Special Issue "Acoustofluidics"

A special issue of Micromachines (ISSN 2072-666X). This special issue belongs to the section "A:Physics".

Deadline for manuscript submissions: closed (10 July 2020).

Special Issue Editor

Dr. Philippe Brunet
E-Mail Website
Guest Editor
Laboratoire MSC, UMR 7057 CNRS, Université Paris Diderot, Paris, France
Interests: soft matter; capillary flows; hydrodynamic instabilities; acoustofluidics; active suspensions

Special Issue Information

Dear colleagues,

We are opening this Special Issue on Acoustofluidics, which aims to encompass recent significant studies in this timely field. The manipulation of fluids and particles, e.g., with piezoelectric transducers of typical frequency between a few kHz and several tens of MHz, has recently received a lot of interest, especially in microfluidics processes. Over the last two decades, it was demonstrated that operative and versatile fluid actuation and/or analysis could be achieved by acoustic waves in micro-channels, thin films, droplets, bubbles, foams, emulsions, or porous membranes.

Still, open questions remain and the optimization of processes requires additional knowledge in both fundamental and practical aspects, for instance in the choice of frequency for optimal conditions within given geometries and a large span of fluids, or strategies for using low power actuation to handle fragile bio-fluids.

This Special Issue on Acoustofluidics aims to cover but is not limited to the following topics:

  • Acoustical tweezers and acoustophoresis, in particular particles, living cells or microorganisms, and bubbles manipulation with acoustics
  • Acoustic streaming and application to mixing (analysis and experimentation)
  • Fluid interface manipulation using ultrasound via streaming and radiation pressure, including atomization, droplet generation, and thin films (analysis and experimentation)
  • Integrated acoustofluidics devices for energy, chemical, biological, and medical applications

Dr. Philippe Brunet
Guest Editor

Manuscript Submission Information

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Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1800 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Microfluidics
  • Acoustophoresis
  • Mixing
  • Acoustic for (Bio)-analyses
  • Acoustic Streaming
  • Radiation Pressure

Published Papers (10 papers)

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Research

Open AccessArticle
Comparison of Acoustic Streaming Flow Patterns Induced by Solid, Liquid and Gas Obstructions
Micromachines 2020, 11(10), 891; https://doi.org/10.3390/mi11100891 - 26 Sep 2020
Viewed by 516
Abstract
In this study, acoustic streaming flows inside micro-channels induced by three different types of obstruction—gaseous bubble, liquid droplet and solid bulge—are compared and investigated experimentally by particle tracking velocimetry (PTV) and numerically using the finite element method (FEM). The micro-channels are made by [...] Read more.
In this study, acoustic streaming flows inside micro-channels induced by three different types of obstruction—gaseous bubble, liquid droplet and solid bulge—are compared and investigated experimentally by particle tracking velocimetry (PTV) and numerically using the finite element method (FEM). The micro-channels are made by poly(dimethylsiloxane) (PDMS) using soft lithography with low-cost micro-machined mold. The characteristic dimensions of the media are 0.2 mm in diameter, and the oscillation generated by piezoelectric actuators has frequency of 12 kHz and input voltages of 40 V. The experimental results show that in all three obstruction types, a pair of counter-rotating vortical patterns were observed around the semi-circular obstructions. The gaseous bubble creates the strongest vortical streaming flow, which can reach a maximum of 21 mm/s, and the largest u component happens at Y/D = 0. The solid case is the weakest of the three, which can only reach 2 mm/s. The liquid droplet has the largest v components and speed at Y/D = 0.5 and Y/D = 0.6. Because of the higher density and incompressibility of liquid droplet compared to the gaseous bubble, the liquid droplet obstruction transfers the oscillation of the piezo plate most efficiently, and the induced streaming flow region and average speed are both the largest of the three. An investigation using numerical simulation shows that the differing interfacial conditions between the varying types of obstruction boundaries to the fluid may be the key factor to these differences. These results suggest that it might be more energy-efficient to design an acoustofluidic device using a liquid droplet obstruction to induce the stronger streaming flow. Full article
(This article belongs to the Special Issue Acoustofluidics)
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Open AccessArticle
PIV-Based Acoustic Pressure Measurements of a Single Bubble near the Elastic Boundary
Micromachines 2020, 11(7), 637; https://doi.org/10.3390/mi11070637 - 29 Jun 2020
Viewed by 471
Abstract
The objective of this paper was to investigate acoustic pressure waves and the transient flow structure emitted from the single bubble near an elastic boundary based on the particle image velocimetry (PIV). A combination of an electric-spark bubble generator and PIV were used [...] Read more.
The objective of this paper was to investigate acoustic pressure waves and the transient flow structure emitted from the single bubble near an elastic boundary based on the particle image velocimetry (PIV). A combination of an electric-spark bubble generator and PIV were used to measure the temporal bubble shapes, transient flow structure, as well as the mid-span deflection of an elastic boundary. Results are presented for three different initial positions near an elastic boundary, which were compared with results obtained using a rigid boundary. A formula relating velocity and pressure was proposed to calculate the acoustic pressure contours surrounding a bubble based on the velocity field of the transient flow structure obtained using PIV. The results show the bubbles near the elastic boundary presented a “mushroom” bubble and an inverted cone bubble. Based on the PIV-measured acoustic pressure contours, a significant pressure difference is found between the elastic boundary and the underside of the bubble, which contributed to the formation of the “mushroom” bubble and inverted cone bubble. Furthermore, the bubbles had opposite migration direction near rigid and elastic boundaries, respectively. In detail, the bubble was repelled away from the elastic boundary and the bubble was attracted by the rigid boundary. The resultant force made up of a Bjerknes force and buoyancy force dominated the migration direction of the bubble. Full article
(This article belongs to the Special Issue Acoustofluidics)
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Open AccessArticle
Acoustic Streaming Generated by Sharp Edges: The Coupled Influences of Liquid Viscosity and Acoustic Frequency
Micromachines 2020, 11(6), 607; https://doi.org/10.3390/mi11060607 - 22 Jun 2020
Cited by 3 | Viewed by 752
Abstract
Acoustic streaming can be generated around sharp structures, even when the acoustic wavelength is much larger than the vessel size. This sharp-edge streaming can be relatively intense, owing to the strongly focused inertial effect experienced by the acoustic flow near the tip. We [...] Read more.
Acoustic streaming can be generated around sharp structures, even when the acoustic wavelength is much larger than the vessel size. This sharp-edge streaming can be relatively intense, owing to the strongly focused inertial effect experienced by the acoustic flow near the tip. We conducted experiments with particle image velocimetry to quantify this streaming flow through the influence of liquid viscosity ν , from 1 mm 2 /s to 30 mm 2 /s, and acoustic frequency f from 500 Hz to 3500 Hz. Both quantities supposedly influence the thickness of the viscous boundary layer δ = ν π f 1 / 2 . For all situations, the streaming flow appears as a main central jet from the tip, generating two lateral vortices beside the tip and outside the boundary layer. As a characteristic streaming velocity, the maximal velocity is located at a distance of δ from the tip, and it increases as the square of the acoustic velocity. We then provide empirical scaling laws to quantify the influence of ν and f on the streaming velocity. Globally, the streaming velocity is dramatically weakened by a higher viscosity, whereas the flow pattern and the disturbance distance remain similar regardless of viscosity. Besides viscosity, the frequency also strongly influences the maximal streaming velocity. Full article
(This article belongs to the Special Issue Acoustofluidics)
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Open AccessArticle
MHz-Order Surface Acoustic Wave Thruster for Underwater Silent Propulsion
Micromachines 2020, 11(4), 419; https://doi.org/10.3390/mi11040419 - 16 Apr 2020
Viewed by 744
Abstract
High frequency (MHz-order) surface acoustic waves (SAW) are able to generate intense fluid flow from the attenuation of acoustic radiation in viscous fluids as acoustic streaming. Though such flows are known to produce a force upon the fluid and an equivalent and opposing [...] Read more.
High frequency (MHz-order) surface acoustic waves (SAW) are able to generate intense fluid flow from the attenuation of acoustic radiation in viscous fluids as acoustic streaming. Though such flows are known to produce a force upon the fluid and an equivalent and opposing force upon the object producing the acoustic radiation, there is no convenient method for measuring this force. We describe a new method to accomplish this aim, noting the potential of these devices in providing essentially silent underwater propulsion by virtue of their use of the sound itself to generate fluid momentum flux. Our example employs a 40 MHz SAW device as a pendulum bob while immersed in a fluid, measuring a 1.5 mN propulsion force from an input power of 5 W power to the SAW device. Supporting details regarding the acoustic streaming profile via particle image velocimetry and an associated theoretical model are provided to aid in the determination of the propulsion force knowing the applied power and fluid characteristics. Finally, a simple model is provided to aid the selection of the acoustic device size to maximize the propulsion force per unit device area, a key figure of merit in underwater propulsion devices. Using this model, a maximum force of approximately 10 mN/cm 2 was obtained from 1 W input power using 40 MHz SAW in water and producing a power efficiency of approximately 50%. Given the advantages of this technology in silent propulsion with such large efficiency and propulsion force per unit volume, it seems likely this method will be beneficial in propelling small autonomous submersibles. Full article
(This article belongs to the Special Issue Acoustofluidics)
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Open AccessArticle
Coalescence Dynamics of Acoustically Levitated Droplets
Micromachines 2020, 11(4), 343; https://doi.org/10.3390/mi11040343 - 26 Mar 2020
Cited by 5 | Viewed by 981
Abstract
The contactless coalescence of a droplet is of paramount importance for physical and industrial applications. This paper describes a coalescence method to be used mid-air via acoustic levitation using an ultrasonic phased array system. Acoustic levitation using ultrasonic phased arrays provides promising lab-on-a-drop [...] Read more.
The contactless coalescence of a droplet is of paramount importance for physical and industrial applications. This paper describes a coalescence method to be used mid-air via acoustic levitation using an ultrasonic phased array system. Acoustic levitation using ultrasonic phased arrays provides promising lab-on-a-drop applications, such as transportation, coalescence, mixing, separation, evaporation, and extraction in a continuous operation. The mechanism of droplet coalescence in mid-air may be better understood by experimentally and numerically exploring the droplet dynamics immediately before the coalescence. In this study, water droplets were experimentally levitated, transported, and coalesced by controlled acoustic fields. We observed that the edges of droplets deformed and attracted each other immediately before the coalescence. Through image processing, the radii of curvature of the droplets were quantified and the pressure difference between the inside and outside a droplet was simulated to obtain the pressure and velocity information on the droplet’s surface. The results revealed that the sound pressure acting on the droplet clearly decreased before the impact of the droplets. This pressure on the droplets was quantitatively analyzed from the experimental data. Our experimental and numerical results provide deeper physical insights into contactless droplet manipulation for futuristic lab-on-a-drop applications. Full article
(This article belongs to the Special Issue Acoustofluidics)
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Open AccessArticle
Microparticle Acoustophoresis in Aluminum-Based Acoustofluidic Devices with PDMS Covers
Micromachines 2020, 11(3), 292; https://doi.org/10.3390/mi11030292 - 11 Mar 2020
Cited by 6 | Viewed by 1038
Abstract
We present a numerical model for the recently introduced simple and inexpensive micromachined aluminum devices with a polydimethylsiloxane (PDMS) cover for microparticle acoustophoresis. We validate the model experimentally for a basic design, where a microchannel is milled into the surface of an aluminum [...] Read more.
We present a numerical model for the recently introduced simple and inexpensive micromachined aluminum devices with a polydimethylsiloxane (PDMS) cover for microparticle acoustophoresis. We validate the model experimentally for a basic design, where a microchannel is milled into the surface of an aluminum substrate, sealed with a PDMS cover, and driven at MHz frequencies by a piezoelectric lead-zirconate-titanate (PZT) transducer. Both experimentally and numerically we find that the soft PDMS cover suppresses the Rayleigh streaming rolls in the bulk. However, due to the low transverse speed of sound in PDMS, such devices are prone to exhibit acoustic streaming vortices in the corners with a relatively large velocity. We predict numerically that in devices, where the microchannel is milled all the way through the aluminum substrate and sealed with a PDMS cover on both the top and bottom, the Rayleigh streaming is suppressed in the bulk thus enabling focusing of sub-micrometer-sized particles. Full article
(This article belongs to the Special Issue Acoustofluidics)
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Open AccessArticle
Fabrication of Silicon Microfluidic Chips for Acoustic Particle Focusing Using Direct Laser Writing
Micromachines 2020, 11(2), 113; https://doi.org/10.3390/mi11020113 - 21 Jan 2020
Cited by 2 | Viewed by 998
Abstract
We have developed a fast and simple method for fabricating microfluidic channels in silicon using direct laser writing. The laser microfabrication process was optimised to generate microfluidic channels with vertical walls suitable for acoustic particle focusing by bulk acoustic waves. The width of [...] Read more.
We have developed a fast and simple method for fabricating microfluidic channels in silicon using direct laser writing. The laser microfabrication process was optimised to generate microfluidic channels with vertical walls suitable for acoustic particle focusing by bulk acoustic waves. The width of the acoustic resonance channel was designed to be 380 µm, branching into a trifurcation with 127 µm wide side outlet channels. The optimised settings used to make the microfluidic channels were 50% laser radiation power, 10 kHz pulse frequency and 35 passes. With these settings, six chips could be ablated in 5 h. The microfluidic channels were sealed with a glass wafer using adhesive bonding, diced into individual chips, and a piezoelectric transducer was glued to each chip. With acoustic actuation at 2.03 MHz a half wavelength resonance mode was generated in the microfluidic channel, and polystyrene microparticles (10 µm diameter) were focused along the centre-line of the channel. The presented fabrication process is especially interesting for research purposes as it opens up for rapid prototyping of silicon-glass microfluidic chips for acoustofluidic applications. Full article
(This article belongs to the Special Issue Acoustofluidics)
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Open AccessArticle
Modeling and Analysis of the Two-Dimensional Axisymmetric Acoustofluidic Fields in the Probe-Type and Substrate-Type Ultrasonic Micro/Nano Manipulation Systems
Micromachines 2020, 11(1), 22; https://doi.org/10.3390/mi11010022 - 24 Dec 2019
Viewed by 832
Abstract
The probe-type and substrate-type ultrasonic micro/nano manipulation systems have proven to be two kinds of powerful tools for manipulating micro/nanoscale materials. Numerical simulations of acoustofluidic fields in these two kinds of systems can not only be used to explain and analyze the physical [...] Read more.
The probe-type and substrate-type ultrasonic micro/nano manipulation systems have proven to be two kinds of powerful tools for manipulating micro/nanoscale materials. Numerical simulations of acoustofluidic fields in these two kinds of systems can not only be used to explain and analyze the physical mechanisms of experimental phenomena, but also provide guidelines for optimization of device parameters and working conditions. However, in-depth quantitative study and analysis of acoustofluidic fields in the two ultrasonic micro/nano manipulation systems have scarcely been reported. In this paper, based on the finite element method (FEM), we numerically investigated the two-dimensional (2D) axisymmetric acoustofluidic fields in the probe-type and substrate-type ultrasonic micro/nano manipulation systems by the perturbation method (PM) and Reynolds stress method (RSM), respectively. Through comparing the simulation results computed by the two methods and the experimental verifications, the feasibility and reasonability of the two methods in simulating the acoustofluidic fields in these two ultrasonic micro/nano manipulation systems have been validated. Moreover, the effects of device parameters and working conditions on the acoustofluidic fields are clarified by the simulation results and qualitatively verified by the experiments. Full article
(This article belongs to the Special Issue Acoustofluidics)
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Open AccessArticle
Local Acoustic Fields Powered Assembly of Microparticles and Applications
Micromachines 2019, 10(12), 882; https://doi.org/10.3390/mi10120882 - 16 Dec 2019
Cited by 1 | Viewed by 892
Abstract
Controllable assembly in nano-/microscale holds considerable promise for bioengineering, intracellular manipulation, diagnostic sensing, and biomedical applications. However, up to now, micro-/nanoscopic assembly methods are severely limited by the fabrication materials, as well as energy sources to achieve the effective propulsion. In particular, reproductive [...] Read more.
Controllable assembly in nano-/microscale holds considerable promise for bioengineering, intracellular manipulation, diagnostic sensing, and biomedical applications. However, up to now, micro-/nanoscopic assembly methods are severely limited by the fabrication materials, as well as energy sources to achieve the effective propulsion. In particular, reproductive manipulation and customized structure is quite essential for assemblies to accomplish a variety of on-demand tasks at small scales. Here, we present an attractive assembly strategy to collect microparticles, based on local acoustic forces nearby microstructures. The micro-manipulation chip is built based on an enhanced acoustic field, which could tightly trap microparticles to the boundaries of the microstructure by tuning the applied driving frequency and voltage. Numerical simulations and experimental demonstrations illustrate that the capturing and assembly of microparticles is closely related to the size of particles, owing to the vibration-induced locally enhanced acoustic field and resultant propulsion force. This acoustic assembly strategy can open extensive opportunities for lab-on-chip systems, microfactories, and micro-manipulators, among others. Full article
(This article belongs to the Special Issue Acoustofluidics)
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Open AccessArticle
Diversity of 2D Acoustofluidic Fields in an Ultrasonic Cavity Generated by Multiple Vibration Sources
Micromachines 2019, 10(12), 803; https://doi.org/10.3390/mi10120803 - 22 Nov 2019
Cited by 1 | Viewed by 1043
Abstract
Two-dimensional acoustofluidic fields in an ultrasonic chamber actuated by segmented ring-shaped vibration sources with different excitation phases are simulated by COMSOL Multiphysics. Diverse acoustic streaming patterns, including aggregation and rotational modes, can be feasibly generated by the excitation of several sessile ultrasonic sources [...] Read more.
Two-dimensional acoustofluidic fields in an ultrasonic chamber actuated by segmented ring-shaped vibration sources with different excitation phases are simulated by COMSOL Multiphysics. Diverse acoustic streaming patterns, including aggregation and rotational modes, can be feasibly generated by the excitation of several sessile ultrasonic sources which only vibrate along radial direction. Numerical simulation of particle trajectory driven by acoustic radiation force and streaming-induced drag force also demonstrates that micro-scale particles suspended in the acoustofluidic chamber can be trapped in the velocity potential well of fluid flow or can rotate around the cavity center with the circumferential acoustic streaming field. Preliminary investigation of simple Russian doll- or Matryoshka-type configurations (double-layer vibration sources) provide a novel method of multifarious structure design in future researches on the combination of phononic crystals and acoustic streaming fields. The implementation of multiple segmented ring-shaped vibration sources offers flexibility for the control of acoustic streaming fields in microfluidic devices for various applications. We believe that this kind of acoustofluidic design is expected to be a promising tool for the investigation of rapid microfluidic mixing on a chip and contactless rotational manipulation of biosamples, such as cells or nematodes. Full article
(This article belongs to the Special Issue Acoustofluidics)
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