1,775 research outputs found
Acoustic radiation- and streaming-induced microparticle velocities determined by micro-PIV in an ultrasound symmetry plane
We present micro-PIV measurements of suspended microparticles of diameters
from 0.6 um to 10 um undergoing acoustophoresis in an ultrasound symmetry plane
in a microchannel. The motion of the smallest particles are dominated by the
Stokes drag from the induced acoustic streaming flow, while the motion of the
largest particles are dominated by the acoustic radiation force. For all
particle sizes we predict theoretically how much of the particle velocity is
due to radiation and streaming, respectively. These predictions include
corrections for particle-wall interactions and ultrasonic thermoviscous
effects, and they match our measurements within the experimental uncertainty.
Finally, we predict theoretically and confirm experimentally that the ratio
between the acoustic radiation- and streaming-induced particle velocities is
proportional to the square of the particle size, the actuation frequency and
the acoustic contrast factor, while it is inversely proportional to the
kinematic viscosity.Comment: 11 pages, 9 figures, RevTex 4-
Ultrasound-induced acoustophoretic motion of microparticles in three dimensions
We derive analytical expressions for the three-dimensional (3D)
acoustophoretic motion of spherical microparticles in rectangular
microchannels. The motion is generated by the acoustic radiation force and the
acoustic streaming-induced drag force. In contrast to the classical theory of
Rayleigh streaming in shallow, infinite, parallel-plate channels, our theory
does include the effect of the microchannel side walls. The resulting
predictions agree well with numerics and experimental measurements of the
acoustophoretic motion of polystyrene spheres with nominal diameters of 0.537
um and 5.33 um. The 3D particle motion was recorded using astigmatism particle
tracking velocimetry under controlled thermal and acoustic conditions in a
long, straight, rectangular microchannel actuated in one of its transverse
standing ultrasound-wave resonance modes with one or two half-wavelengths. The
acoustic energy density is calibrated in situ based on measurements of the
radiation dominated motion of large 5-um-diam particles, allowing for
quantitative comparison between theoretical predictions and measurements of the
streaming induced motion of small 0.5-um-diam particles.Comment: 13 pages, 8 figures, Revtex 4.
Acoustic Tweezing and Patterning of Concentration Fields in Microfluidics
We demonstrate theoretically that acoustic forces acting on inhomogeneous
fluids can be used to pattern and manipulate solute concentration fields into
spatio-temporally controllable configurations stabilized against gravity. A
theoretical framework describing the dynamics of concentration fields that
weakly perturb the fluid density and speed of sound is presented and applied to
study manipulation of concentration fields in rectangular-channel acoustic
eigenmodes and in Bessel-function acoustic vortices. In the first example,
methods to obtain horizontal and vertical multi-layer stratification of the
concentration field at the end of a flow-through channel are presented. In the
second example, we demonstrate acoustic tweezing and spatio-temporal
manipulation of a local high-concentration region in a lower-concentration
medium, thereby extending the realm of acoustic tweezing to include
concentration fields.Comment: Revtex, 9 pages, 5 eps figure
Current methods for characterising mixing and flow in microchannels
This article reviews existing methods for the characterisation of mixing and flow in microchannels, micromixers and microreactors. In particular, it analyses the current experimental techniques and methods available for characterising mixing and the associated phenomena in single and multiphase flow. The review shows that the majority of the experimental techniques used for characterising mixing and two-phase flow in microchannels employ optical methods, which require optical access to the flow, or off-line measurements. Indeed visual measurements are very important for the fundamental understanding of the physics of these flows and the rapid advances in optical measurement techniques, like confocal scanning laser microscopy and high resolution stereo micro particle image velocimetry, are now making full field data retrieval possible. However, integration of microchannel devices in industrial processes will require on-line measurements for process control that do not necessarily rely on optical techniques. Developments are being made in the areas of non-intrusive sensors, magnetic resonance techniques, ultrasonic spectroscopy and on-line flow through measurement cells. The advances made in these areas will certainly be of increasing interest in the future as microchannels are more frequently employed in continuous flow equipment for industrial applications
On the forces acting on a small particle in an acoustical field in a viscous fluid
We calculate the acoustic radiation force from an ultrasound wave on a
compressible, spherical particle suspended in a viscous fluid. Using
Prandtl--Schlichting boundary-layer theory, we include the kinematic viscosity
of the solvent and derive an analytical expression for the resulting radiation
force, which is valid for any particle radius and boundary-layer thickness
provided that both of these length scales are much smaller than the wavelength
of the ultrasound wave (mm in water at MHz frequencies). The acoustophoretic
response of suspended microparticles is predicted and analyzed using parameter
values typically employed in microchannel acoustophoresis.Comment: 12 pages, 4 figure
Three-dimensional phenomena in microbubble acoustic streaming
Ultrasound-driven oscillating micro-bubbles have been used as active
actuators in microfluidic devices to perform manifold tasks such as mixing,
sorting and manipulation of microparticles. A common configuration consists on
side-bubbles, created by trapping air pockets in blind channels perpendicular
to the main channel direction. This configuration consists of acoustically
excited bubbles with a semi-cylindrical shape that generate significant
streaming flow. Due to the geometry of the channels, such flows have been
generally considered as quasi two-dimensional. Similar assumptions are often
made in many other microfluidic systems based on \emph{flat} micro-channels.
However, in this paper we show that microparticle trajectories actually present
a much richer behavior, with particularly strong out-of-plane dynamics in
regions close to the microbubble interface. Using Astigmatism Particle Tracking
Velocimetry, we reveal that the apparent planar streamlines are actually
projections of a \emph{streamsurface} with a pseudo-toroidal shape. We
therefore show that acoustic streaming cannot generally be assumed as a
two-dimensional phenomenon in confined systems. The results have crucial
consequences for most of the applications involving acoustic streaming as
particle trapping, sorting and mixing.Comment: 5 pages, 4 high quality figures. Accepted for Publication in Phys.
Rev. Applied, March 201
Numerical and experimental analysis of a hybrid material acoustophoretic device for manipulation of microparticles.
Acoustophoretic microfluidic devices have been developed for accurate, label-free, contactless, and non-invasive manipulation of bioparticles in different biofluids. However, their widespread application is limited due to the need for the use of high quality microchannels made of materials with high specific acoustic impedances relative to the fluid (e.g., silicon or glass with small damping coefficient), manufactured by complex and expensive microfabrication processes. Soft polymers with a lower fabrication cost have been introduced to address the challenges of silicon- or glass-based acoustophoretic microfluidic systems. However, due to their small acoustic impedance, their efficacy for particle manipulation is shown to be limited. Here, we developed a new acoustophoretic microfluid system fabricated by a hybrid sound-hard (aluminum) and sound-soft (polydimethylsiloxane polymer) material. The performance of this hybrid device for manipulation of bead particles and cells was compared to the acoustophoretic devices made of acoustically hard materials. The results show that particles and cells in the hybrid material microchannel travel to a nodal plane with a much smaller energy density than conventional acoustic-hard devices but greater than polymeric microfluidic chips. Against conventional acoustic-hard chips, the nodal line in the hybrid microchannel could be easily tuned to be placed in an off-center position by changing the frequency, effective for particle separation from a host fluid in parallel flow stream models. It is also shown that the hybrid acoustophoretic device deals with smaller temperature rise which is safer for the actuation of bioparticles. This new device eliminates the limitations of each sound-soft and sound-hard materials in terms of cost, adjusting the position of nodal plane, temperature rise, fragility, production cost and disposability, making it desirable for developing the next generation of economically viable acoustophoretic products for ultrasound particle manipulation in bioengineering applications
Efficient production of hybrid bio-nanomaterials by continuous microchannel emulsification: Dye-doped SiO2 and Au-PLGA nanoparticles
A novel microfluidic system was designed to produce in a continuous manner hybrid nanomaterials using the microchannel double w/o/w emulsification technique. Double w/o/w nanoemulsions were produced combining two inter-digital micromixers that afford working in continuous flow and with a high reproducibility and control on the reaction conditions. High throughput production of two hybrid nanomaterials, dye-doped SiO2 (4 mg/min) and Au-loaded poly(lactic-co-glycolic) acid (PLGA) (168 mg/min) nanoparticles, were achieved, showing the resulting nanomaterials excellent and reproducible optical properties and tunable loading. These hybrid nanomaterials could be potentially used in different biomedical applications. In addition, the microfluidic system designed for carrying out double emulsification enabled to decrease the particle size distribution of dye-doped SiO2 nanoparticles (NPs) up to 20 nm and to improve the Au NPs loading efficiency in Au-loaded PLGA hybrid nanoparticles. The excellent control achieved in the Au NPs loading allowed tuning the payload on demand. Finally, the microfluidic system designed in this work overpasses the productivity described in previously published batch-type reactors, while assuring the same properties of the resulting hybrid nanomaterials
A meso-scale ultrasonic milli-reactor enables gas–liquid-solid photocatalytic reactions in flow
The handling of solid reagents, catalysts and by-products is a daunting challenge in continuous-flow micro- and milli-reactors. Suspensions tend to settle over time leading to irrevocable clogging of the reaction channels. Herein, we describe our efforts to develop an ultrasonic milli-reactor which can handle such challenging solid-containing transformations. The reactor consists of a Langevin-type transducer, a sonotrode and an irradiating cylinder, on which a coiled glass capillary (12.88 mL) was attached. The ultrasonic milli-reactor was combined with an LED illuminating box and its efficacy was showcased in the photocatalytic aerobic oxidation of benzyl alcohol enabled by TiO2 particles exposed to UV-A irradiation. Ultrasound irradiation generates cavitation bubbles and causes a vigorous oscillation of both the cavitation and the Taylor bubbles. This improves the liquid mixing, the gas–liquid mass transfer and ensures resuspension of the settled particles. Moreover, these effects enhance the photon absorption by the semiconductor catalyst, which has an overall positive effect on the photocatalytic transformation
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