Acoustofluidics 24: theory and experimental measurements of acoustic interaction force.

The motion of small objects in acoustophoresis depends on the acoustic radiation force and torque. These are nonlinear phenomena originating from wave scattering, and consist of primary and secondary components. The primary radiation force is the force acting on an object due to the incident field, in the absence of other objects. The secondary component, known as acoustic interaction force, accounts for the interaction among objects, and contributes to the clustering patterns of objects, as commonly observed in experiments. In this tutorial, the theory of acoustic interaction forces is presented using the force potential and partial-wave expansion approaches, and the distinguishing features of these forces such as rotational coupling and non-reciprocity are described. Theoretical results are compared to experimental measurements of interaction forces using a glass micro-capillary setup to explain the practical challenges. Finally, the phenomenon of clustering patterns induced by the close-range interaction of objects is demonstrated to point out the considerations about multiple collision and the predicted clustering patterns entirely due to the interaction force. Understanding the principles of acoustic interaction enables us to develop novel acoustofluidic applications beyond the typical processing of large populations of particles and with focus on the controlled manipulation of small clusters.

Assessment of in-cabin noise of wide-body aircrafts.

The aviation industry has seen dramatic growth over the decades till the recent disruption due to the COVID-19 pandemic. Moreover, long-haul routes with a distance of more than 4000 km are common for major airlines worldwide. Therefore, aircraft cabin noise assessment is essential, especially in long-haul flights, for passenger and flight crew health wellness. In this paper, the cabin noise of five wide-body aircraft, namely Airbus A330-300ER, A350-900, A380-800, and Boeing B777-200ER and B787-900, was recorded using a calibrated in-house developed smartphone application. The sound pressure levels of in-cabin noise have been measured on two different decibel scales, namely, A-weighted [dB(A)] and C-weighted scales [dB(C)]. The sound pressure levels of Airbus A380-800 were lowest among selected models, while the in-cabin pressure level values of Airbus A350-900 were maximum. However, the difference in decibel levels between the aircraft is minimal as it is within 3 dB.

Generalized potential theory for close-range acoustic interactions in the Rayleigh limit.

Under an external acoustic field, particles experience radiation forces that bring them to certain trapping locations, such as pressure or velocity nodes for the case of plane standing wave. Due to acoustical interactions, particles form clusters on reaching those trapping locations. In this work, by using the far-field evaluation of scattered fields, a generalized force potential is formulated that gives both the primary and interaction forces for particles with size much smaller than the wavelength (Rayleigh limit). The generalized potential for the primary force is the same as the Gorkov's potential. The interaction potential and forces between a pair of particles at the zero-primary-force locations are studied for the two cases of planar and nonplanar (Bessel) standing waves. It was found that the interaction forces are predominantly dependent on the product of the external acoustic field and the scattered fields from the adjacent particles. Besides the line formation, other cluster shapes are shown to be plausible for three solid particles agglomerating under a plane standing wave. The mutual interaction force between particles of different material properties was found to be not equal and opposite in general, suggesting that they do not form an action and reaction pair. From the interaction patterns due to the nonplanar field of a Bessel standing wave, it is inferred that many cluster configurations are possible since particles near the stable trapping locations attract each other from more than one direction. The advantage of using the generalized force potential is that it provides physical insight for the acoustical manipulation of small particles in any external field with arbitrary wave front, such as those used in acoustic holography.

Experimental measurement of interparticle acoustic radiation force in the Rayleigh limit.

Acoustophoresis is a form of contact-free particle manipulation in microfluidic devices. The precision of manipulation can be enhanced with better understanding of the acoustic radiation force. In this paper we present the measurements of interparticle radiation force between a pair of polystyrene beads in the Rayleigh limit. The study is conducted for three different sizes of beads and the experimental results are of the same order of magnitude when compared with theoretical predictions. However, the experimental values are larger than the theoretical values. The trend of a decrease in the magnitude of the interparticle radiation force with decreasing particle size and increasing center-to-center distance between the particles is also observed experimentally. The experiments are conducted in the specific scenario where the pair of beads are in close proximity, but not in contact with each other, and the beads are approaching the pressure nodal plane with the center-to-center line aligned perpendicular to the incident wave. This scenario minimizes the presence of the primary radiation force, allowing accurate measurement of the interparticle force. The attractive nature of the interparticle force is observed, consistent with theoretical predictions.

Effects of viscosity and acoustic streaming on the interparticle radiation force between rigid spheres in a standing wave.

The total acoustic radiation force acting on interacting spheres in a viscous fluid consists of the primary and secondary forces. The primary force pushes rigid spheres to the pressure node due to the incident standing wave. The secondary force is the interparticle force caused by the interaction between spheres in the standing wave. In this study, an algorithm based on the multipole series expansion and Stokeslet method is proposed for calculating the primary and secondary radiation forces acting on a pair of spheres in a viscous fluid. It is concluded that the acoustical interaction between a pair of spheres is considerably stronger in a viscous fluid compared to the inviscid case due to the streaming effects in the viscous fluid. For spheres located far from each other, the interaction becomes considerably weak; thus, the spheres move mainly due to the primary radiation force.

A hybrid dielectrophoretic system for trapping of microorganisms from water.

Assessment of the microbial safety of water resources is among the most critical issues in global water safety. As the current detection methods have limitations such as high cost and long process time, new detection techniques have transpired among which microfluidics is the most attractive alternative. Here, we show a novel hybrid dielectrophoretic (DEP) system to separate and detect two common waterborne pathogens, Escherichia coli (E. coli), a bacterium, and Cryptosporidium parvum (C. parvum), a protozoan parasite, from water. The hybrid DEP system integrates a chemical surface coating with a microfluidic device containing inter-digitated microelectrodes to impart positive dielectrophoresis for enhanced trapping of the cells. Trimethoxy(3,3,3-trifluoropropyl) silane, (3-aminopropyl)triethoxysilane, and polydiallyl dimethyl ammonium chloride (p-DADMAC) were used as surface coatings. Static cell adhesion tests showed that among these coatings, the p-DADMAC-coated glass surface provided the most effective cell adhesion for both the pathogens. This was attributed to the positively charged p-DADMAC-coated surface interacting electrostatically with the negatively charged cells suspended in water leading to increased cell trapping efficiency. The trapping efficiency of E. coli and C. parvum increased from 29.0% and 61.3% in an uncoated DEP system to 51.9% and 82.2% in the hybrid DEP system, respectively. The hybrid system improved the cell trapping by encouraging the formation of cell pearl-chaining. The increment in trapping efficiency in the hybrid DEP system was achieved at an optimal frequency of 1 MHz and voltage of 2.5 Vpp for C. parvum and 2 Vpp for E. coli, the latter is lower than 2.5 Vpp and 7 Vpp, respectively, utilized for obtaining similar efficiency in an uncoated DEP system.

Numerical study of interparticle radiation force acting on rigid spheres in a standing wave.

Acoustic radiation force can be used to move micro-sized particles, such as cells, in microfluidic devices. Although the number of particles in a microfluidic device is large, typically 2.5% (weight/volume), the acoustic force acting on a particle is commonly calculated using an analytical formula for a single particle in infinite medium. The interparticle forces are typically ignored as these are not easily accounted for and calculated with simple closed-form solutions. Based on the isothermal theory for an ideal fluid, a numerical scheme is hereby proposed to calculate the total radiation force, including the interparticle forces. The method uses the multipole series expansion and the weighted residual method to solve the governing Helmholtz equation with the necessary boundary conditions on the particle surface. The effect of different parameters on the primary and interparticle forces is studied using the proposed numerical scheme. It is shown that, near the pressure node, the interparticle forces are dominant and configurations of the spheres are determined by the interparticle forces. The proposed numerical scheme can be used for various sizes of spherical particles.

Numerical calculation of acoustic radiation forces acting on a sphere in a viscous fluid.

In this work, a numerical scheme based on multipoles and Stokeslet is proposed for calculating the radiation force acting on a single rigid sphere in a viscous fluid. First-order velocity and pressure are obtained from the multipole series solution, and the volumetric force in the acoustic streaming is subsequently calculated from the first-order velocity and pressure. The acoustic streaming equations are solved using the Stokeslet method within a finite domain descretized by tetrahedral elements. The boundary conditions for streaming are imposed using the weighted residue method to obtain the unknown coefficients in the multipole series expansion for the second-order velocity potentials. The radiation forces obtained from this multipole-Stokeslet method match well with Doinikov's series solution, for a wide range of the sphere size. Compared to the complicated series solution, the multipole-Stokeslet method can be easily implemented without the evaluation of the semi-infinite integrals.

Numerical analysis of flow induced noise propagation in supercavitating vehicles at subsonic speeds.

Flow supercavitation begins when fluid is accelerated over a sharp edge, usually at the nose of an underwater vehicle, where phase change occurs and causes low density gaseous cavity to gradually envelop the whole object (supercavity) and thereby enabling higher speeds of underwater vehicles. The process of supercavity inception/development by means of "natural cavitation" and its sustainment through ventilated cavitation result in turbulence and fluctuations at the water-vapor interface that manifest themselves as major sources of hydrodynamic noise. Therefore in the present context, three main sources are investigated, namely, (1) flow generated noise due to turbulent pressure fluctuations around the supercavity, (2) small scale pressure fluctuations at the vapor-water interface, and (3) pressure fluctuations due to direct impingement of ventilated gas-jets on the supercavity wall. An understanding of their relative contributions toward self-noise is very crucial for the efficient operation of high frequency acoustic sensors that facilitate the vehicle's guidance system. Qualitative comparisons of acoustic pressure distribution resulting from aforementioned sound sources are presented by employing a recently developed boundary integral method. By using flow data from a specially developed unsteady computational fluid dynamics solver for simulating supercavitating flows, the boundary-element method based acoustic solver was developed for computing flow generated sound.

Assessment of airflow ventilation in human nasal cavity and maxillary sinus before and after targeted sinonasal surgery: a numerical case study.

In this study, we evaluated the effects of targeted sinonasal surgery on nasal and maxillary sinus airflow patterns. A patient, who underwent right balloon sinuplasty and left uncinectomy for recurrent maxillary sinus barometric pressure, and concomitant septoplasty and bilateral inferior turbinate reduction for deviated nasal septum and inferior turbinate hypertrophy, was selected. Two 3D models representing both pre- and post-operative sinonasal morphology were constructed. The models were then used to evaluate nasal and maxillary sinus airflow patterns during respiration at ventilation rates of 7.5 L/min, 15 L/min and 30 L/min using computational fluid dynamics. The results showed that septoplasty and inferior turbinate reduction increased the nasal volume by 13.6%. The airflow patterns in the nasal cavity showed reasonably decreased resistance and slightly more even flow partitioning after the operation. Maxillary sinus ventilation significantly increased during inspiration in the left sinus after uncinectomy, and during expiration in right sinus after balloon sinuplasty. This study demonstrates computational fluid dynamics simulation is a tool in the investigation of outcomes after targeted, minimally invasive sinonasal surgery.

Pressure compensated fiber laser hydrophone: modeling and experimentation.

A pressure compensated metal diaphragm based fiber laser hydrophone configuration that can provide good sensitivity, large bandwidth, and sea state zero noise floor is proposed in this paper. A simplified theoretical model of the proposed sensor configuration is developed in which the acoustic elements of the sensor configuration are modeled using a four-pole acoustic transfer matrix and the structural elements are modeled as second order single degree of freedom elements. This model is then used to optimize the design parameters of the sensor system to achieve the performance objectives. An axisymmetric finite element analysis of the sensor configuration is also carried out to validate the results from the simplified theoretical model. Prototype sensors were fabricated and hydrostatic testing in a pressure vessel validated the static pressure compensation performance of the sensor. Frequency dependent sensitivity of the sensor system was measured through acoustic testing in a water tank. The prototype sensor gave a flat frequency response up to 5 kHz and experimental results compared well with theoretical predictions. The sensor has an acceleration rejection figure on the order of 0 dB ref 1 m/s(2) Pa and the pressure compensation approach worked reasonably well up to a hydrostatic pressures equivalent to a depth of 50 m.

Inspirational airflow patterns in deviated noses: a numerical study.

This study attempts to evaluate the effects of deviation of external nose to nasal airflow patterns. Four typical subjects were chosen for model reconstruction based on computed tomography images of undeviated, S-shaped deviated, C-shaped deviated and slanted deviated noses. To study the hypothetical influence of deviation of external nasal wall on nasal airflow (without internal blockage), the collapsed region along the turbinate was artificially reopened in all the three cases with deviated noses. Computational fluid dynamics simulations were carried out in models of undeviated, original deviated and reopened nasal cavities at both flow rates of 167 and 500 ml/s. The shape of the anterior nasal roof was found to be collapsed on one side of the nasal airways in all the deviated noses. High wall shear stress region was found around the collapsed anterior nasal roof. The nasal resistances in cavities with deviated noses were considerably larger than healthy nasal cavity. Patterns of path-line distribution and wall shear stress distribution were similar between original deviated and reopened models. In conclusion, the deviation of an external nose is associated with the collapse of one anterior nasal roof. The crooked external nose induced a larger nasal resistance compared to the undeviated case, while the internal blockage of the airway along the turbinates further increased it.

Shear flow of an electrically charged fluid by ion concentration polarization: scaling laws for electroconvective vortices.

We consider electroconvective fluid flows initiated by ion concentration polarization (ICP) under pressure-driven shear flow, a scenario often found in many electrochemical devices and systems. Combining scaling analysis, experiment, and numerical modeling, we reveal unique behaviors of ICP under shear flow: a unidirectional vortex structure, its height selection, and vortex advection. Determined by both the external pressure gradient and the electric body force, the dimensionless height of the sheared electroconvective vortex is shown to scale as (ϕ(2)/U(HP))(1/3), which is a clear departure from the previous diffusion-drift model prediction. To the best of our knowledge, this is the first microscopic characterization of ion concentration polarization under shear flow, and it firmly establishes electroconvection as the mechanism for an overlimiting current in realistic, large-area ion exchange membrane systems such as electrodialysis. The new scaling law has significant implications on the optimization of electrodialysis and other electrochemical systems.

Direct numerical simulation of electroconvective instability and hysteretic current-voltage response of a permselective membrane.

We present a systematic, multiscale, fully detailed numerical modeling for dynamics of fluid flow and ion transport covering Ohmic, limiting, and overlimiting current regimes in conductance of ion-selective membrane. By numerically solving the Poisson-Nernst-Planck-Navier-Stokes equations, it is demonstrated that the electroconvective instability, arising from the electric field acting upon the extended space charge layer, and the induced strong vortical fluid flow are the dominant factors of the overlimiting current in the planar membrane system. More importantly, at the transition between the limiting and the overlimiting current regimes, hysteresis of electric current is identified. The hysteresis demonstrates the important role of the electroconvective flow in enhancing of current in electrolyte systems with ion-selective membrane.

Effect of accessory ostia on maxillary sinus ventilation: a computational fluid dynamics (CFD) study.

We evaluated, by CFD simulation, effects of accessory ostium (AO) on maxillary sinus ventilation. A three-dimensional nasal model was constructed from an adult CT scan with two left maxillary AOs (sinus I) and one right AO (sinus II), then compared to an identical control model with all AOs sealed (sinuses III and IV). Transient simulations of quiet inspiration and expiration at 15 L/min, and nasal blow at 48 L/min, were calculated for both models using low-Reynolds-number turbulent analysis. At low flows, ventilation rates in sinuses with AOs (I ≈ 0.46 L/min, II ≈ 0.54 L/min), were both more than a magnitude higher than sinuses without AOs (II I ≈ 0.019 L/min, IV ≈ 0.020 L/min). Absence of AO almost completely prevented sinus ventilation. Increased ventilation of sinuses with AOs is complex. Under high flow conditions mimicking nose blowing, in sinuses II, III, and IV, the sinus flow rate increased. In contrast, the airflow direction through sinus I reversed between inspiration and expiration, while it remained almost constant throughout the respiration cycle in sinus II. CFD simulation demonstrated that AOs markedly increase maxillary sinus airflow rates and alter sinus air circulation patterns. Whether these airflow changes impact maxillary sinus physiology or pathophysiology is unknown.

Cell separation and transportation between two miscible fluid streams using ultrasound.

This paper presents a two-stream microfluidic system for transporting cells or micro-sized particles from one fluid stream to another by acoustophoresis. The two fluid streams, one being the original suspension and the other being the destination fluid, flow parallel to each other in a microchannel. Using a half-wave acoustic standing wave across the channel width, cells or particles with positive acoustic contrast factors are moved to the destination fluid where the pressure nodal line lies. By controlling the relative flow rate of the two fluid streams, the pressure nodal line can be maintained at a specific offset from the fluid interface within the destination fluid. Using this transportation method, particles or cells of different sizes and mechanical properties can be separated. The cells experiencing a larger acoustic radiation force are separated and transported from the original suspension to the destination fluid stream. The other particles or cells experiencing a smaller acoustic radiation force continue flowing in the original solution. Experiments were conducted to demonstrate the effective separation of polystyrene microbeads of different sizes (3 µm and 10 µm) and waterborne parasites (Giardia lamblia and Cryptosporidium parvum). Diffusion occurs between the two miscible fluids, but it was found to have little effects on the transport and separation process, even when the two fluids have different density and speed of sound.

Characterization and separation of Cryptosporidium and Giardia cells using on-chip dielectrophoresis.

Dielectrophoresis (DEP) has been shown to have significant potential for the characterization of cells and could become an efficient tool for rapid identification and assessment of microorganisms. The present work is focused on the trapping, characterization, and separation of two species of Cryptosporidium (C. parvum and C. muris) and Giardia lambia (G. lambia) using a microfluidic experimental setup. Cryptosporidium oocysts, which are 2-4 µm in size and nearly spherical in shape, are used for the preliminary stage of prototype development and testing. G. lambia cysts are 8-12 µm in size. In order to facilitate effective trapping, simulations were performed to study the effects of buffer conductivity and applied voltage on the flow and cell transport inside the DEP chip. Microscopic experiments were performed using the fabricated device and the real part of Clausius-Mossotti factor of the cells was estimated from critical voltages for particle trapping at the electrodes under steady fluid flow. The dielectric properties of the cell compartments (cytoplasm and membrane) were calculated based on a single shell model of the cells. The separation of C. muris and G. lambia is achieved successfully at a frequency of 10 MHz and a voltage of 3 Vpp (peak to peak voltage).

Dielectrophoretic capture voltage spectrum for measurement of dielectric properties and separation of cancer cells.

In this paper, a new dielectrophoresis (DEP) method based on capture voltage spectrum is proposed for measuring dielectric properties of biological cells. The capture voltage spectrum can be obtained from the balance of dielectrophoretic force and Stokes drag force acting on the cell in a microfluidic device with fluid flow and strip electrodes. The method was demonstrated with the measurement of dielectric properties of human colon cancer cells (HT-29 cells). From the capture voltage spectrum, the real part of Clausius-Mossotti factor of HT-29 cells for different frequencies of applied electric field was obtained. The dielectric properties of cell interior and plasma membrane were then estimated by using single-shell dielectric model. The cell interior permittivity and conductivity were found to be insensitive to changes in the conductivity of the medium in which the cells are suspended, but the measured permittivity and conductivity of cell membrane were found to increase with the increase of medium conductivity. In addition, the measurement of capture voltage spectrum was found to be useful in providing the optimum operating conditions for separating HT-29 cells from other cells (such as red blood cells) using dielectrophoresis.

Passive movement of human soft palate during respiration: A simulation of 3D fluid/structure interaction.

This study reconstructed a three dimensional fluid/structure interaction (FSI) model to investigate the compliance of human soft palate during calm respiration. Magnetic resonance imaging scans of a healthy male subject were obtained for model reconstruction of the upper airway and the soft palate. The fluid domain consists of nasal cavity, nasopharynx and oropharynx. The airflow in upper airway was assumed as laminar and incompressible. The soft palate was assumed as linear elastic. The interface between airway and soft palate was the FSI interface. Sinusoidal variation of velocity magnitude was applied at the oropharynx corresponding to ventilation rate of 7.5L/min. Simulations of fluid model in upper airway, FSI models with palatal Young's modulus of 7539Pa and 3000Pa were carried out for two cycles of respiration. The results showed that the integrated shear forces over the FSI interface were much smaller than integrated pressure forces in all the three directions (axial, coronal and sagittal). The total integrated force in sagittal direction was much smaller than that of coronal and axial directions. The soft palate was almost static during inspiration but moved towards the posterior pharyngeal wall during expiration. In conclusion, the displacement of human soft palate during respiration was mainly driven by air pressure around the surface of the soft palate with minimal contribution of shear stress of the upper airway flow. Despite inspirational negative pressure, expiratory posterior movement of soft palate could be another factor for the induction of airway collapse.

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.