291 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
Localized plasmons in point contacts
Using a hydrodynamic model of the electron fluid in a point contact geometry
we show that localized plasmons are likely to exist near the constriction. We
attempt to relate these plasmons with the recent experimental observation of
deviations of the quantum point contact conductance from ideal integer
quantization. As a function of temperature this deviation exhibits an activated
behavior, exp(-T_a/T), with a density dependent activation temperature T_a of
the order of 2 K. We suggest that T_a can be identified with the energy needed
to excite localized plasmons, and we discuss the conductance deviations in
terms of a simple theoretical model involving quasiparticle lifetime broadening
due to coupling to the localized plasmons.Comment: 5 pages (Latex) including 1 postscript figur
A sharp-interface model of electrodeposition and ramified growth
We present a sharp-interface model of two-dimensional ramified growth during
quasi-steady electrodeposition. Our model differs from previous modeling
methods in that it includes the important effects of extended space-charge
regions and nonlinear electrode reactions. The model is validated by comparing
its behavior in the initial stage with the predictions of a linear stability
analysis.Comment: RevTex, 14 pages, 12 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.
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
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
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.
The Spectrum of the two dimensional Hubbard model at low filling
Using group theoretical and numerical methods we have calculated the exact
energy spectrum of the two-dimensional Hubbard model on square lattices with
four electrons for a wide range of the interaction strength. All known
symmetries, i.e.\ the full space group symmetry, the SU(2) spin symmetry, and,
in case of a bipartite lattice, the SU(2) pseudospin symmetry, have been taken
explicitly into account. But, quite remarkably, a large amount of residual
degeneracies remains giving strong evidence for the existence of a yet unknown
symmetry. The level spacing distribution and the spectral rigidity are found to
be in close to but not exact agreement with random matrix theory. In contrast,
the level velocity correlation function presents an unexpected exponential
decay qualitatively different from random matrix behavior.Comment: 4 pages, latex (revtex), 3 uuencoded postscript figure
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