181 research outputs found
Rhythmic cluster generation in strongly driven colloidal dispersions
We study the response of a nematic colloidal dispersion of rods to a driven
probe particle which is dragged with high speed through the dispersion
perpendicular to the nematic director. In front of the dragged particle,
clusters of rods are generated which rhythmically grow and dissolve by
rotational motion. We find evidence for a mesoscopic cluster-cluster
correlation length, {\em independent} of the imposed drag speed. Our results
are based on non-equilibrium Brownian dynamics computer simulations and in line
with a dynamical scaling theory.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let
Concentration Dependen Sedimentation of Collidal Rods
In the first part of this paper, an approximate theory is developed for the
leading order concentration dependence of the sedimentation coefficient for
rod-like colloids/polymers/macromolecules. To first order in volume fraction
of rods, the sedimentation coefficient is written as .
For large aspect ratio L/D (L is the rod length, D it's thickness) is
found to very like . This
theoretical prediction is compared to experimental results. In the second part,
experiments on {\it fd}-virus are described, both in the isotropic and nematic
phase. First order in concentration results for this very long and thin
(semi-flexible) rod are in agreement with the above theoretical prediction.
Sedimentation profiles for the nematic phase show two sedimentation fronts.
This result indicates that the nematic phase becomes unstable with the respect
to isotropic phase during sedimentation.Comment: Submitted to J. Chem. Phys. See related webpage
http://www.elsie.brandeis.ed
Structure and equation of state of interaction site models for disc-shaped lamellar colloids
We apply RISM (Reference Interaction Site Model) and PRISM (polymer-RISM)
theories to calculate the site-site pair structure and the osmotic equation of
state of suspensions of circular or hexagonal platelets (lamellar colloids)
over a range of ratios of the particle diameter over thickness. Despite the
neglect of edge effects, the simpler PRISM theory yields results in good
agreement with the more elaborate RISM calculations, provided the correct form
factor, characterizing the intramolecular structure of the platelets, is used.
The RISM equation of state is sensitive to the number of sites used to model
the platelets, but saturates when the hard spheres, associated with the
interaction sites, nearly touch; the limiting equation of state agrees
reasonably well with available simulation data for all densities up to the
isotropic-nematic transition. When properly scaled with the second virial
coefficient, the equations of state of platelets with different aspect ratios
nearly collapse on a single master curve.Comment: 10 Pages, 11 Figures, Typesetted using RevTeX
Self-organized dynamics and the transition to turbulence of confined active nematics
We study how confinement transforms the chaotic dynamics of bulk
microtubule-based active nematics into regular spatiotemporal patterns. For
weak confinements, multiple continuously nucleating and annihilating
topological defects self-organize into persistent circular flows of either
handedness. Increasing confinement strength leads to the emergence of distinct
dynamics, in which the slow periodic nucleation of topological defects at the
boundary is superimposed onto a fast procession of a pair of defects. A defect
pair migrates towards the confinement core over multiple rotation cycles, while
the associated nematic director field evolves from a distinct double spiral
towards a nearly circularly symmetric configuration. The collapse of the defect
orbits is punctuated by another boundary-localized nucleation event, that sets
up long-term doubly-periodic dynamics. Comparing experimental data to a
theoretical model of an active nematic, reveals that theory captures the fast
procession of a pair of defects, but not the slow spiral
transformation nor the periodic nucleation of defect pairs. Theory also fails
to predict the emergence of circular flows in the weak confinement regime. The
developed confinement methods are generalized to more complex geometries,
providing a robust microfluidic platform for rationally engineering
two-dimensional autonomous flows
Nematic Ordering of Rigid Rods in a Gravitational Field
The isotropic-to-nematic transition in an athermal solution of long rigid
rods subject to a gravitational (or centrifugal) field is theoretically
considered in the Onsager approximation. The new feature emerging in the
presence of gravity is a concentration gradient which coupled with the nematic
ordering. For rodlike molecules this effect becomes noticeable at centrifugal
acceleration g ~ 10^3--10^4 m/s^2, while for biological rodlike objects, such
as tobacco mosaic virus, TMV, the effect is important even for normal
gravitational acceleration conditions. Rods are concentrated near the bottom of
the vessel which sometimes leads to gravity induced nematic ordering. The
concentration range corresponding to phase separation increases with increasing
g. In the region of phase separation the local rod concentration, as well as
the order parameter, follow a step function with height.Comment: Full article http://prola.aps.org/abstract/PRE/v60/i3/p2973_
Measurements of Protein-Protein Interactions by Size Exclusion Chromatography
A method is presented for determining second virial coefficients B_2 of
protein solutions from retention time measurements in size exclusion
chromatography (SEC). We determine B_2 by analyzing the concentration
dependance of the chromatographic partition coefficient. We show the ability of
this method to track the evolution of B_2 from positive to negative values in
lysozyme and bovine serum albumin solutions. Our SEC results agree
quantitatively with data obtained by light scattering.Comment: 18 pages including 1 table and 5 figure
Isotropic-nematic phase transition in suspensions of filamentous virus and the neutral polymer Dextran
We present an experimental study of the isotropic-nematic phase transition in
an aqueous mixture of charged semi-flexible rods (fd virus) and neutral polymer
(Dextran). A complete phase diagram is measured as a function of ionic strength
and polymer molecular weight. At high ionic strength we find that adding
polymer widens the isotropic-nematic coexistence region with polymers
preferentially partitioning into the isotropic phase, while at low ionic
strength the added polymer has no effect on the phase transition. The nematic
order parameter is determined from birefringence measurements and is found to
be independent of polymer concentration (or equivalently the strength of
attraction). The experimental results are compared with the existing
theoretical predictions for the isotropic-nematic transition in rods with
attractive interactions.Comment: 8 Figures. To be published in Phys. Rev. E. For more information see
http://www.elsie.brandeis.ed
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