Acoustic Pressure Node Research Articles

Design of Reactive Rectangular Expansion Chambers for Broadband Acoustic Attenuation Performance based on Optimal Port Location

This paper analyses the transmission loss (TL) performance of rectangular expansion chambers having a single-inlet and single-outlet (SISO) or single-inlet and double-outlet (SIDO) by means of a 3-D semi-analytical formulation based on the modal expansion and the Green’s function approach. To this end, the acoustic field inside the rigid-wall rectangular chamber is obtained as the orthogonal modal solution of the 3-D homogeneous Helmholtz equation. The SISO/SIDO rectangular chamber system is characterised using the uniform piston-driven model in terms of the impedance matrix parameters (equivalently, the acoustic pressure response function) obtained by computing the average of the 3-D Green’s function over the surface area of the inlet/outlet ports modelled as rigid pistons oscillating with uniform velocity. The TL graphs computed using the 3-D semi-analytical formulation are found to be in an excellent agreement with those obtained from the 3-D FEA for SISO test cases, thereby validating the technique presented here. A parametric investigation is conducted to study the effect of arbitrary locations of the inlet/outlet ports on the chamber surface on the TL graph. This results in the formulation of guidelines towards designing the axially short and long SISO/SIDO rectangular chambers exhibiting a broadband TL performance in terms of optimal angular and axial location of ports on the appropriate acoustic pressure nodes. In addition, characteristic features of the TL spectrum of a general reciprocal and conservative single-inlet and multiple-outlet muffler system, such as (1) the effect of interchanging the position of inlet and outlet ports and (2) analysis of peaks and troughs, are proved analytically by means of the scattering matrix parameters. These features are corroborated through the analysis of the TL graphs (obtained using the 3-D semi-analytical formulation) of the SIDO rectangular chambers.

Enhancing Macrophage Drug Delivery Efficiency via Co-Localization of Cells and Drug-Loaded Microcarriers in 3D Resonant Ultrasound Field.

In this study, a novel synthetic 3D molecular transfer system which involved the use of model drug calcein-AM-encapsulated poly(lactic-co-glycolic acid) microspheres (CAPMs) and resonant ultrasound field (RUF) with frequency of 1 MHz and output intensity of 0.5 W/cm2 for macrophage drug delivery was explored. We hypothesized that the efficiency of CAPMs-mediated drug delivery aided by RUF can be promoted by increasing the contact opportunities between cells and the micrometer-sized drug carriers due to effects of acoustic radiation forces generated by RUF. Through the fluoromicroscopic and flow cytometric analyses, our results showed that both DH82 macrophages and CAPMs can be quickly brought to acoustic pressure nodes within 20 sec under RUF exposure, and were consequently aggregated throughout the time course. The efficacy of cellular uptake of CAPMs was enhanced with increased RUF exposure time where a 3-fold augmentation (P < 0.05) was obtained after 15 min of RUF exposure. We further demonstrated that the enhanced CAPM delivery efficiency was mainly contributed by the co-localization of cells and CAPMs resulting from the application of the RUF, rather than from sonoporation. In summary, the developed molecular delivery approach provides a feasible means for macrophage drug delivery.

A theoretical and experimental study on flow characterisation in an acoustically excited chamber

Flame–acoustic wave interactions have been studied widely in the combustion community; however the whole physicochemical mechanism is still not clear. The present research aims to analyse the acoustic model inside an enclosed combustion chamber and to gain more detailed data to further study flame/acoustic interactions theoretically. The acoustic coupling can be calculated by using linear acoustic equations. The theoretical acoustic model has been developed to analyse the acoustic response for the present square tube and acoustical system. A good agreement between experimental measurements and theoretical predictions were proved by measuring the pressure and velocity fields of acoustic wave. The first four measured harmonic frequencies match well with theoretical prediction. The measured acoustic pressure node and anti-node region were also the same as theoretical prediction. However, due to the neglect of pressure drop loss in the upper end of the tube, the theoretical prediction data was from 23% to 43.3%, higher than that of the experimental measurements.

Dynamical behaviors of cavitation bubble under acoustic standing wave field

Considering the compressibility of liquid, we investigate the dynamical behaviors of a cavitation bubble in an acoustic standing wave field by regarding water as a work medium. The motion state of the cavitation bubble at each position is simulated in the standing wave field, the influences of the primary Bjerknes force on the motion direction of the cavitation bubble at each position are also simulated with different relevant parameters. The results show that in the standing wave field, the motion state of the cavitation bubble is divided into three aspects: the cavitation bubble is of steady-state cavitation near the pressure antinode; the cavitation bubble is of transient cavitation at the position deviating from the pressure antinode; in the vicinity of the acoustic pressure node, the cavitation bubble has been moving to the acoustic pressure node due to the primary Bjerknes force, so the phenomenon of cavitation does not occur. In the standing wave field, when the acoustic pressure amplitude exceeds its upper limit, the primary Bjerknes force makes the cavitation bubble move to pressure node, it is not conducive to the occurrence of cavitation. When the acoustic frequency is smaller than the bubble resonant frequency, the primary Bjerknes force will make more cavitation bubbles move to acoustic pressure node with the increase of the acoustic pressure, so this is not conducive to the occurrence of cavitation. Especially, the height of the liquid level should not be a quarter of acoustic wavelength. For a given acoustic frequency, the larger the initial radius of cavitation bubble, the more favorable the occurrence of cavitation is. But when the initial radius of cavitation bubble exceeds the resonant radius of acoustic frequency, the bubble will be pushed to pressure node. That is to say, the acoustic pressure amplitude, the acoustic frequency, and the initial radius of cavitation bubble each have a corresponding limit. Moreover, the lower limit is conducive to the occurrence of the phenomenon of cavitation.

An analytical study of the acoustic force implication on the settling velocity of non‐spherical particles in the incompressible Newtonian fluid

AbstractIn this paper, the effect of the acoustic radiation force on the acceleration motion of a vertically falling non‐spherical particle in the incompressible Newtonian fluid is investigated. The using of the acoustic radiation force for increasing the falling velocity of particles (especially non‐spherical particles) has the most important implication on increasing the efficiency of particulate control devices such as gravity settling chambers. In this study, the motion of the non‐spherical particles in the fluids such as water can be described by the force balance equation (Basset–Boussinesq–Ossen equation) plus the acoustic radiation force term. The main difficulty in the solution of this equation lies in the nonlinear term due to the nonlinearity nature of the drag coefficient. The settling velocity was calculated by using the differential transformation method, which is an analytical solution technique. The effect of some parameters such as the particle's sphericity (0.3, 0.5, and 0.7), the amplitude of the acoustic pressure wave (200, 400, and 600 KPa), the distance from the acoustic pressure node (0.2, 0.4, and 0.6 m), and the acoustic source frequency (20, 40, and 60 KHz) on the settling velocity as a function of time was fully studied in this paper. © 2014 Curtin University of Technology and John Wiley &amp; Sons, Ltd.

Particle Motion in a Macroscale, Multiwavelength Acoustic Field

Particle motion due to ultrasonic acoustic radiation in a macroscale, multiwavelength acoustic chamber is investigated and compared with available theories. Primary acoustic radiation force theory has been extensively developed to predict single particle motion in a microscale, single-node acoustic chamber/channel. There is a need to investigate the applicability of this theory to macroscale, multiwavelength acoustic channels utilizing the acoustic radiation force for separating polydispersed particles. A particle-tracking velocimetry (PTV) approach for measuring individual particle motion is developed specifically to track particles as they densify at an acoustic pressure node. Particle motion is tracked over the lifetime of their motion to a node. Good agreement between the experimental and theoretical results is observed in the early stages of particle motion, where particles can be considered individually. Only in the densified region of the acoustic pressure node is there some mismatch with theory. The acoustic energy density of the acoustic chamber, a parameter intrinsically associated with the system by the theory, is also determined experimentally for different conditions and shown to be constant for all investigated system settings. The investigation demonstrates the capability of available theory in predicting the motion of polydispersed particles in macroscale, multiwavelength acoustic chambers.

Laser vibrometry characterisation of a microfluidic lab-on-a-chip device: a preliminary investigation

Since their original inception as ultrasound contrast agents, potential applications of microbubbles have evolved to encompass molecular imaging and targeted drug delivery. As these areas develop, so does the need to understand the mechanisms behind the interaction of microbubbles both with biological tissue and with other microbubbles. There is therefore a metrological requirement to develop a controlled environment in which to study these processes. Presented here is the design and characterisation of such a system, which consists of a microfluidic chip, specifically developed for manipulating microbubbles using both optical and acoustic trapping. A laser vibrometer is used to observe the coupling of acoustic energy into the chip from a piezoelectric transducer bonded to the surface. Measurement of the velocity of surface waves on the chip is investigated as a potential method for inferring the nature of the acoustic fields excited within the liquid medium of the device. Comparison of measured surface wavelengths with wave types suggests the observation of anti-symmetric Lamb or Love-Kirchhoff waves. Further visual confirmation of the acoustic fields through bubble aggregation highlights differences between the model and experimental results in predicting the position of acoustic pressure nodes in relation to excitation frequency.

An On-Chip, Multichannel Droplet Sorter Using Standing Surface Acoustic Waves

The emerging field of droplet microfluidics requires effective on-chip handling and sorting of droplets. In this work, we demonstrate a microfluidic device that is capable of sorting picoliter water-in-oil droplets into multiple outputs using standing surface acoustic waves (SSAW). This device integrates a single-layer microfluidic channel with interdigital transducers (IDTs) to achieve on-chip droplet generation and sorting. Within the SSAW field, water-in-oil droplets experience an acoustic radiation force and are pushed toward the acoustic pressure node. As a result, by tuning the frequency of the SSAW excitation, the position of the pressure nodes can be changed and droplets can be sorted to different outlets at rates up to 222 droplets s(-1). With its advantages in simplicity, controllability, versatility, noninvasiveness, and capability to be integrated with other on-chip components such as droplet manipulation and optical detection units, the technique presented here could be valuable for the development of droplet-based micro total analysis systems (μTAS).

Varying the agglomeration position of particles in a micro-channel using Acoustic Radiation Force beyond the resonance condition

It is well-known that particles can be focused at mid-height of a micro-channel using Acoustic Radiation Force (ARF) tuned at the resonance frequency (h=λ/2). The resonance condition is a strong limitation to the use of acoustophoresis (particles manipulation using acoustic force) in many applications. In this study we show that it is possible to focus the particles anywhere along the height of a micro-channel just by varying the acoustic frequency, in contradiction with the resonance condition. This result has been thoroughly checked experimentally. The different physical properties as well as wall materials have been changed. The wall materials is finally the only critical parameters. One of the specificity of the micro-channel is the thickness of the carrier and reflector layer. A preliminary analysis of the experimental results suggests that the acoustic focusing beyond the classic resonance condition can be explained in the framework of the multilayered resonator proposed by Hill [1]. Nevertheless, further numerical studies are needed in order to confirm and fully understand how the acoustic pressure node can be moved over the entire height of the micro channel by varying the acoustic frequency. Despite some uncertainties about the origin of the phenomenon, it is robust and can be used for improved acoustic sorting or manipulation of particles or biological cells in confined set-ups.

On-chip measurements of cell compressibility via acoustic radiation

Measurements of mechanical properties of biological cells are of great importance because changes in these properties can be strongly associated with the progression of cell differentiation and cell diseases. Although state of the art methods, such as atomic force microscopy, optical tweezers and micropipette aspiration, have been widely used to measure the mechanical properties of biological cells, all these methods involve direct contact with the cell and the measurements could be affected by the contact or any local deformation. In addition, all these methods typically deduced the Young's modulus of the cells based on their measurements. Herein, we report a new method for fast and direct measurement of the compressibility or bulk modulus of various cell lines on a microchip. In this method, the whole cell is exposed to acoustic radiation force without any direct contact. The method exploits the formation of an acoustic standing wave within a straight microchannel. When the polystyrene beads and cells are introduced into the channel, the acoustic radiation force moves them to the acoustic pressure node and the movement speed is dependent on the compressibility. By fitting the experimental and theoretical trajectories of the beads and the cells, the compressibility of the cells can be obtained. We find that the compressibility of various cancer cells (MCF-7: 4.22 ± 0.19 × 10(-10) Pa(-1), HEPG2: 4.28 ± 0.12 × 10(-10) Pa(-1), HT-29: 4.04 ± 0.16 × 10(-10) Pa(-1)) is higher than that of normal breast cells (3.77 ± 0.09 × 10(-10) Pa(-1)) and fibroblast cells (3.78 ± 0.17 × 10(-10) Pa(-1)). This work demonstrates a novel acoustic-based method for on-chip measurements of cell compressibility, complementing existing methods for measuring the mechanical properties of biological cells.

Flow-Acoustic Coupling in T-Junctions: Effect of T-Junction Geometry

The flow-acoustic coupling mechanism in a T-junction, which combines flows from two branches, forming the “cross-bar” of the T-junction, into one pipe, forming the “stem” of the T-junction, is investigated experimentally. The T-junction has a step pipe expansion at its inlets. The shear layer separating from this step expansion is found to excite intense acoustic resonances over multiple ranges of flow velocity. The excited acoustic mode is confined to the branch pipes and has an acoustic pressure node at the centerline of the T-junction. The length of the expansion section of the T-junction is found to control the frequency of the shear layer oscillation and therefore determines the ranges of flow velocity over which acoustic resonances are excited. Introducing asymmetry in the T-junction expansion length has shown little influence on the excitation of acoustic resonance. An additional T-junction arrangement made of rectangular cross-sectional ducts is also investigated to facilitate a flow visualization study of unsteady flow structures in the T-junction during acoustic resonance, and thereby improve understanding of the acoustic resonance mechanism and the nature of the aero-acoustic sources in the T-junction.

Performance of a quarter-wavelength particle concentrator

A series of devices have been investigated which use acoustic radiation forces to concentrate micron sized particles. These multi-layered resonators use a quarter-wavelength resonance in order to position an acoustic pressure node close to the top surface of a fluid layer such that particles migrate towards this surface. As flow-through devices, it is then possible to collect a concentrate of particulates by drawing off the particle stream and separating it from the clarified fluid and so can operate continuously as opposed to batch processes such as centrifugation. The methods of construction are described which include a micro-fabricated, wet-etched device and a modular device fabricated using a micro-mill. These use silicon and macor, a machinable glass ceramic, as a carrier layer between the transducer and fluid channel, respectively. Simulations using an acoustic impedance transfer model are used to determine the influence of various design parameters on the acoustic energy density within the fluid layer and the nodal position. Concentration tests have shown up to 4.4-, 6.0- and 3.2-fold increases in concentration for 9, 3 and 1 μm diameter polystyrene particles, respectively. The effect of voltage and fluid flow rates on concentration performance is investigated and helps demonstrate the various factors which determine the increase in concentration possible.

Applications of ultrasound streaming and radiation force in biosensors

Direct radiation force (DRF) and acoustic streaming provide the main influences on the behaviour of particles in aqueous suspension in an ultrasound standing wave (USW). The direct radiation force, which drives suspended particles towards and concentrates them in acoustic pressure node planes, has been applied to rapidly transfer cells in small scale analytical separators. The DRF also significantly increased the sensitivity of latex agglutination test (LAT) by concentrating the particles of an analytical sample in the pressure node positions and hence greatly increasing the antibody-antigen encounter rate. Capture of biotinylated particles and spores on a coated acoustic reflector in a quarter wavelength USW resonator was DRF-enhanced by 70- and 100-fold, respectively compared to the situation in the absence of ultrasound. Acoustic streaming has been successfully employed for mixing small analytical samples. Cavitation micro-streaming substantially enhanced, through mixing, DNA hybridization and the capture efficiency of Escherichia coli K12 on the surface of immunomagnetic beads. Acoustic streaming induced in longitudinal standing wave and flexural plate wave immuno-sensors increased the detection of antigens by a factor of five and three times, respectively. Combined DRF and acoustic streaming effects enhanced the rate of the reaction between suspended mixture of cells and retroviruses. The examples of a biochip and an ultrasonic immuno-sensor implementing the DRF and acoustic streaming effects are also described in the review.

ASYMPTOTIC FORMULATION FOR AN ACOUSTICALLY DRIVEN FIELD INSIDE A RECTANGULAR CAVITY WITH A WELL-DEFINED CONVECTIVE MEAN FLOW MOTION

When small amplitude harmonic pressure waves are introduced inside an enclosure where the classic cavity flow is already established, a strong coupling ensues between the mean and time-dependent fields. In addition to the mean flow influence, the presence of solid boundaries gives rise to a strong coupling between acoustic and vortical waves that prescribes the propagation of disturbances. Based on a well-defined mean flow structure, the purpose of this report is to arrive at closed-form expressions for the two-dimensional, time-dependent velocity and vorticity fields, in order to elucidate the nature and extent of intrinsic flow coupling. On that account, regular perturbation tools are used on the vorticity transport equation derived from the linearised Navier–Stokes equations. After an inviscid expansion is achieved, successive approximations that use viscous correction multipliers are employed to derive the vorticity in a manner to satisfy the existing boundary conditions. At the outset, unsteady vorticity is found to originate at the walls due to the acoustic pressure gradient normal to the inflow direction. In consequence, most intense vortices are initiated in regions that coincide with acoustic pressure nodes. Conversely, zero vorticity streaks emanate from acoustic velocity nodes and are carried downstream by the bulk fluid motion. From the vorticity formulation, both components of the time-dependent velocity vector are derived in a manner to satisfy continuity and momentum conservation. The axial velocity component is found to be the most significant, exhibiting characteristics associated with oscillatory flows. In addition to comparisons with numerical simulations, a limiting process validation against the existing exact solution in the event of no mean flow transmission is carried out. In closing, an error assessment is included to quantify both magnitude and order of the global error accumulated in the final asymptotic expressions.

Response of propellant combustion to a turbulent acoustic boundary layer

An analysis of the transitional and turbulent reactive acoustic boundary layer on a homogeneous, solid-propellant surface is conducted to investigate potential mechanisms of combustion instability. The theoretical approach utilizes a low Reynolds number turbulence closure model and a finite-difference solution procedure for an equation system of parabolic form. A new technique is developed for the condensed-phase thermal layer, in which the propellant space is mapped onto the gas space and efficiently solved using the same adaptive numerical grid. Results are obtained at an acoustic pressure node in the absence of a mean axial flow. The results indicate that acoustically induced transition can occur at relatively low acoustic pressure amplitudes, propellant response to harmonic axial velocity fluctuations is both rectified and displays a mean augmentation (D-C shift), and that nominal propellant combustion parameters lead to increasing susceptibility to acoustic transition at elevated mean pressures. As OQ Bg cp €« / H h h^ k ks L® n q R Reac Res Ret Ru r

Tensile strength and visible ultrasonic cavitation of superfluid4He

Motion picture photographs of cavitation in He II revealed new characteristics pertinent to the liquid's tensile strength and bubble dynamics. A cylindrical acoustic standing wave with a frequency of 50.58 kHz induced the cavitation in He II at a temperature of 2.09 K. Analysis of light diffracted by the sound gave measurements of the acoustic pressure amplitude which were used both for selecting the best drive frequency and for obtaining the tensile strength. Bubbles appeared to originate on pressure antinodes, expanded to a diameter of 0.5–1.0 mm in about 0.3 msec, and eventually fragmented into smaller bubbles. They originated where the negative pressure extremum was as small as −0.6 bar (+0, −50%), a tensile strength much smaller than the predictions of theories developed for the homogeneous nucleation of bubbles in classical liquids. The bubble fragments were frequently nonspherical and had widths of 0.1–0.2 mm. Small bubbles also displayed an unexpected preference to originate on the surface of a stainless steel tube inserted in the sound field. Subsequent to nucleation, bubbles were frequently attracted to acoustic pressure nodes in agreement with a theory of vibrations and forces originally developed for bubbles in normal liquids. Attempts to detect first and second sound radiated by cavitation are described.