrasonic standing wave. In most cases devices have been designed to move particles to either the centre or (more recently) the side of a flow channel using ultrasonic frequencies that produce a half or quarter wavelength over the channel, respectively.It is demonstrated here that by rapidly switching back and forth between half and quarter wavelength frequencies - mode-switching - a new agglomeration position is established that permits beads to be brought to any arbitrary point between the half and quarter-wave nodes. This new agglomeration position is effectively a position of stable equilibrium. This has many potential applications, particularly in cell sorting and manipulation. It should also enable precise control of agglomeration position to be maintained regardless of manufacturing tolerances, temperature variations, fluid medium characteristics and particle concentration.     

Acoustic metasurface for refracted wave manipulation

Here we present a design of a transmitted acoustic metasurface based on a single row of Helmholtz resonators with varying geometric parameters. The proposed metasurface can not only steer an acoustic beam as expected from the generalized Snell's law of refraction, but also exhibits various interesting properties and potential applications such as insulation of two quasi-intersecting transmitted sound waves, ultrasonic Bessel beam generator, frequency broadening effect of anomalous refraction and focusing.     

Investigation of two-dimensional acoustic resonant modes in a particle separator

Within an acoustic standing wave particles experience acoustic radiation forces, a phenomenon which is exploited in particle or cell manipulation devices. When developing such devices, one-dimensional acoustic characteristics corresponding to the transducer(s) are typically of most importance and determine the primary radiation forces acting on the particles. However, radiation forces have also been observed to act in the lateral direction, perpendicular to the primary radiation force, forming striated patterns. These lateral forces are due to lateral variations in the acoustic field influenced by the geometry and materials used in the resonator. The ability to control them would present an advantage where their effect is either detrimental or beneficial to the particle manipulation process. The two-dimensional characteristics of an ultrasonic separator device have been modelled within a finite element analysis (FEA) package. The fluid chamber of the device, within which the standing wave is produced, has a width to height ratio of approximately 30:1 and it is across the height that a half-wavelength standing wave is produced to control particle movement. Two-dimensional modal analyses have calculated resonant frequencies which agree well with both the one-dimensional modelling of the device and experimentally measured frequencies. However, these two-dimensional analyses also reveal that these modes exhibit distinctive periodic variations in the acoustic pressure field across the width of the fluid chamber. Such variations lead to lateral radiation forces forming particle bands (striations) and are indicative of enclosure modes. The striation spacings predicted by the FEA simulations for several modes compare well with those measured experimentally for the ultrasonic particle separator device. It is also shown that device geometry and materials control enclosure modes and therefore the strength and characteristics of lateral radiation forces, suggesting the potential use of FEA in designing for the control of enclosure modes in similar particle manipulator devices.     

Finite-difference and finite-volume methods for nonlinear standing ultrasonic waves in fluid media

In the framework of the application of high-power ultrasonics in industrial processing in fluid media, the mathematical prediction of the acoustical parameters inside resonators should improve the development of practical systems. This can be achieved by the use of numerical tools able to treat the nonlinear acoustics involved in these phenomena. In particular, effects like nonlinear distortion and nonlinear attenuation are fundamental in applications. In this paper, three one-dimensional numerical models in the time domain for calculating the nonlinear acoustic field inside a one-dimensional resonant cavity are presented and compared. They are based on the finite-difference and the finite-volume methods. These different algorithms solve the differential equations, from the linear up to the strongly nonlinear case (including weak shock). Some physical results obtained from the modelling of ultrasonic waves and a comparison of the efficiency of the different algorithms are presented.     

Panel of resonators with variable resonance frequency for noise control

The article focuses on acoustic resonators made of perforated sheets bonded onto honeycomb cavities. This kind of resonators can be used in adverse conditions such as high temperature, dirt and mechanical constraints. For all these reasons, they are, for example, widely used in aeronautic applications. The acoustic properties are directly linked to the size, shape and porosity of holes and to the thickness of air gaps. Unfortunately, the acoustic absorption of these resonators is selective in frequency and conventional acoustic resonators are only well adapted to tonal noises. In case of variable tonal noise, the efficiency is limited if the resonators are not tunable. One common solution is to control the depth of cavities based on the noise to be attenuated. This article proposes another technology of tunable resonators with only a very small mass and size increase. It consists of two superposed and identically perforated plates associated with cavities. One plate is fixed and bonded to the cavities and the other plate is mobile. The present concept enables to change the internal shapes of the holes of the perforated layers. The article describes this system and gives a theoretical model of the normal incidence acoustic impedance that allows to predict the acoustic behavior, in particular the resonance frequency. The model shows that the resonance frequency varies with hole profiles and that the absorption peak moves towards the lower frequencies. The proposed model is validated by measurements on various configurations of resonators tested in an impedance tube. The perspectives of this work are to adapt the hole profiles using an actuator in order to perform active control of impedance.     

厚度模压电超声换能器无源声学材料研究进展*

厚度模压电超声换能器作为超声波发射、接收以及电信号间转换的载体,是超声成像与检测系统的核心器件,一般由压电层、匹配层和背衬层3部分组成。超声换能器的性能一定程度上决定着整体超声设备的性能,影响了其在工业、医学、军事等领域的应用。该换能器的关键性能指标(带宽、灵敏度)除了受到压电层的影响,还与匹配层、背衬层等无源声学材料的设计密切相关。该文综述了近年来厚度模压电超声换能器无源声学材料(匹配层、背衬层和声透镜)的研究进展,提出了当前该类材料面临的难题和解决途径,并对其未来发展方向进行了展望。     

Effectiveness of T-shaped acoustic resonators in low-frequency sound transmission control of a finite double-panel partition

Double-panel partitions are widely used for sound insulation purposes. Their insulation efficiency is, however, deteriorated at low frequencies due to the structural and acoustic resonances. To tackle this problem, this paper proposes the use of long T-shaped acoustic resonators in a double-panel partition embedded along the edges. In order to facilitate the design and assess the performance of the structure, a general vibro-acoustic model, characterizing the interaction between the panels, air cavity, and integrated acoustic resonators, is developed. The effectiveness of the technique as well as the optimal locations of the acoustic resonators is examined at various frequencies where the system exhibits different coupling characteristics. The measured optimal locations are also compared with the predicted ones to verify the developed theory. Finally, the performance of the acoustic resonators in broadband sound transmission control is demonstrated.     

Design and analysis of an acoustic demultiplexer exploiting negative density,negative bulk modulus and extra-ordinary transmission of membrane-based acoustic metamaterial

A typical membrane-based acoustic metamaterial has been reconfigured into an array of semi-flexi-walled membrane-based Helmholtz resonators to design a novel 4 port device which demultiplexes an incoming broadband acoustic signal into frequency ranges where negative modulus, density and extra-ordinary transmission behaviour exist. It has two ports exhibiting identical transfer functions and for input given in any of these ports, the device operates as a 3 port demultiplexer. The device is asymmetric in its functionality both in its degree of demultiplexing and bandwidths of the demultiplexed spectrum. Theoretical analysis of this acoustic demux is carried out by modelling it as a combination of two independent physical systems and is further validated by full-wave analysis.     

Design of an Acoustic Levitator for Three-Dimensional Manipulation of Numerous Particles

We present a design of an acoustic levitator consisting of three pairs of opposite transducer arrays.Three orthogonal standing waves create a large number of acoustic traps at which the particles are levitated in mid-air.By changing the phase difference of transducer arrays,three-dimensional manipulation of particles is successfully realized.Moreover,the relationship between the translation of particles and the phase difference is experimentally investigated,and the result is in agreement with the theoretical calculation.This design can expand the application of acoustic levitation in many fields,such as biomedicine,ultrasonic motor and new materials processing.     

Influence of atmospheric refraction on the performance of an outdoor ultrasonic pulse compression system

Nowadays, many ultrasonic sensory systems are being developed to operate outdoors, where they are finding a variety of applications, such as local positioning, vehicle navigation or obstacle detection. To assure the reliable operation of these systems under any meteorological condition, it is necessary to achieve a thorough comprehension of the effects that the different atmospheric phenomena can have on the propagation of these acoustic waves. This paper deals with one of these phenomena, atmospheric refraction, and its influence on the performance of an ultrasonic system whose signals are detected by matched filtering.     

Interplay of local resonances and Bragg band gaps in acoustic waveguides with periodic detuned resonators

This letter is concerned with acoustic wave propagation and transmission in acoustic waveguides with periodically grafted detuned Helmholtz resonators. The interplay of local resonances and Bragg band gaps in such periodic systems is examined. It is shown that, when the resonant frequencies of the resonators are tuned close to a Bragg band gap, the behavior of the Bragg band gap can be affected dramatically. Particularly, by introducing appropriately tuned resonators, the bandwidth of a Bragg band gap can be reduced to zero, leading to a very narrow pass band with great wave attenuation performance near both band edges. The band formation mechanisms of such periodic waveguides are further examined, providing explicit formulae to locate the band edge frequencies of all the band gaps, as well as the conditions to achieve very narrow pass bands in such periodic waveguides.     

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。     

Opto-acoustic sensing of fluids and bioparticles with optomechanofluidic resonators

Opto-mechano-fluidic resonators (OMFRs) are a unique optofluidics platform that can measure the acoustic properties of fluids and bioanalytes in a fully-contained microfluidic system. By confining light in ultra-high-Q whispering gallery modes of OMFRs, optical forces such as radiation pressure and electrostriction can be used to actuate and sense structural mechanical vibrations spanning MHz to GHz frequencies. These vibrations are hybrid fluid-shell modes that entrain any bioanalyte present inside. As a result, bioanalytes can now reflect their acoustic properties on the optomechanical vibrational spectrum of the device, in addition to optical property measurements with existing optofluidics techniques. In this work, we investigate acoustic sensing capabilities of OMFRs using computational eigenfrequency analysis. We analyze the OMFR eigenfrequency sensitivity to bulk fluid-phase materials as well as nanoparticles, and propose methods to extract multiple acoustic parameters from multiple vibrational modes. The new informational degrees-of-freedom provided by such opto-acoustic measurements could lead to surprising new sensor applications in the near future.     

On optimal design of half-wave resonators for acoustic damping in an enclosure

Acoustic design parameters of a half-wave resonator are studied experimentally for purely acoustic tuning of the resonator. According to the standard acoustic-test procedures, acoustic-pressure signals in the model enclosure with the resonators are measured. Based on the signals, quantitative acoustic properties of damping factor and sound absorption coefficient are evaluated and thereby, the acoustic-damping capacity of the resonator is characterized. Sound absorption coefficient has the advantages of the damping factor in various aspects. The coefficient indicates clearly the tuning frequency of the resonator, absorption effectiveness as a function of frequency, and overall damping capacity. The diameter and the number of a half-wave resonator, its distribution, and the blockage ratio at its inlet are selected as design parameters for optimal tuning of the resonator in the model enclosure. The resonators with larger diameter have the advantage of those with smaller one with respect to purely acoustic damping at the tuning frequency. The optimum number of resonators or the optimum open-area ratio decreases as boundary absorption decreases. When the open-area ratio exceeds the optimum value, over-damping appears, leading to a decrease in peak absorption coefficient and a broadening of absorption bandwidth. Blockage at the resonator inlet controls both peak absorption coefficient and its absorption bandwidth and it can be considered one of design factors for acoustic tuning.     

Surface impedance design with ground corrugation for mitigation of large-calibre gun blast noise

The surface impedance design approach is proposed for mitigating large-calibre gun blast noise. Surrounding the blast noise, we employ a group of concentric trenches with critical depths to dampen the propagation of the acoustic wave. These trenches behave like quarter-wavelength resonators and produce acoustic soft surfaces at their openings. The sound pressure is then mitigated over these soft surfaces by destructive interference and the wave attenuates rapidly along the ground surface. To evaluate the overall acoustic performance of such a design, we develop an efficient numerical solver by treating the geometry as a body of revolution (BOR). The symmetry of the structure in the revolution direction allows the 3D boundary integral equation (BIE) for acoustic wave scattering to be reduced to a 2D integral equation by the use of Fourier series expansions. Numerical experiments show that this model can effectively suppress the acoustic wave propagation horizontally and the reduction can reach about 15 dB for large-calibre gun noise with very low-frequency components.     

Transfer matrix methods for sound attenuation in resonators with perforated intruding inlets

A resonator with perforated intruding inlet (PII) is a superior silencer element, since the use of perforated inlet extensions can dramatically improve the acoustic performance. In this work, both a one-dimensional (1D) and a two-dimensional (2D) transfer matrix methods are developed to predict the transmission loss of the resonator without considering the mean flow. Based on the two groups of comparisons with tests, it is found out that the applicability of 1D method is limited by the resonator geometry even when the frequency is below the cut-off value of plane wave. Whereas the 2D approach is much more accurate while predicting the transmission losses within entire frequency range. Subsequently, five groups of resonators are chosen to determine the effects of structure parameters to transmission loss based on the 2D approach. The resonant frequency decreases and more resonant peaks appear when the length of inlet extension increases. A higher perforation rate leads to a shift of resonant peak towards higher frequencies. Besides, better acoustic performance could be obtained with the perforation being properly designed. Reducing the inlet/outlet radius can obviously improve the transmission loss without changing the frequency of resonant peak. The theories and conclusions in this study can be used for the design and optimization of resonators in various engineering applications.