494 research outputs found
Kinetic pathways of the Nematic-Isotropic phase transition as studied by confocal microscopy on rod-like viruses
We investigate the kinetics of phase separation for a mixture of rodlike
viruses (fd) and polymer (dextran), which effectively constitutes a system of
attractive rods. This dispersion is quenched from a flow-induced fully nematic
state into the region where the nematic and the isotropic phase coexist. We
show experimental evidence that the kinetic pathway depends on the overall
concentration. When the quench is made at high concentrations, the system is
meta-stable and we observe typical nucleation-and-growth. For quenches at low
concentration the system is unstable and the system undergoes a spinodal
decomposition. At intermediate concentrations we see the transition between
both demixing processes, where we locate the spinodal point.Comment: 11 pages, 6 figures, accepted in J. Phys.: Condens. Matter as
symposium paper for the 6th Liquid Matter Conference in Utrech
Dynamic Monte Carlo Simulations of Anisotropic Colloids
We put forward a simple procedure for extracting dynamical information from
Monte Carlo simulations, by appropriate matching of the short-time diffusion
tensor with its infinite-dilution limit counterpart, which is supposed to be
known. This approach --discarding hydrodynamics interactions-- first allows us
to improve the efficiency of previous Dynamic Monte Carlo algorithms for
spherical Brownian particles. In a second step, we address the case of
anisotropic colloids with orientational degrees of freedom. As an illustration,
we present a detailed study of the dynamics of thin platelets, with emphasis on
long-time diffusion and orientational correlations.Comment: 12 pages, 9 figure
Effective Confinement as Origin of the Equivalence of Kinetic Temperature and Fluctuation-Dissipation Ratio in a Dense Shear Driven Suspension
We study response and velocity autocorrelation functions for a tagged
particle in a shear driven suspension governed by underdamped stochastic
dynamics. We follow the idea of an effective confinement in dense suspensions
and exploit a time-scale separation between particle reorganization and
vibrational motion. This allows us to approximately derive the
fluctuation-dissipation theorem in a "hybrid" form involving the kinetic
temperature as an effective temperature and an additive correction term. We
show numerically that even in a moderately dense suspension the latter is
negligible. We discuss similarities and differences with a simple toy model, a
single trapped particle in shear flow
Colloids dragged through a polymer solution: experiment, theory and simulation
We present micro-rheological measurments of the drag force on colloids pulled
through a solution of lambda-DNA (used here as a monodisperse model polymer)
with an optical tweezer. The experiments show a violation of the
Stokes-Einstein relation based on the independently measured viscosity of the
DNA solution: the drag force is larger than expected. We attribute this to the
accumulation of DNA infront of the colloid and the reduced DNA density behind
the colloid. This hypothesis is corroborated by a simple drift-diffusion model
for the DNA molecules, which reproduces the experimental data surprisingly
well, as well as by corresponding Brownian dynamics simulations.Comment: 9 pages, 13 figures, 3 table
Chiral separation in microflows
Molecules that only differ by their chirality, so called enantiomers, often
possess different properties with respect to their biological function.
Therefore, the separation of enantiomers presents a prominent challenge in
molecular biology and belongs to the ``Holy Grail'' of organic chemistry. We
suggest a new separation technique for chiral molecules that is based on the
transport properties in a microfluidic flow with spatially variable vorticity.
Because of their size the thermal fluctuating motion of the molecules must be
taken into account. These fluctuations play a decisive role in the proposed
separation mechanism
On the interplay between sedimentation and phase separation phenomena in two-dimensional colloidal fluids
Colloidal particles that are confined to an interface effectively form a
two-dimensional fluid. We examine the dynamics of such colloids when they are
subject to a constant external force, which drives them in a particular
direction over the surface. Such a situation occurs, for example, for colloidal
particles that have settled to the bottom of their container, when the
container is tilted at an angle, so that they `sediment' to the lower edge of
the surface. We focus in particular on the case when there are attractive
forces between the colloids which causes them to phase separate into regions of
high density and low density and we study the influence of this phase
separation on the sedimentation process. We model the colloids as Brownian
particles and use both Brownian dynamics computer simulations and dynamical
density functional theory (DDFT) to obtain the time evolution of the ensemble
average one-body density profiles of the colloids. We consider situations where
the external potential varies only in one direction so that the ensemble
average density profiles vary only in this direction. We solve the DDFT in
one-dimension, by assuming that the density profile only varies in one
direction. However, we also solve the DDFT in two-dimensions, allowing the
fluid density profile to vary in both the - and -directions. We find that
in certain situations the two-dimensional DDFT is clearly superior to its
one-dimensional counterpart when compared with the simulations and we discuss
this issue.Comment: 17 pages, 10 figures, submitted to Molecular Physic
A dynamic density functional theory for particles in a flowing solvent
We present a dynamic density functional theory (dDFT) which takes into accou
nt the advection of the particles by a flowing solvent. For potential flows we
can use the same closure as in the absence of solvent flow. The structure of
the resulting advected dDFT suggests that it could be used for non-potential
flows as well. We apply this dDFT to Brownian particles (e.g., polymer coils)
in a solvent flowing around a spherical obstacle (e.g., a colloid) and compare
the results with direct simulations of the underlying Brownian dynamics.
Although numerical limitations do not allow for an accurate quantitative
check of the advected dDFT both show the same qualitative features. In contrast
to previous works which neglected the deformation of the flow by the obstacle,
we find that the bow-wave in the density distribution of particles in front of
the obstacle as well as the wake behind it are reduced dramatically. As a
consequence the friction force exerted by the (polymer) particles on the
colloid can be reduced drastically.Comment: 7 pages, 5 figures, 2 tables, submitte
Successful Treatment of Early Endometrial Carcinoma by Local Delivery of Levonorgestrel: A Case Report
We describe a case of a 67-year-old Caucasian woman with an early, moderately-differentiated adenocarcinoma of the endometrium. A levonorgestrel-releasing intrauterine system was inserted, which she tolerated well. A full D&C, following removal of the device, was performed after 9 months, confirming absence of tumoral tissue. Examination after 24 months showed a very thin endometrium, indicating complete remission
Reentrance effect in the lane formation of driven colloids
Recently it has been shown that a strongly interacting colloidal mixture
consisting of oppositely driven particles, undergoes a nonequilibrium
transition towards lane formation provided the driving strength exceeds a
threshold value. We predict here a reentrance effect in lane formation: for
fixed high driving force and increasing particle densities, there is first a
transition towards lane formation which is followed by another transition back
to a state with no lanes. Our result is obtained both by Brownian dynamics
computer simulations and by a phenomenological dynamical density functional
theory.Comment: 4 pages, 2 figure
Dense colloidal suspensions under time-dependent shear
We consider the nonlinear rheology of dense colloidal suspensions under a
time-dependent simple shear flow. Starting from the Smoluchowski equation for
interacting Brownian particles advected by shearing (ignoring fluctuations in
fluid velocity) we develop a formalism which enables the calculation of
time-dependent, far-from-equilibrium averages. Taking shear-stress as an
example we derive exactly a generalized Green-Kubo relation, and an equation of
motion for the transient density correlator, involving a three-time memory
function. Mode coupling approximations give a closed constitutive equation
yielding the time-dependent stress for arbitrary shear rate history. We solve
this equation numerically for the special case of a hard sphere glass subject
to step-strain.Comment: 4 page
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