512 research outputs found
Brownian particles in transient polymer networks
We discuss the thermal motion of colloidal particles in transient polymer networks. For particles that are physically bound to the surrounding chains, light-scattering experiments reveal that the submillisecond dynamics changes from diffusive to Rouse-like upon crossing the network formation threshold. Particles that are not bound do not show such a transition. At longer time scales the mean-square displacement (MSD) exhibits a caging plateau and, ultimately, a slow diffusive motion. The slow diffusion at longer time scales can be related to the macroscopic viscosity of the polymer solutions. Expressions that relate the caging plateau to the macroscopic network elasticity are found to fail for the cases presented here. The typical Rouse scaling of the MSD with the square root of time, as found in experiments at short time scales, is explained by developing a bead-spring model of a large colloidal particle connected to several polymer chains. The resulting analytical expressions for the MSD of the colloidal particle are shown to be consistent with experimental findings
The growth of a semigroup and its Cayley transform
Let be the infinitesimal generator of an exponentially stable, strongly continuous semigroup on a Hilbert space. We show that the powers of the Cayley transform of are bounded by a constant times . The proof is based on Lyapunov equations
Coherent and Incoherent Vortex Flow States in Crossed Channels
We examine vortex flow states in periodic square pinning arrays with one row
and one column of pinning sites removed to create an easy flow crossed channel
geometry. When a drive is simultaneously applied along both major symmetry axes
of the pinning array such that vortices move in both channels, a series of
coherent flow states develop in the channel intersection at rational ratios of
the drive components in each symmetry direction when the vortices can cross the
intersection without local collisions. The coherent flow states are correlated
with a series of anomalies in the velocity force curves, and in some cases can
produce negative differential conductivity. The same general behavior could
also be realized in other systems including colloids, particle traffic in
microfluidic devices, or Wigner crystals in crossed one-dimensional channels.Comment: 5 pages, 4 postscript figure
Wall slip and flow of concentrated hard-sphere colloidal suspensions
We present a comprehensive study of the slip and flow of concentrated
colloidal suspensions using cone-plate rheometry and simultaneous confocal
imaging. In the colloidal glass regime, for smooth, non-stick walls, the solid
nature of the suspension causes a transition in the rheology from
Herschel-Bulkley (HB) bulk flow behavior at large stress to a Bingham-like slip
behavior at low stress, which is suppressed for sufficient colloid-wall
attraction or colloid-scale wall roughness. Visualization shows how the
slip-shear transition depends on gap size and the boundary conditions at both
walls and that partial slip persist well above the yield stress. A
phenomenological model, incorporating the Bingham slip law and HB bulk flow,
fully accounts for the behavior. Microscopically, the Bingham law is related to
a thin (sub-colloidal) lubrication layer at the wall, giving rise to a
characteristic dependence of slip parameters on particle size and
concentration. We relate this to the suspension's osmotic pressure and yield
stress and also analyze the influence of van der Waals interaction. For the
largest concentrations, we observe non-uniform flow around the yield stress, in
line with recent work on bulk shear-banding of concentrated pastes. We also
describe residual slip in concentrated liquid suspensions, where the vanishing
yield stress causes coexistence of (weak) slip and bulk shear flow for all
measured rates
The Effect of Pinning on Drag in Coupled One-Dimensional Channels of Particles
We consider a simple model for examining the effects of quenched disorder on
drag consisting of particles interacting via a Yukawa potential that are placed
in two coupled one-dimensional channels. The particles in one channel are
driven and experience a drag from the undriven particles in the second channel.
In the absence of pinning, for a finite driving force there is no pinned phase;
instead, there are two dynamical regimes of completely coupled or locked flow
and partially coupled flow. When pinning is added to one or both channels, we
find that a remarkably rich variety of dynamical phases and drag effects arise
that can be clearly identified by features in the velocity force curves. The
presence of quenched disorder in only the undriven channel can induce a pinned
phase in both channels. Above the depinning transition, the drag on the driven
particles decreases with increasing pinning strength, and for high enough
pinning strength, the particles in the undriven channel reach a reentrant
pinned phase which produces a complete decoupling of the channels. We map out
the dynamic phase diagrams as a function of pinning strength and the density of
pinning in each channel. Our results may be relevant for understanding drag
coupling in 1D Wigner crystal phases, and the effects we observe could also be
explored using colloids in coupled channels produced with optical arrays,
vortices in nanostructured superconductors, or other layered systems where drag
effects arise.Comment: 6 pages, 5 postscript figure
Mode locking of vortex matter driven through mesoscopic channels
We investigated the driven dynamics of vortices confined to mesoscopic flow
channels by means of a dc-rf interference technique. The observed mode-locking
steps in the -curves provide detailed information on how the number of rows
and lattice structure in the channel change with magnetic field. Minima in flow
stress occur when an integer number of rows is moving coherently, while maxima
appear when incoherent motion of mixed and row configurations is
predominant. Simulations show that the enhanced pinning at mismatch originates
from quasi-static fault zones with misoriented edge dislocations induced by
disorder in the channel edges.Comment: some minor changes were made, 4 pages, 4 figures, accepted for
publication in Phys. Rev. Let
Colloidal gels under shear: Strain rate effects
Attractive colloidal particles are trapped in metastable states such as colloidal gels at high attraction strengths and attractive glasses and high volume fractions. Under shear such states flow via a two step yielding process that relates to bond and cluster or cage breaking. We discuss the way the structural properties and related stress response are affected by the shear rate. At low rates colloidal gels yield during start-up shear essentially in a single step, exhibiting a single stress overshoot due to creation of compact flowing clusters. With increasing shear rate a second stress overshoot, linked with further cluster breaking up to individual particles, is becoming more pronounced. We further present the age dependence of the two step yielding and wall slip effects often taking place during rheological experiments of colloidal gels. The latter is related both with the shear rate dependent gel structure as well as the time evolution of the near wall structure
Multiple shear-banding transitions in a supramolecular polymer solution
We report on the nonlinear rheology of a reversible supramolecular polymer based on hydrogen bonding. The coupling between the flow-induced chain alignment and breakage and recombination of bonds between monomers leads to a very unusual flow behavior. Measured velocity profiles indicate three different shear-banding regimes upon increasing shear rate, each with different characteristics. While the first of these regimes has features of a mechanical instability, the second shear-banding regime is related to a shear-induced phase separation and the appearance of birefringent textures. The shear-induced phase itself becomes unstable at very high shear rates, giving rise to a third banding regime
Dynamic ordering and frustration of confined vortex rows studied by mode-locking experiments
The flow properties of confined vortex matter driven through disordered
mesoscopic channels are investigated by mode locking (ML) experiments. The
observed ML effects allow to trace the evolution of both the structure and the
number of confined rows and their match to the channel width as function of
magnetic field. From a detailed analysis of the ML behavior for the case of
3-rows we obtain ({\it i}) the pinning frequency , ({\it ii}) the onset
frequency for ML ( ordering velocity) and ({\it iii}) the
fraction of coherently moving 3-row regions in the channel. The
field dependence of these quantities shows that, at matching, where is
maximum, the pinning strength is small and the ordering velocity is low, while
at mismatch, where is small, both the pinning force and the ordering
velocity are enhanced. Further, we find that , consistent
with the dynamic ordering theory of Koshelev and Vinokur. The microscopic
nature of the flow and the ordering phenomena will also be discussed.Comment: 10 pages, 7 figure, submitted to PRB. Discussion has been improved
and a figure has been adde
Glass Rheology: From mode-coupling theory to a dynamical yield criterion
The mode coupling theory (MCT) of glasses, while offering an incomplete
description of glass transition physics, represents the only established route
to first-principles prediction of rheological behavior in nonergodic materials
such as colloidal glasses. However, the constitutive equations derivable from
MCT are somewhat intractable, hindering their practical use and also their
interpretation. Here, we present a schematic (single-mode) MCT model which
incorporates the tensorial structure of the full theory. Using it, we calculate
the dynamic yield surface for a large class of flows
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