Streaming Velocity Research Articles

Enhanced Acoustic Mixing in Silicon-Based Chips with Sharp-Edged Micro-Structures

The small dimensions of microfluidic channels allow for fast diffusive or passive mixing, which is beneficial for time-sensitive applications such as chemical reactions, biological assays, and the transport of to-be-detected species to sensors. In microfluidics, the need for fast mixing within milliseconds arises primarily because these devices are often used in fields where rapid and efficient mixing significantly impacts the performance and outcome of the processes. Active mixing with acoustics in microfluidic devices involves using acoustic waves to enhance the mixing of fluids within microchannels. Using sharp corners and wall patterns in acoustofluidic devices significantly enhances the mixing by acoustic streaming around these features. The streaming patterns around the sharp edges are particularly effective for the mixing because they can produce strong lateral flows that rapidly homogenize liquids. This work presents extensive characterizations of the effect of sharp-edged structures on acoustic mixing in bulk acoustic wave (BAW) mode in a silicon microdevice. The effect of side wall patterns in different angles and shapes, their positions, the type of piezoelectric transducer, and its amplitude and frequency have been studied. Following the patterning of the channel walls, a mixing time of 25 times faster was reached, compared to channels with smooth side walls exhibiting conventional BAW behavior. The average locally determined acoustic streaming velocity inside the channel becomes 14 times faster if sharp corners of 10° are added to the wall.

The Supersonic Project: Early Star Formation with the Streaming Velocity

At high redshifts (z ≳ 12), the relative velocity between baryons and dark matter (the so-called streaming velocity) significantly affects star formation in low-mass objects. Streaming substantially reduces the abundance of low-mass gas objects while simultaneously allowing for the formation of supersonically induced gas objects (SIGOs) and their associated star clusters outside of dark matter halos. Here, we present a study of the population-level effects of streaming on star formation within both halos and SIGOs in a set of simulations with and without streaming. Notably, we find that streaming actually enhances star formation within individual halos of all masses at redshifts between z = 12 and z = 20. This is demonstrated both as an increased star formation rate per object as well as an enhancement of the Kennicutt–Schmidt relation for objects with streaming. We find that our simulations are consistent with some observations at high redshift, but on a population level, they continue to underpredict star formation relative to the majority of observations. Notably, our simulations do not include feedback and so can be taken as an upper limit on the star formation rate, exacerbating these differences. However, simulations of overdense regions (both with and without streaming) agree with observations, suggesting a strategy for extracting information about the overdensity and streaming velocity in a given survey volume in future observations.

Influence of a transverse magnetic field on wakefield oscillations around a charged dust grain in complex plasma

In the presence of ion streaming, the potential around dust particles immersed in plasma becomes anisotropic. In this scenario, the repulsive Debye–Hückel potential is superimposed with an attractive wake potential. This work presents an analytical study of the complex behavior of such a wake potential in the presence of a magnetic field (oriented transversely to the ion flow) and ion-neutral collisions using linear response formalism, both in subsonic and supersonic regimes. The amplitude and periodicity of this potential are found to be controlled by the interplay among ion streaming velocity, ion cyclotron frequency, and ion-neutral collision frequency. Due to the tunable nature of this potential, it is possible to control the crystal formation, phase transitions, and transport properties of dusty plasma by adjusting the external magnetic field. The study also reveals that the wake potential almost disappears in a collision-dominant regime.

Numerical investigation of acoustic streaming vortices in cylindrical tube arrays

Abstract Acoustic streaming has a significant effect on accelerating material mixing and flow field disturbance. To explore the characteristics of acoustic streaming in the cylindrical tube array field under the action of an acoustic wave, we derive the dimensionless acoustic streaming control equation and establish a numerical calculation model of acoustic streaming. The effects of acoustic incidence angle, acoustic Reynolds number, and Strouhal number on the acoustic streaming vortex flow field in the tube array were investigated. The numerical results show that with the change in acoustic parameters, the acoustic streaming in the tube array presents rich changes in the vortex flow field, and there are flow field phenomena such as shrinking, merging, tearing, and splitting of the vortex structure. Toward the walls of each tube, there is a strong acoustic streaming flow velocity. Besides, there is also a large streaming velocity on the interface of the adjacent acoustic streaming vortices. The inner streaming vortex structure in the acoustic boundary layer decreases with the increase in the acoustic Reynolds number, but the intensity of the inner streaming vortex and outer streaming vortex increases rapidly, and the disturbance effect of the flow field is enhanced. With the increase in the dimensionless acoustic frequency (or Strouhal number), although the structure and intensity of the inner streaming vortex decrease, the velocity gradient on the wall of the cylindrical tube increases, which is beneficial to destroy the flow boundary layer of the cylindrical tube wall and accelerate the instability of the wall flow field.

Microparticle focusing and micromixing with two-dimensional acoustic waves

Acoustofluidics has recently been popularized as a crucial element of lab-on-a-chip (LoC) platforms to efficiently manipulate microparticles and continuous matter alike. In this study, a three-dimensional (3D) numerical model is proposed to simulate the focusing of polystyrene microparticles with three diameters and micromixing of dilute species using two orthogonally oriented standing waves, contrasting them with one-dimensional (1D) waves. The limiting velocity method is modified to explore the 3D acoustic streaming in a symmetric microchannel. In contrast to 1D standing acoustic waves, the simultaneous excitation of two orthogonal waves generates an acoustic streaming velocity field that does not counteract the radiation force. The obtained results show that the focusing efficiency of 5-μm particles reaches 97% with two dimensional (2D) standing acoustic waves, which was unachievable using 1D waves. Moreover, by reducing the flow rate to 1 μL min−1, the focusing of critical microparticle diameter peaked at 94%, indicating an approximately 9% improvement over a flow rate of 2.5 μL min−1. Increasing the viscosity of the background fluid resulted in 16% better 2D focusing with a single vortex compared to other cases, and higher amplitudes did not change focusing efficiency with a single vortex, while reducing efficiency in other cases. Finally, using 2D acoustic waves remarkably improved the mixing efficiency of dilute species, underscoring the advantage of 2D acoustic waves over their 1D counterpart. The proposed numerical model can play a meaningful role in cutting fabrication costs of next-generation LoC devices by identifying the most crucial parameters influencing acoustofluidic matter transport.

Solar wind effect on the multi-fluid plasma expansion in the Venusian upper ionosphere

Inspired by the observations suggesting that at altitudes of about 1000 km the interaction between solar wind streams and Venus’ ionosphere plasma leads to ions acceleration and outflow, the influence of different solar wind physical parameters, such as densities, temperatures and initial streaming velocities, has been studied. The ionosphere plasma system consists of two positive ion populations O+, H+ and electrons along with the solar wind streaming protons and electrons. We calculated the generated oxygen and hydrogen ions flow velocities and the electric fields. In addition, we calculated rough estimates for the escaping flux of ion populations (O+, H+) from Venus’ ionosphere and compared them to observations. To a large extent, we found that the estimates match. We also discuss the relevance of ionospheric ion acceleration and outflow from Venus’ upper.

Model-based investigation of a novel decontamination technology: Ultrasound assisted aerosolized hydrogen peroxide

This work investigates the feasibility of a novel decontamination technology that employs ultrasonic acoustic streaming to improve aerosol distribution inside isolator chambers. For this purpose, a numerical model consisting of three simulations (i) acoustic pressure, (ii) fluid flow, and (iii) particle tracing, is developed. Acoustic air streaming is examined for ultrasound source pressure levels between 131 and 136 dB. Maximum streaming velocity reached values of the order 10−3 and 10−2m/s which agrees with previous experimental and numerical investigations. The effect of air velocity field on the deposition of the decontamination agent on the isolator floor was assessed for aerosol with droplet diameter between 10 and 20μm. The results show that a more homogeneous distribution of deposited droplets is obtained with configurations that have sound sources placed at a distance from each other. Designs with a homogeneous aerosol deposition are expected to have a better decontamination outcome.

Velocity Acoustic Oscillations on Cosmic Dawn 21 cm Power Spectrum as a Probe of Small-scale Density Fluctuations

We investigate the feasibility of using the velocity acoustic oscillations (VAO) features on the Cosmic Dawn 21 cm power spectrum to probe small-scale density fluctuations. In the standard cold dark matter (CDM) model, Population III stars form in minihalos and affect the 21 cm signal through Lyα and X-ray radiation. Such a process is modulated by the relative motion between dark matter and baryons, generating the VAO wiggles on the 21 cm power spectrum. In the fuzzy or warm dark matter models for which the number of minihalos is reduced, the VAO wiggles are weaker or even fully invisible. We investigate the wiggle features in the CDM with different astrophysical models and in different dark matter models. We find that (1) in the CDM model the relative streaming velocities can generate the VAO wiggles for broad ranges of parameters f *, ζ X , and f esc,LW ζ LW, though for different parameters the wiggles would appear at different redshifts and have different amplitudes. (2) For the axion model with m a ≲ 10−19 eV, the VAO wiggles are negligible. In the mixed model, the VAO signal is sensitive to the axion fraction. For example, the wiggles almost disappear when f a ≳ 10% for m a = 10−21 eV. Therefore, the VAO signal can be an effective indicator for small-scale density fluctuations and a useful probe of the nature of dark matter. The Square Kilometre Array-low with ∼2000 hr observation time has the ability to detect the VAO signal and constrain dark matter models.

Bubble oscillations at low frequency ultrasound for biological applications

Bubbles oscillating in the presence of ultrasound is commonly employed in biomedical applications for drug delivery, ultrasound enhanced thrombolysis, and the transport and manipulation of cells. This is possible because bubbles tend to interact with the ultrasound to undergo periodic shape changes known as shape-mode oscillation, concomitant with the generation of liquid agitation or streaming. This phenomenon is examined both experimentally and theoretically on a single bubble at a frequency of (45 ± 1) kHz. Effects of ultrasonic frequency and power on the flowfield were explored. Experiments revealed different trends in the development of liquid streaming velocities at different acoustic forcing conditions (5.53, 6.80 and 7.02 Vpp), with lowest (0.5 mm/s) and highest (1.1 mm/s) values of time-averaged mean streaming velocity occurring at 6.80 Vpp and 7.02 Vpp, respectively. Simulations captured the simultaneous evolution of bubble-shapes that helped create flow vortices in the liquid surrounding the bubble. These vortices collectively responsible in generating signature patterns in the liquid for a dominant shape-mode of the bubble, and could also generate localised shear stresses for practical application. The velocity and pressure profiles in the liquid around the bubble confirmed the connection of the applied and reflected soundwaves in driving this phenomenon.

A Global Semianalytic Model of the First Stars and Galaxies Including Dark Matter Halo Merger Histories

We present a new self-consistent semianalytic model of the first stars and galaxies to explore the high-redshift (z ≥ 15) Population III (PopIII) and metal-enriched star formation histories. Our model includes the detailed merger history of dark matter halos generated with Monte Carlo merger trees. We calibrate the minimum halo mass for PopIII star formation from recent hydrodynamical cosmological simulations that simultaneously include the baryon–dark matter streaming velocity, Lyman–Werner (LW) feedback, and molecular hydrogen self-shielding. We find an overall increase in the resulting star formation rate density (SFRD) compared to calibrations based on previous simulations (e.g., the PopIII SFRD is over an order of magnitude higher at z = 35−15). We evaluate the effect of the halo-to-halo scatter in this critical mass and find that it increases the PopIII stellar mass density by a factor ∼1.5 at z ≥ 15. Additionally, we assess the impact of various semianalytic/analytic prescriptions for halo assembly and star formation previously adopted in the literature. For example, we find that models assuming smooth halo growth computed via abundance matching predict SFRDs similar to the merger tree model for our fiducial model parameters, but that they may underestimate the PopIII SFRD in cases of strong LW feedback. Finally, we simulate subvolumes of the Universe with our model both to quantify the reduction in total star formation in numerical simulations due to a lack of density fluctuations on spatial scales larger than the simulation box, and to determine spatial fluctuations in SFRD due to the diversity in halo abundances and merger histories.

Acoustic streaming-based calibration of ultrasound transducers

The accurate determination of the transfer function of ultrasound transducers is important for their design and operational performance. However, conventional methods for quantifying the transfer function, such as hydrophone measurements, radiation force balance, and pulse-echo measurements, are costly and complex due to specialized equipment required. In this study, we introduce a novel approach to estimate the transfer function of ultrasound transducers by measuring the acoustic streaming velocity generated by the transducer. We utilize an experimental setup consisting of a water tank with a millimeter scale, an ink-filled syringe, and a camera for recording the streaming phenomenon. Through streaming velocity measurements in the frequency range from 2 to 8 MHz, we determined the transfer function of an unfocused circular transducer with a center frequency of 5 MHz and a radius of 5.6 mm. We compared the performance of our method with hydrophone and pulse-echo measurements. At the center frequency, we measured a transmit efficiency of 1.9 kPa/V using the streaming approach, while hydrophone and pulse-echo measurements yielded transmit efficiencies of 2.1 kPa/V and 1.8 kPa/V, respectively. These findings demonstrate that the proposed method for estimating the transfer function of ultrasound transducers achieves a sufficient level of accuracy comparable to pulse-echo and hydrophone measurements.

Experimental study of acoustic streaming induced by a sharp edge at different frequencies and vibrating amplitudes

Acoustic streaming is the time-averaged flow induced by acoustic waves inside the fluid medium. Much attention has been paid to the streaming flow at the microscale, with the rapid development of micro-fluidics and significant demand for the microscale manipulation of fluid or particles. Recently, the streaming flow at the audible or lower frequency (10 Hz~10 kHz) has been found to be closely associated with local structures, like a sharp edge in the micro-channel. By its strong magnitude and low cost, this kind of streaming flow has been applied in various fields. However, the mechanisms behind this non-classical Rayleigh streaming are still not very clear, though its high sensitivity to the thickness of the acoustic boundary-layer and unstable streaming pattern under high forcing amplitude have been demonstrated. In this study, experimental work has been conducted, with the help of the particle imaginary velocimetry platform, to reveal the influence of frequency and vibrating amplitude on the streaming flow field around a sharp edge with 90?, and its characterized spatial dimension. The scaling law concerning the vibration amplitude and streaming velocity has been come up with, and the parameter frequency is also included. The expression f ?1/6va2~vsy,max demonstrates a good prediction in terms of the streaming magnitude, in comparison with experimental results.

Enhanced acoustic streaming effects via sharp-edged 3D microstructures.

Acoustofluidic micromanipulation is an important tool for biomedical research, where acoustic forces offer the ability to manipulate fluids, cells, and particles in a rapid, biocompatible, and contact-free manner. Of particular interest is the investigation of acoustically driven sharp edges, where high tip velocity magnitudes and strong acoustic potential gradients drive rapid motion. Whereas prior devices utilizing 2D sharp edges have demonstrated promise for micromanipulation activities, taking advantage of 3D structures has the potential to increase their performance and the range of manipulation activities. In this work, we investigate high-magnitude acoustic streaming fields in the vicinity of sharp-edged, sub-wavelength 3D microstructures. We numerically model and experimentally demonstrate this in fabricating parametrically configured 3D microstructures whose tip-angle and geometry influence acoustic streaming velocities and the complexity of streaming vortices, finding that the simulated and realized velocities and streaming patterns are both tunable and a function of microstructure shape. These sharp-edge interfaces hold promise for biomedical studies benefiting from precise and targeted micromanipulation.

The acoustic streaming effects and transmission mechanisms of a micro-cavity acoustic black hole structure with an abrupt cross-section

This paper investigates the acoustic streaming effects (ASEs) and mechanisms behind the transmittance of sound in a metallic micro-cavity acoustic black hole (ABH) structure with an abrupt cross-section and examines the sound field flow characteristics inside the micro-cavity ABH under the sound excitation, such as the velocity, acceleration and pressure fields. And the sound transmission mechanisms are characterized by the ASEs which can be obtained by solving Navier–Stokes equations. The numerical results show that the sharp increase in the velocity and acceleration at the ABH tip position is the main reason for the focusing of the sound energy. And the dramatic increase in the tip cross-section reduces the acoustic streaming velocity, which is the main reason for the attenuation of the sound energy. Additionally, the thermoviscous effect of the acoustic boundary layer can also dissipate the low-frequency sound energy. The sound insulation experiment shows that the proposed micro-cavity ABH structure has a sound transmission loss (STL) of over 15[Formula: see text]dB in the low-frequency regime. This research reveals the mechanisms of the ASE’s work on the sound transmission properties of the micro-cavity ABH and provides new insight into low-frequency sound wave suppression. The ABH structure proposed in this paper has excellent strength, bearing capacity and long lifecycle, so it can be applied in the construction industry through its integrated design of structure and performance.

Characterizing bulk-driven acoustic streaming in air

The time-independent fluid flows induced by the attenuation of acoustic energy known as acoustic streaming have been experimentally measured in air using particle image velocimetry for two high powered ultrasonic transducers. The literature declares that these bulk-driven “Eckart” streaming flows are observed as turbulent jets in the direction of the acoustic wave propagation. The aim of this work is to increase the understanding of the coupling between the acoustic and fluid domains and to validate these well-established theoretical relationships. Langevin horns and focused arrays of parking sensor transducers, which operate at ∼26 and 40 kHz, respectively, and are both capable of producing sound pressure levels of up to 170dB, were used to assess the relationship between the first order acoustic field and the second order streaming velocity field. Streaming velocities of the order of 0.2 and 0.3 m/s were measured for the Langevin horn and the focused array, respectively. Two-step COMSOL Multiphysics models were created for both transducers, first solving the acoustic fields in the frequency domain, then using the results to drive a streaming volume force in a stationary fluid mechanics study. Both laminar and turbulent k-ε fluid mechanics models were compared to the experimental results.

Formation of first star clusters under the supersonic gas flow – I. Morphology of the massive metal-free gas cloud

ABSTRACT We performed 42 simulations of first star formation with initial supersonic gas flows relative to the dark matter at the cosmic recombination era. Increasing the initial streaming velocities led to delayed halo formation and increased halo mass, enhancing the mass of the gravitationally shrinking gas cloud. For more massive gas clouds, the rate of temperature drop during contraction, in other words, the structure asymmetry, becomes more significant. When the maximum and minimum gas temperature ratios before and after contraction exceed ∼10, the asymmetric structure of the gas cloud prevails, inducing fragmentation into multiple dense gas clouds. We continued our simulations until 105 yr after the first dense core formation to examine the final fate of the massive star-forming gas cloud. Among the 42 models studied, we find the simultaneous formation of up to four dense gas clouds, with a total mass of about $2254\, \mathrm{ M}_\odot$. While the gas mass in the host halo increases with increasing the initial streaming velocity, the mass of the dense cores does not change significantly. The star formation efficiency decreases by more than one order of magnitude from ϵIII ∼ 10−2 to 10−4 when the initial streaming velocity, normalized by the root mean square value, increases from 0 to 3.

Study of micro-scale flow characteristics under surface acoustic waves

As an effective tool for contactless manipulation of submicrometer scale objects, the controllability of acoustic streaming velocity and flow field morphology determines the accuracy of object migration and the completeness of three dimensional (3D) imaging. This paper proposes an equivalent acoustic streaming driving force model that is applicable to both two dimensional (2D) and 3D calculations and constructs a numerical method for submicrometer microsphere migration and rotation velocity in acoustic streaming. The results show that the relationship between the peripheral vortex size Lp/wc and the relative acoustic streaming velocity vas/vf satisfies Lp/wc = 0.125vas/vf0.36 under certain geometrical conditions. Reducing the spatial confinement and increasing the inter-vortex distance will increase the energy release efficiency, reduce the pressure gradient distribution and convective dissipation rates, increase the vortex intensity and radiation range, and consequently, increase the vortex characteristic size. In complex 3D vortex flow fields, suspended objects are affected by velocity distributions and exhibit motions such as cross-flow lines and rotation. For larger vortex structure sizes, full 3D imaging is more favorable due to the increased rotation speed and period of motion along the orbit of the submicrometer microspheres. This study helps us to reveal the modulation mechanism of acoustic streaming field flow characteristics, enrich the basic theory of alternating orbital motion and forces on objects in vortex structures, and provide guidance for acoustic flow-based contactless object manipulation.

Electrostatic wave propagation and self-streaming effect in an electron-hole plasma

Electrostatic nonlinear waves which transfer energy through the semiconductor are investigated. A quantum hydrodynamic plasma system composed of self-streaming electrons and holes is examined. The basic equations are reduced to one evolution equation called a modified nonlinear Schrödinger (mNLS) equation. The stability and instability regions are studied with respect to the wavenumber and different plasma effects such as degenerate pressure, Bohm potential, and collisions. The mNLS equation is solved analytically to obtain three kinds of nonlinear envelope wave packet modes. It is found that there are different regions of stability and instability depending on various quantum effects. The electrons’ and holes’ self-streaming velocity is studied and manipulated for the three types of nonlinear envelope waves ‘dark soliton, bright soliton, and rogue wave’. The dark envelope wave packet is generated in a stable region. When the electrons and holes streaming velocities become faster, the wave amplitude becomes taller and the pulses have higher frequency. The bright envelope wave packet exists in the unstable region. For low streaming velocities, the rogue wave amplitude becomes shorter, however, when the streaming velocities reach a critical value the amplitude increases suddenly six times. The self-heating could be produced as the tunneling electrons and holes exchange their energy with the lattice, which may decrease the lifetime of the semiconductors. The present results are helpful in realizing the physical solution to the intrinsic heating problem in semiconductors.

Starbursts in low-mass haloes at Cosmic Dawn. I. The critical halo mass for star formation

ABSTRACT The first stars, galaxies, star clusters, and direct-collapse black holes are expected to have formed in low-mass (∼105–109 M⊙) haloes at Cosmic Dawn (z ∼ 10–30) under conditions of efficient gas cooling, leading to gas collapse towards the centre of the halo. The halo mass cooling threshold has been analysed by several authors using both analytical models and numerical simulations, with differing results. Since the halo number density is a sensitive function of the halo mass, an accurate model of the cooling threshold is needed for (semi-)analytical models of star formation at Cosmic Dawn. In this paper, the cooling threshold mass is calculated (semi-)analytically, considering the effects of H2-cooling and formation (in the gas phase and on dust grains), cooling by atomic metals, Lyman-α cooling, photodissociation of H2 by Lyman–Werner photons (including self-shielding by H2), photodetachment of H− by infrared photons, photoevaporation by ionization fronts, and the effect of baryon streaming velocities. We compare the calculations to several high-resolution cosmological simulations, showing excellent agreement. We find that in regions of typical baryon streaming velocities, star formation is possible in haloes of mass ≳ 1–2 × 106 M⊙ for z ≳ 20. By z ∼ 8, the expected Lyman–Werner background suppresses star formation in all minihaloes below the atomic cooling threshold (Tvir = 104 K). The halo mass cooling threshold increases by another factor of ∼4 following reionization, although this effect is slightly delayed (z ∼ 4–5) because of effective self-shielding.

Impact of dark matter-baryon relative velocity on the 21-cm forest

We study the effect of the relative velocity between the dark matter (DM) and the baryon on the 21cm forest signals. The DM-baryon relative velocity arises due to their different evolutions before the baryon-photon decoupling epoch and it gives an additional anisotropic pressure that can suppress the perturbation growth. It is intriguing that the scales $k\sim {\cal O}(10\sim 10^3)h/\mathrm{Mpc}$ at which the matter power spectrum is affected by such a streaming velocity turns out to be the scale at which the 21cm forest signal is sensitive to. We demonstrate that the 21cm absorption line abundance can decrease by more than a factor of a few due to the small-scale matter power spectrum suppression caused by the DM-baryon relative velocity.