240 research outputs found
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
Scaling behavior of optimally structured catalytic microfluidic reactors
In this study of catalytic microfluidic reactors we show that, when optimally
structured, these reactors share underlying scaling properties. The scaling is
predicted theoretically and verified numerically. Furthermore, we show how to
increase the reaction rate significantly by distributing the active porous
material within the reactor using a high-level implementation of topology
optimization.Comment: 4 pages, 5 eps figure
Forces acting on a small particle in an acoustical field in a thermoviscous fluid
We present a theoretical analysis of the acoustic radiation force on a single
small particle, either a thermoviscous fluid droplet or a thermoelastic solid
particle, suspended in a viscous and heat-conducting fluid medium. Our analysis
places no restrictions on the length scales of the viscous and thermal boundary
layer thicknesses and relative to the
particle radius , but it assumes the particle to be small in comparison to
the acoustic wavelength . This is the limit relevant to scattering of
sound and ultrasound waves from micrometer-sized particles. For particles of
size comparable to or smaller than the boundary layers, the thermoviscous
theory leads to profound consequences for the acoustic radiation force. Not
only do we predict forces orders of magnitude larger than expected from
ideal-fluid theory, but for certain relevant choices of materials, we also find
a sign change in the acoustic radiation force on different-sized but otherwise
identical particles. This phenomenon may possibly be exploited in handling of
submicrometer-sized particles such as bacteria and vira in lab-on-a-chip
systems.Comment: Revtex, 23 pages, 4 eps figure
Acoustic interaction forces between small particles in an ideal fluid
We present a theoretical expression for the acoustic interaction force
between small spherical particles suspended in an ideal fluid exposed to an
external acoustic wave. The acoustic interaction force is the part of the
acoustic radiation force on one given particle involving the scattered waves
from the other particles. The particles, either compressible liquid droplets or
elastic microspheres, are considered to be much smaller than the acoustic
wavelength. In this so-called Rayleigh limit, the acoustic interaction forces
between the particles are well approximated by gradients of pair-interaction
potentials with no restriction on the inter-particle distance. The theory is
applied to studies of the acoustic interaction force on a particle suspension
in either standing or traveling plane waves. The results show aggregation
regions along the wave propagation direction, while particles may attract or
repel each other in the transverse direction. In addition, a mean-field
approximation is developed to describe the acoustic interaction force in an
emulsion of oil droplets in water.Comment: 11 pages, 5 eps figures, RevTex 4.
Concentration polarization, surface currents, and bulk advection in a microchannel
We present a comprehensive analysis of salt transport and overlimiting
currents in a microchannel during concentration polarization. We have carried
out full numerical simulations of the coupled Poisson-Nernst-Planck-Stokes
problem governing the transport and rationalized the behaviour of the system. A
remarkable outcome of the investigations is the discovery of strong couplings
between bulk advection and the surface current; without a surface current, bulk
advection is strongly suppressed. The numerical simulations are supplemented by
analytical models valid in the long channel limit as well as in the limit of
negligible surface charge. By including the effects of diffusion and advection
in the diffuse part of the electric double layers, we extend a recently
published analytical model of overlimiting current due to surface conduction.Comment: 15 pages, 11 figures, Revtex 4.
Three-Dimensional Numerical Modeling of Acoustic Trapping in Glass Capillaries
Acoustic traps are used to capture and handle suspended microparticles and
cells in microfluidic applications. A particular simple and much-used acoustic
trap consists of a commercially available, millimeter-sized, liquid-filled
glass capillary actuated by a piezoelectric transducer. Here, we present a
three-dimensional numerical model of the acoustic pressure field in the liquid
coupled to the displacement field of the glass wall, taking into account mixed
standing and traveling waves as well as absorption. The model predicts
resonance modes well suited for acoustic trapping, their frequencies and
quality factors, the magnitude of the acoustic radiation force on a single test
particle as a function of position, and the resulting acoustic retention force
of the trap. We show that the model predictions are in agreement with published
experimental results, and we discuss how improved and more stable acoustic
trapping modes might be obtained using the model as a design tool.Comment: 13 pages, 15 pdf figures, pdfLatex/Revte
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