Pulsed Acoustic Fields Research Articles

Apparatus for Physical Modeling of the Electroseismic Effect of the First Kind

The paper describes a modified apparatus for physically modeling the electroseismic effect of the first kind in rocks. The apparatus makes it possible to simulate the effect when a rock sample is exposed to an electric field with and without current in the sample. Two methods can be used for measuring changes in the acoustic field propagation velocity. The first method excites a pulsed acoustic field and measures the time it takes for the field to propagate from the source to the receiver; the second measures the phase modulation of a sinusoidal acoustic field when the sample is exposed to an electric field. The first method has two modifications: in the first, only an electric field without an active current component is generated in a sample; in the second, both an electric field and an active current component are generated. The apparatus includes a signal generator that excites coherent electric and acoustic fields in rock samples. Field coherence makes it possible to apply an interference-immune phase method for measuring the velocity of a sinusoidal acoustic field and to decrease the sensitivity threshold for a change in velocity from 0.2 to 0.02%. The authors present results of modeling the effect using saltwater-saturated limestone and sandstone samples (four each) with a mineralization factor of 1%. When the electric field was switched on, all samples demonstrated an approximately 0.2% decrease in acoustic field velocity. In the 2–200 kHz frequency range, the velocity decrease does not depend on frequency for all limestone and sandstone samples. It is shown that the modified apparatus can reliably detect the electroseismic effect without a current in a sample, despite the fact that its value exceeds the sensitivity threshold by only 20–25 dB. Field coherence makes it possible to measure the relaxation time of the acoustic field velocity after an electric field without a current in a sample is switched on and off. The authors demonstrate that after the electric field is switched on, the relaxation time of the acoustic field velocity does not exceed 2 ms, and after it is switched off, this value is 10–20 ms. The relaxation time difference can be used to assess the nonlinearity of the electroseismic effect of the first kind.

Effects of ultrasound pulse parameters on cavitation properties of flowing microbubbles under physiologically relevant conditions

Acoustic cavitation from ultrasound-driven microbubbles can induce diverse bioeffects that are useful in clinical therapy. However, lack of control over the cavitation activity of flowing microbubbles results in unwanted treatment regions in the targeted tissue, which influences the therapeutic efficacy and bio-safety. The aim of this study is to understand the relationship between the ultrasound pulse parameters and cavitation properties of flowing microbubbles, including the type (and transition between types), threshold, intensity and temporal distribution of cavitation. An in vitro physiological-flow phantom was fabricated, in which the microbubbles had a constant velocity, and were sonicated to a 1-MHz focused transducer at a wide range of peak negative pressures (PNPs) (0.10–1.28 MPa), pulse repetition frequencies (PRFs) (1–200 Hz) and pulse lengths (PLs) (10–400 μs). The signals from the flowing bubbles were passively detected by another 7.5-MHz plane transducer. From detailed time- and frequency-domain analysis, we found 1). The occurrence of stable cavitation (SC) and inertial cavitation (IC) depended on PNP and PL when the PRF was below a critical value (PRF threshold) that related to the fluid velocity and PNP full width at half maximum diameter of the transducer. 2) Below the PRF threshold, the PL had no influence on the temporal distribution of SC intensity; however, above the PRF threshold, the SC properties depended on the PL because of acoustically-driven diffusion. Specifically, at shorter PLs, the SC intensity had a uniform temporal distribution and was independent of the PRF; at longer PLs, the SC intensity correlated negatively with the PRF. 3) Below the PRF threshold, the IC properties were independent of the PRF. Increasing the PRF above the PRF threshold caused the IC intensity to decrease with a non-uniform temporal distribution. These results indicate that the fluid velocity and a pulsed acoustic field influence the number and properties of the replenished bubbles into the targeted region, resulting in the change of cavitation properties. In future therapeutic applications, the physiological fluid conditions must be taken into consideration to design reasonable pulse parameters and achieve desirable cavitation properties.

Metaparticles of Pulsed Wave Fields

Properties of elementary particles of pulsed (localized) electromagnetic and acoustic wave fields directionally radiated by aperture sources (antennas, lasers, acoustic diaphragms) in free space and homogeneous isotropic media, for which the term metaparticle is used, are considered. The structure of the wave field and the energy transferred by the metaparticles are presented. The results obtained for the presented scalar theory of metaparticles of pulsed transverse electromagnetic (TEM) waves and longitudinal elastic acoustic waves can be extended to vector pulsed wave modes guided by layered inhomogeneous guiding media and guiding technical devices (waveguides).

On measurement of acoustic pulse arrival angles using a vertical array

We consider a recently developed method to analyze the angular structure of pulsed acoustic fields in an underwater sound channel. The method is based on the Husimi transform that allows us to approximately link a wave field with the corresponding ray arrivals. The advantage of the method lies in the possibility of its practical realization by a vertical hydrophone array crossing only a small part of the oceanic depth. The main aim of the present work is to find the optimal parameter values of the array that ensure good angular accuracy and sufficient reliability of the algorithm to calculate the arrival angles. Broadband pulses with central frequencies of 80 and 240 Hz are considered. It is shown that an array with a length of several hundred meters allows measuring the angular spectrum with an accuracy of up to 1 degree. The angular resolution is lowered with an increase of the sound wavelength due to the fundamental limitations imposed by the uncertainty relation.

Computational dynamics of acoustically driven microsphere systems.

We propose a computational framework for the self-consistent dynamics of a microsphere system driven by a pulsed acoustic field in an ideal fluid. Our framework combines a molecular dynamics integrator describing the dynamics of the microsphere system with a time-dependent integral equation solver for the acoustic field that makes use of fields represented as surface expansions in spherical harmonic basis functions. The presented approach allows us to describe the interparticle interaction induced by the field as well as the dynamics of trapping in counter-propagating acoustic pulses. The integral equation formulation leads to equations of motion for the microspheres describing the effect of nondissipative drag forces. We show (1) that the field-induced interactions between the microspheres give rise to effective dipolar interactions, with effective dipoles defined by their velocities and (2) that the dominant effect of an ultrasound pulse through a cloud of microspheres gives rise mainly to a translation of the system, though we also observe both expansion and contraction of the cloud determined by the initial system geometry.

Importance of Bubble Size Control in Ultrasonic Surface Cleaning by Pulsed High-Frequency Sound Fields

The occurrence of acoustic cavitation is incorporating a multitude of interdependent effects that strongly depend on the bubble size. Therefore, bubble size control would be beneficial for industrial processes that rely on acoustic cavitation. A pulsed acoustic field can result in bubble size control and bubble recycling, which strongly enhances cavitation activity. In this paper, we present experimentally determined bubble size distributions for different power densities, which are interpreted in the frame of numerical calculations of the oscillatory responses of the bubbles to the intermittent driving sound field. The distributions are found to be shaped by the size dependent interplay between bubble pulsations, rectified diffusion, coalescence and the development of parametrically amplified shape instabilities. Furthermore, a relation between cleaning and acoustic cavitation activity is established. The results show that the changed acoustic conditions bubbles experience in close vicinity to a solid surface, need to be taken into account. The influence of the inertial type of cavitation activity on the structural damage that is occurring during the cleaning process, is evaluated. It is found that the damage to the fragile structures that ought to be cleaned, is significantly reduced, when the applied power is kept below values, for which inertial cavitation is observed.

Nanoparticle Removal with Megasonics: A Review

Nanoparticle removal obtained without inflicting damage to fragile device elements remains a big challenge. The feasibility of physical cleans is assessed and boundary conditions are outlined. An overview of megasonic cleaning process improvements is given. In order to reduce damage without reducing particle removal frequencies during high frequency ultrasound cleaning processes, cavitation needs to be better controlled. This is partly achieved by (1) using pulsed acoustic fields which makes it possible to control the average bubble size and, at the same time, maximize the number of resonant bubbles, by (2) increasing the dissolved gas concentration and lowering the surface tension which facilitates bubble formation and, finally, by (3) introducing traveling waves to transport bubbles to the surface which needs to be cleaned.

Pulsed acoustic field radiation in a laterally bounded layered fluid

The acoustic field radiation from an impulsive source located in a discretely layered and laterally bounded fluid is investigated with the aid of the generalized-ray theory. The main ingredient of the presented approach is the Cagniard–de Hoop technique in combination with the method of images that provide exact space–time expressions for the activated acoustic wavefield components. Illustrative numerical examples are presented.

Trapping of a mie sphere by acoustic pulses: effects of pulse length.

The acoustic counterpart of optical tweezers shows great promise as a single-particle manipulator using a highly focused acoustic beam. Understanding the dependence of the trapping performance of the acoustic beam on the acoustic pulse length may facilitate its development and extend the applications. Herein, we propose a ray-based model for the time-course simulation of instantaneous forces exerted on single Mie spheres by highly focused acoustic pulses of arbitrary lengths. The simulations considered single fat/lipid spheres with a density of 950 kg/m3 and speed of sound of 1450 m/s, suspended in water and located on the beam axis. Simulation was used to establish the spatial and temporal pressure data of pulsed acoustic fields transmitted from a 100-MHz transducer with a half-power bandwidth of 50% and an f-number of 1. The instantaneous intensity vectors were calculated to represent rays for estimating forces exerted by consecutive wave-particle interactions. The results suggest that short acoustic pulses can exert negative forces pulling spheres beyond the focus in the direction opposite to that of wave propagation. Varying the excitation pulse duration has no effect on the region where the exerted forces are averagely negative. Lengthening the excitation pulse duration rapidly increases the amplitude of the average force. A smaller sphere experiences a greater average force when the spatial length of a transmitted acoustic pulse is comparable to the sphere diameter. The amplitude of the instantaneous force can be maximized as long as the acoustic pulse length is longer than the sphere diameter. Regulating the relation between acoustic pulse length and sphere size may be advantageous in particle sorting applications.

Evaluation and interpretation of bubble size distributions in pulsed megasonic fields

The occurrence of acoustic cavitation is incorporating a multitude of interdependent effects that strongly depend on the bubble size. Therefore, bubble size control would be beneficial for biological and industrial processes that rely on acoustic cavitation. A pulsed acoustic field can result in bubble size control and the repeated dissolution and reactivation (“recycling”) of potentially active bubbles. As a consequence, a pulsed field can strongly enhance cavitation activity. In this paper, we present a modified methodology for the evaluation of the active bubble size distribution by means of a combination of cavitation noise measurements and ultrasonic pulsing. The key component of this modified methodology is the definition of an upper size limit, below which bubbles—in between subsequent pulses—have to dissolve, in order to be sustainably recycled. This upper limit makes it possible to explain and link the enhancement of cavitation activity to a bubble size distribution. The experimentally determined bubble size distributions for different power densities are interpreted in the frame of numerical calculations of the oscillatory responses of the bubbles to the intermittent driving sound field. The distributions are found to be shaped by the size dependent interplay between bubble pulsations, rectified diffusion, coalescence, and the development of parametrically amplified shape instabilities. Also, a phenomenological reactivation-deactivation model is proposed to explain and quantify the observed enhancement of cavitation activity under pulsed, with respect to continuous sonication. In this model, the pulse-duration determines the magnitude of the reactivation of partially dissolved bubbles and the deactivation of activated bubbles by coalescence. It is shown that the subsequent recycling of previously active bubbles leads to an accumulation of cavitation activity, which saturates after a certain number of pulses. The model is fitted to the experimental data for the cavitation activity measured by means of ultraharmonic cavitation noise as a function of the pulse duration. Measurements of the development of the cavitation noise and sonochemiluminescence over a sequence of pulses for different pulse durations and separations confirm the general validity of the proposed model. Size distributions of the larger, inactive bubbles that were extracted from High-speed images of the cavitation field, relate the deactivation of activated bubbles by coalescence to the increase in volume concentrations of larger bubbles as observed by others.

Influence of Dissolved CO<sub>2</sub> on Bubble Activity in Pulsed Acoustic Fields

The continuous miniaturization of electronic building blocks in the semiconductor industry imposes more stringent requirements on the different cleaning processes. Purely chemical particle cleaning is based on weak etching of the substrate. This etching reduces the attractive van de Waals interaction between particle and substrate and at the same time, electrostatic repulsion ensures the removal of particles. This technique is not applicable anymore for future technology nodes, since an unacceptable large substrate loss (up to 3 nm) is necessary to obtain high particle removal efficiencies with pure chemical cleaning alone [. As a result, an additional physical force has to be considered to overcome this limitation. Several physical cleaning techniques exist, but all of them suffer from too much damage creation when fragile structures are cleaned. Currently, the industry is more focusing on spray cleaning and state-of-the-art spray tools show a high control over droplet size and droplet velocity [2]. Despite all of the advancements in spray cleaning, damage creation of fragile elements remains an issue, which could partially be attributed to the chaotic behavior of the water layer on the wafer surface [3]. Therefore, megasonic cleaning is still considered as a possible alternative to reduce damage formation during a physical cleaning process. Recently, it has been shown that the acoustic pressure amplitude can be reduced while maintaining the same particle removal efficiency level. This is achieved by (1) using pulsed acoustic fields which makes it possible to control the average bubble size and maximize the number of resonant bubbles, by (2) increasing the dissolved gas concentration which facilitates bubble nucleation and, finally, by (3) introducing traveling waves to transport bubbles to the wafer surface which needs to be cleaned. These conditions are briefly discussed and are applied during the investigation of the influence of dissolved CO2on bubble activity. Dissolved CO2is particularly interesting since it has been reported that sonoluminescence (i.e. strong bubble collapse) as well as damage formation is reduced when CO2is added to the cleaning liquid [4,5]. Here, it is shown that also particle removal efficiencies (PREs) diminish with increasing CO2concentrations.

Controlled cell aggregation in a pulsed acoustic field

Cell aggregation in ultrasonic resonators can be obtained in a few seconds. Hundreds even thousands of cells can be levitated in suspension and generate 2D or 3D aggregates. Nevertheless, the aggregation rate and the 2D or 3D configurations of the resultant aggregates are very difficult to control. This work reports on a novel way of generating and controlling particle and cell aggregates using pulsed ultrasound. This technique specifically explores (in addition to the ultrasound wave, frequency and amplitude) the time of ultrasound application, i.e. the number of pulses as well as the pulse repetition frequency. We demonstrate that with pulsed ultrasound, particles and/or cells levitate in suspension, as with continuous ultrasound, and the aggregation rate can be modified in a controlled manner. By carefully tuning the number of pulses and the repetition frequency, the 3-D and 2-D configurations of the aggregates can be selectively generated. In addition, pulsed ultrasound limits transducer heating, thus allowing for higher acoustic energies than those currently employed with continuous ultrasound.

Mechanisms for saturation of nonlinear pulsed and periodic signals in focused acoustic beams

Acoustic fields of powerful ultrasound sources with Gaussian spatial apodization and initial excitation in the form of a periodic wave or single pulse are examined based on the numerical solution of the Khokhlov-Zabolotskaya-Kuznetsov equation. The influence of nonlinear effects on the spatial structure of focused beams, as well as on the limiting values of the acoustic field parameters is compared. It is demonstrated that pressure saturation in periodic fields is mainly due to the effect of nonlinear absorption at a shock front, while in pulsed fields is due to the effect of nonlinear refraction. The limiting attainable values for the peak positive pressure in periodic fields turned out to be higher than the analogous values in pulsed acoustic fields. The total energy in a beam of periodic waves decreases with the distance from the source faster than in the case of a pulsed field, but it becomes concentrated within much smaller spatial region in the vicinity of the focus. These special features of nonlinear effect manifestation provide an opportunity to use pulsed beams for more efficient delivery of wave energy to the focus and to use periodic beams for attaining higher values of pressure in the focal region.

An iterative method for the computation of nonlinear, wide-angle, pulsed acoustic fields of medical diagnostic transducers

The development and optimization of medical ultrasound transducers and imaging modalities require a computational method that accurately predicts the nonlinear acoustic pressure field. A prospective method should provide the wide-angle, pulsed field emitted by an arbitrary planar source distribution and propagating in a three-dimensional, large scale domain holding a nonlinear acoustic medium. In this paper, a method is presented that is free of any assumed wavefield directionality. The nonlinear acoustic wave equation is solved by treating the nonlinear term as a contrast source. This formulation leads to an iterative scheme that involves the repetitive solution of a linear wave problem through Green's function method. It is shown that accurate field predictions may be obtained within a few iterations. Moreover, by employing a dedicated numerical convolution technique, the method allows for a discretization down to two points per wavelength or period of the highest frequency of interest. The performance of the method is evaluated through a number of nonlinear field predictions for pulsed transducers with various geometries. The results demonstrate the directional independence of the method. Moreover, comparison with results from several existing methods shows that the method accurately predicts the nonlinear field for weak to moderate nonlinearity.

Extension of the iterative nonlinear contrast source method to nonlinear media exhibiting tissue-like attenuation.

Recently, the Iterative Nonlinear Contrast Source (INCS) method has been developed to compute the pulsed acoustic pressure field in a nonlinear and lossless medium that extends over a very large, 3-D domain. This method solves the nonlinear wave equation by considering the nonlinear term as a distributed contrast source in a linear background medium. The full nonlinear wave field follows by convolving the background Green’s function with successive estimates of the contrast source. It has been shown that by spatiotemporal filtering of the convolution factors, the method yields accurate field predictions even for a discretization approaching two points per smallest wavelength or period. The current paper discusses how attenuation, in particular tissue-like power law losses, may be incorporated into the method through the introduction of a causal compressibility relaxation function. The latter gives the background the desired loss behavior. Fields in nonlinear media with different power law losses are presented, as computed by the extended INCS method. Moreover, the ability of both the lossless and the extended INCS method to evaluate fields at the shock formation distance is shown. Especially the latter results show the significance of using a proper, tissue-like power law. [Work supported by STW and NCF.]

Multimode Air-Coupled Excitation of Micromechanical Structures

The development of an acoustic measurement system for multimode air-coupled excitation and detection of micrometer-scale cantilever structures, which are, for example, used in micro-electromechanical systems (MEMSs), is detailed and reported for the first time. The source of noncontact vibrational excitation is a pulsed acoustic field generated by an air-coupled transducer. In the experimental system, the transient response of the cantilever beam is obtained at various points along the beam axis to extract its resonance frequencies and corresponding mode shapes. We demonstrate that measurable amplitudes of vibrations can be obtained at various excitation levels in the megahertz range, and higher harmonics of vibration of a microbeam can be excited by the air-coupled mechanism from a distance on the order of 10 mm. In the specific utilizations of the reported system, resonance frequencies and mode shapes can be related to the mechanical properties and geometric attributes (dimensions and defects), as well as the residual stress state in a microstructural element using various established computational and experimental inverse techniques. Another potential application area of the reported system is in the sensors for detecting the bending stiffness of deposited films on cantilever oscillators (in addition to its film mass loading) to increase the detection sensitivity and selectivity in a single sensing element.

Non-destructive acoustic defect detection in drug tablets

For physical defect detection in drug tablets, a non-destructive and non-contact technique based on air coupled excitation and interferometric detection is presented. Physical properties and mechanical integrity of drug tablets can often affect their therapeutic and structural functions. The monitoring for defects and the characterization of tablet mechanical properties therefore have been of practical interest for solid oral dosage forms. The presented monitoring approach is based on the analysis of the transient vibrational responses of an acoustically excited tablet in both in temporal and spectral domains. The pulsed acoustic field exciting the tablet is generated by an air-coupled transducer. Using a laser vibrometer, the out-of-plane vibrational transient response of the tablet is detected and acquired in a non-contact manner. The physical state of the tablet is evaluated based on the spectral properties of these transient responses. In the current study, the effectiveness of three types of simple similarity measures is evaluated for their potential uses as defect detection norms, and for their potential use in quantifying the extent of tablet defect is discussed. It is found that these quantities can not only be used for identification of defective tablets, but could also provide a measure for the extent of the damage.

Optical probing of pulsed, focused ultrasonic fields using a heterodyne interferometer

The pulsed acoustic field of a piezoelectric focused transducer transmitted in water has been investigated by using an optical heterodyne interferometer. The probe beam is reflected by a thin membrane that follows the motion of the fluid particle. Absolute measurements of the mechanical displacement have been performed in the frequency range 5–15 MHz with a spatial resolution better than 0.1 mm and a sensitivity of 0.1 nm with a 20 MHz detection bandwidth that corresponds to a minimum detectable acoustic energy density of 2 μJ/m2. The experimental results all agree quantitatively with diffraction theory predictions of impulse response at the focus of the transducer.

Forces acting in the direction of propagation in pulsed ultrasound fields

The authors consider some non-thermal effects resulting from absorption of acoustic energy from an ultrasound beam. An experimental investigation of the location of the 'source pump', responsible for the generation of streaming in high amplitude diagnostic fields in water, is reported. Acoustically transparent membranes were inserted in the ultrasound field in order to restrict the streaming volume. It is shown that the major contribution to an acoustic stream is generated in the region near to the focus of a transducer where the intensity in the beam and the degree of non-linear distortion are both high. A simple model of non-linear propagation is used to predict the magnitude of the maximum pressure gradient induced in a medium by the absorption of acoustic energy from a beam. Propagation in water, in tissue and in amniotic fluid are considered. Within the limitations of this model it is shown that the pressure gradients induced in pulsed acoustic fields do not result in the ultimate shear stress of tissue being exceeded.

Quasi-real time imaging of pulsed acoustic fields

Quasi-real time imaging of pulsed acoustic fields