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Henrik Bruus

Publications and source records attributed to Henrik Bruus.

At least 19 recordsLinked to original sources
arXiv2026

Increased throughput in antisymmetrically actuated acoustofluidic flow-through devices

Separation of low-abundance biological objects requires high throughput for practical use of an acoustofluidic system. Increasing the flow rate helps in achieving high-throughput if the acoustic energy density can be increased proportionally, and this may be possible with an efficient coupling of the transducer to the device. In particular, antisymmetric actuation using two electrodes with opposite phases is theoretically proven to enhance the acoustic energy density of the device. In this work, we study the symmetric and antisymmetric actuation mechanisms of an acoustofluidic system using both experiments and three-dimensional numerical simulations. The acoustic focusability experiments show that under the same electrical input power, the antisymmetric actuation mode performs better than the symmetric actuation, quantified in terms of the normalized width of the band formed by the focused particles. Numerical simulations of this particle bandwidth are performed for both actuation modes, and the results suggest that the antisymmetric actuation mode is more robust than the symmetric one, being weakly dependent of the geometric symmetry properties of the system. The simulation results corroborate the experimental findings, which indicate that the antisymmetric actuation increases the acoustophoretic efficiency and robustness for high-throughput applications.

physics.flu-dyn↗
arXiv2025

Boundary-layer modeling of polymer-based acoustofluidic devices

In fluid-filled microchannels embedded in solid devices and driven by MHz ultrasound transducers, the thickness of the viscous boundary layer in the fluid near the confining walls is typically 3 to 4 orders of magnitude smaller than the acoustic wavelength and 5 orders of magnitude smaller than the longest dimension of the device. This large span in length scale renders direct numerical simulations of such devices prohibitively expensive in terms of computer memory requirements, and consequently, the so-called boundary-layer models are introduced. In such models, approximate analytical expressions of the boundary-layer fields are found and inserted in the governing equations and boundary conditions for the remaining bulk fields. Since the bulk fields do not vary across the boundary layers, they can be computed numerically using the resulting boundary-layer model without resolving the boundary layers. However, current boundary-layer models are only accurate for hard solids (e.g. glass and silicon) with relatively small oscillation amplitudes of the confining wall, and they fail for soft solids (e.g. polymers) with larger wall oscillations. In this work, we extend the boundary-layer model of Bach and Bruus, J. Acoust. Soc. Am. 144, 766 (2018) to enable accurate simulation of soft-walled devices. The extended model is validated by comparing (1) with direct numerical simulations in three and two dimensions of tiny sub-mm and larger mm-sized polymer devices, respectively, and (2) with previously published experimental data.

physics.flu-dyn↗
arXiv2024

Miscible fluids patterning and micro-manipulation using vortex-based single-beam acoustic tweezers

Vortex-based single-beam tweezers have the ability to precisely and selectively move a wide range of objects, including particles, bubbles, droplets, and cells with sizes ranging from the millimeter to micrometer scale. In 2017, Karlsen and Bruus [Phys. Rev. Appl. 7, 034017 (2017)] theoretically suggested that these tweezers could also address one of the most challenging issues: the patterning and manipulation of miscible fluids. In this paper, we experimentally demonstrate this ability using acoustic vortex beams generated by interdigital transducer-based active holograms. The experimental results are supported by a numerical model based on acoustic body force simulations. This work paves the way for the precise shaping of chemical concentration fields, a crucial factor in numerous chemical and biological processes.

physics.flu-dyn↗
arXiv2023

Acoustic radiation force on a heated spherical particle in a fluid including scattering and microstreaming from a standing ultrasound wave

Analytical expressions are derived for the time-averaged, quasi-steady, acoustic radiation force on a heated, spherical, elastic, solid microparticle suspended in a fluid and located in an axisymmetric incident acoustic wave. The heating is assumed to be spherically symmetric, and the effects of particle vibrations, sound scattering, and acoustic microstreaming are included in the calculations of the acoustic radiation force. It is found that changes in the speed of sound of the fluid due to temperature gradients can significantly change the force on the particle, particularly through perturbations to the microstreaming pattern surrounding the particle. For some fluid-solid combinations, the effects of particle heating even reverse the direction of the force on the particle for a temperature increase at the particle surface as small as 1 K.

physics.flu-dyn↗
arXiv2022

The acoustic radiation force on a spherical thermoviscous particle in a thermoviscous fluid including scattering and microstreaming

We derive general analytical expressions for the time-averaged acoustic radiation force on a small spherical particle suspended in a fluid and located in an axisymmetric incident acoustic wave. We treat the cases of the particle being either an elastic solid or a fluid particle. The effects of particle vibrations, acoustic scattering, acoustic microstreaming, heat conduction, and temperature-dependent fluid viscosity are all included in the theory. Acoustic streaming inside the particle is also taken into account for the case of a fluid particle. No restrictions are placed on the widths of the viscous and thermal boundary layers relative to the particle radius. We compare the resulting acoustic radiation force with that obtained from previous theories in the literature, and we identify limits, where the theories agree, and specific cases of particle and fluid materials, where qualitative or significant quantitative deviations between the theories arise.

physics.flu-dyn↗
arXiv2022

Determination of the complex-valued elastic moduli of polymers by electrical impedance spectroscopy for ultrasound applications

A method is presented for the determination of complex-valued compression and shear elastic moduli of polymers for ultrasound applications. The resulting values, which are scarcely reported in the literature, are found with uncertainties typically around 1 % (real part) and 6 % (imaginary part). The method involves a setup consisting of a cm-radius, mm-thick polymer ring glued concentrically to a disk-shaped piezoelectric transducer. The ultrasound electrical impedance spectrum of the transducer is computed numerically and fitted to measured values as an inverse problem in a wide frequency range, typically from 500 Hz to 5 MHz, both on and off resonance. The method was validated experimentally by ultrasonic through-transmission around 1.9 MHz. Experimentally, the method is arguably simple and low cost, and it is not limited to specific geometries and crystal symmetries. Moreover, by involving off-resonance frequencies, it allows for determining the imaginary parts of the elastic moduli, equivalent to attenuation coefficients. Finally, the method has no obvious frequency limitations before severe attenuation sets in above 100 MHz.

physics.flu-dyn↗
arXiv2022

Constant-power versus constant-voltage actuation in frequency sweeps for acoustofluidic applications

Supplying a piezoelectric transducer with constant voltage or constant power during a frequency sweep can lead to different results in the determination of the acoustofluidic resonance frequencies, which are observed when studying the acoustophoretic displacements and velocities of particles suspended in a liquid-filled microchannel. In this work, three cases are considered: (1) Constant input voltage into the power amplifier, (2) constant voltage across the piezoelectric transducer, and (3) constant average power dissipation in the transducer. For each case, the measured and the simulated responses are compared, and good agreement is obtained. It is shown that Case 1, the simplest and most frequently used approach, is largely affected by the impedance of the used amplifier and wiring, so it is therefore not suitable for a reproducible characterization of the intrinsic properties of the acoustofluidic device. Case 2 strongly favors resonances at frequencies yielding the lowest impedance of the piezoelectric transducer, so small details in the acoustic response at frequencies far from the transducer resonance can easily be missed. Case 3 provides the most reliable approach, revealing both the resonant frequency, where the power-efficiency is the highest, as well as other secondary resonances across the spectrum.

physics.flu-dyn↗
arXiv2021

Numerical study of acoustic cell trapping above elastic membrane disks driven in higher-harmonic modes by thin-film transducers with patterned electrodes

Excitations of MHz acoustic modes are studied numerically in 10-um-thick silicon disk membranes with a radius of 100 and 500 um actuated by an attached 1-um-thick (AlSc)N thin-film transducer. It is shown how higher-harmonic membrane modes can be excited selectively and efficiently by appropriate patterning of the transducer electrodes. When filling the half-space above the membrane with a liquid, the higher-harmonic modes induce acoustic pressure fields in the liquid with interference patterns that result in the formation of a single, strong trapping region located 50 - 100 um above the membrane, where a single suspended cell can be trapped in all three spatial directions. The trapping strength depends on the acoustic contrast between the cell and the liquid, and as a specific example it is shown by numerical simulation that by using a 60% iodixanol solution, a cancer cell can be held in the trap.

physics.flu-dyn↗
arXiv2021

A transition from boundary- to bulk-driven acoustic streaming due to nonlinear thermoviscous effects at high acoustic energy densities

Acoustic streaming is studied in a rectangular microfluidic channel. It is demonstrated theoretically, numerically, and experimentally with good agreement, frictional heating can alter the streaming pattern qualitatively at high acoustic energy densities E_ac above 500 J/m^3. The study shows, how as a function of increasing E_ac at fixed frequency, the traditional boundary-driven four streaming rolls created at a half-wave standing-wave resonance, transition into two large streaming rolls. This nonlinear transition occurs because friction heats up the fluid resulting in a temperature gradient, which spawns an acoustic body force in the bulk that drives thermoacoustic streaming.

physics.flu-dyn↗
arXiv2021

Theory and modeling of nonperturbative effects at high acoustic energy densities in thermoviscous acoustofluidics

A theoretical model of thermal boundary layers and acoustic heating in microscale acoustofluidic devices is presented. It includes effective boundary conditions allowing for simulations in three dimensions. The model is extended by an iterative scheme to incorporate nonlinear thermoviscous effects not captured by standard perturbation theory. The model predicts that the dominant nonperturbative effects in these devices are due to the dependency of thermoacoustic streaming on gradients in the steady temperature induced by a combination of internal frictional heating, external heating, and thermal convection. The model enables simulations in a nonperturbative regime relevant for design and fabrication of high-throughput acoustofluidic devices.

physics.flu-dyn↗
arXiv2021

Acoustophoresis in polymer-based microfluidic devices: modeling and experimental validation

A finite-element model is presented for numerical simulation in three dimensions of acoustophoresis of suspended microparticles in a microchannel embedded in a polymer chip and driven by an attached piezoelectric transducer at MHz frequencies. In accordance with the recently introduced principle of whole-system ultrasound resonances, an optimal resonance mode is identified that is related to an acoustic resonance of the combined transducer-chip-channel system and not to the conventional pressure half-wave resonance of the microchannel. The acoustophoretic action in the microchannel is of comparable quality and strength to conventional silicon-glass or pure glass devices. The numerical predictions are validated by acoustic focusing experiments on 5-um-diameter polystyrene particles suspended inside a microchannel, which was milled into a PMMA-chip. The system was driven anti-symmetrically by a piezoelectric transducer, driven by a 30-V peak-to-peak AC-voltage in the range from 0.5 to 2.5 MHz, leading to acoustic energy densities of 13 J/m^3 and particle focusing times of 6.6 s.

physics.flu-dyn↗
arXiv2021

Fast microscale acoustic streaming driven by a temperature-gradient-induced non-dissipative acoustic body force

We study acoustic streaming in liquids driven by a non-dissipative acoustic body force created by light-induced temperature gradients. This thermoacoustic streaming produces a velocity amplitude approximately 50 times higher than boundary-driven Rayleigh streaming and 90 times higher than Rayleigh-Benard convection at a temperature gradient of 10 K/mm in the channel. Further, Rayleigh streaming is altered by the acoustic body force at a temperature gradient of only 0.5 K/mm. The thermoacoustic streaming allows for modular flow control and enhanced heat transfer at the microscale. Our study provides the groundwork for studying microscale acoustic streaming coupled with temperature fields.

physics.flu-dyn↗
arXiv2021

Numerical study of the coupling layer between transducer and chip in acoustofluidic devices

We study by numerical simulation in two and three dimensions the coupling layer between the transducer and the microfluidic chip in ultrasound acoustofluidic devices. The model includes the transducer with electrodes, the microfluidic chip with a liquid-filled microchannel, and the coupling layer between the transducer and the chip. We consider two commonly used coupling materials, solid epoxy glue and viscous glycerol, as well as two commonly used device types, glass capillary tubes and silicon-glass chips. We study how acoustic resonances in ideal devices without a coupling layer is either sustained or attenuated as a coupling layer of increasing thickness is inserted. We establish a simple criterion based on the phase of the acoustic wave for whether a given zero-layer resonance is sustained or attenuated by the addition of a coupling layer. Finally, we show that by controlling the thickness and the material, the coupling layer can be used as a design component for optimal and robust acoustofluidic resonances.

physics.flu-dyn↗
arXiv2021

Theory and simulation of AC electroosmotic suppression of acoustic streaming

Acoustic handling of nanoparticles in resonating acoustofluidic devices is often impeded by the presence of acoustic streaming. For micrometer-sized acoustic chambers, this acoustic streaming is typically driven from the fluid-solid interface by viscous shear-stresses generated by the acoustic actuation. AC electroosmosis is another boundary-driven streaming phenomena routinely used in microfluidic devices for handling of particle suspensions in electrolytes. Here, we study how streaming can be suppressed by combining ultrasound acoustics and AC electroosmosis. Based on a theoretical analysis of the electrokinetic problem, we are able to compute numerically a form of the electrical potential at the fluid-solid interface, which is suitable for suppressing a typical acoustic streaming pattern associated with a standing acoustic half-wave. In the linear regime, we even derive an analytical expression for the electroosmotic slip velocity at the fluid-solid interface, and use this as a guiding principle for developing models in the experimentally more relevant nonlinear regime that occurs at elevated driving voltages. We present simulation results for an acoustofluidic device, showing how implementing a suitable AC electroosmosis results in a suppression of the resulting streaming in the bulk of the device by two orders of magnitude.

physics.flu-dyn↗
arXiv2021

Bulk acoustofluidic devices driven by thin-film transducers and whole-system resonance modes

In acoustofluidics, acoustic resonance modes for fluid and microparticle handling are traditionally excited by bulk piezoelectric transducers. In this work, we demonstrate by numerical simulation in three dimensions (3D) that integrated piezoelectric thin-film transducers constituting less than 0.1% of the device work equally well. The simulations are done using a well-tested and experimentally validated numerical model. Our proof-of-concept example is a water-filled straight channel embedded in a mm-sized glass chip with a 1-um thick thin-film transducer made of (Al,Sc)N. We compute the acoustic energy, streaming, and radiation force, and show that it is comparable to that of a conventional silicon-glass device actuated by a bulk PZT transducer. The ability of the thin-film transducer to create the desired acoustofluidic effects in bulk acoustofluidic devices rely on three physical aspects: The in-plane-expansion of the thin-film transducer under the orthogonal applied electric field, the acoustic whole-system resonance of the device, and the high Q-factor of the elastic solid constituting the bulk part of the device. Consequently, the thin-film device is surprisingly insensitive to the Q-factor and resonance properties of the thin-film transducer.

physics.flu-dyn↗
arXiv2020

Theory of pressure acoustics with thermoviscous boundary layers and streaming in elastic cavities

We present an effective thermoviscous theory of acoustofluidics including pressure acoustics, thermoviscous boundary layers, and streaming for fluids embedded in elastic cavities. By including thermal fields, we thus extend the effective viscous theory by Bach and Bruus, J. Acoust. Soc. Am. 144, 766 (2018). The acoustic temperature field and the thermoviscous boundary layers are incorporated analytically as effective boundary conditions and time-averaged body forces on the thermoacoustic bulk fields. Because it avoids resolving the thin boundary layers, the effective model allows for numerical simulation of both thermoviscous acoustic and time-averaged fields in 3D models of acoustofluidic systems. We show how the acoustic streaming depends strongly on steady and oscillating thermal fields through the temperature dependency of the material parameters, in particular the viscosity and the compressibility, affecting both the boundary conditions and spawning additional body forces in the bulk. We also show how even small steady temperature gradients (1 K/mm) induce gradients in compressibility and density that may result in very high streaming velocities (1 mm/s) for moderate acoustic energy densities (100 J/m^3).

physics.flu-dyn↗
arXiv2020

Fabrication, characterization, and simulation of glass devices with AlN-thin-film-transducers for excitation of ultrasound resonances

We present fabrication of 570-um-thick, millimeter-sized soda-lime-silicate float glass blocks with a 1-um-thick AlN-thin-film piezoelectric transducer sandwiched between thin metallic electrodes and deposited on the top surface. The electro-mechanical properties are characterized by electrical impedance measurements in the frequency range from 0.1 to 10 MHz with a peak-to-peak voltage of 0.5 V applied to the electrodes. We measured the electrical impedance spectra of 35 devices, all of width 2 mm, but with 9 different lengths ranging from 2 to 6 mm and with 2-7 copies of each individual geometry. Each impedance spectrum exhibits many resonance peaks, of which we carefully measured the 5 most prominent ones in each spectrum. We compare the resulting 173 experimental resonance frequencies with the simulation result of a finite-element-method model that we have developed. When using material parameters from the manufacturer, we obtain an average relative deviation of the 173 simulated resonance frequencies from the experimental ones of (-4.2 +/-0.04)%. When optimizing the values of the Young's modulus and the Poisson ratio of the float glass in the simulation, this relative deviation decreased to (-0.03 +/- 0.04)%. Our results suggest a method for an accurate in-situ determination of the acoustic parameters at ultrasound frequencies of any elastic solid onto which a thin-film transducer can be attached

physics.app-ph↗