2,035 research outputs found
Jamming, two-fluid behaviour and 'self-filtration' in concentrated particulate suspensions
We study the flow of model experimental hard sphere colloidal suspensions at
high volume fraction driven through a constriction by a pressure
gradient. Above a particle-size dependent limit , direct microscopic
observations demonstrate jamming and unjamming--conversion of fluid to solid
and vice versa--during flow. We show that such a jamming flow produces a
reduction in colloid concentration downstream of the constriction.
We propose that this `self-filtration' effect is the consequence of a
combination of jamming of the particulate part of the system and continuing
flow of the liquid part, i.e. the solvent, through the pores of the jammed
solid. Thus we link the concept of jamming in colloidal and granular media with
a 'two-fluid'-like picture of the flow of concentrated suspensions. Results are
also discussed in the light of Osborne Reynolds' original experiments on
dilation in granular materials.Comment: 4 pages, 3 figure
Familial vasculitides: granulomatosis with polyangitis and microscopic polyangitis in two brothers with differing anti-neutrophil cytoplasm antibody specificity
Anti-neutrophil cytoplasm antibody (ANCA)-associated vasculitis (AAV) is a group of rare autoimmune diseases. Although the aetiology of AAV is uncertain, it is likely that genetic and environmental factors contribute. We report the unusual case of two brothers presenting with AAV with differing clinical pictures and differing ANCA specificity. There is a recently identified difference in genetic risk factors associated with ANCA specificity, making it surprising that first-degree relatives develop AAV with differing clinical and serological features. Our report illustrates the complex aetiology of AAV and suggests that further research on the interaction of genetic and environmental factors is needed
Quantum lost property: a possible operational meaning for the Hilbert-Schmidt product
Minimum error state discrimination between two mixed states \rho and \sigma
can be aided by the receipt of "classical side information" specifying which
states from some convex decompositions of \rho and \sigma apply in each run. We
quantify this phenomena by the average trace distance, and give lower and upper
bounds on this quantity as functions of \rho and \sigma. The lower bound is
simply the trace distance between \rho and \sigma, trivially seen to be tight.
The upper bound is \sqrt{1 - tr(\rho\sigma)}, and we conjecture that this is
also tight. We reformulate this conjecture in terms of the existence of a pair
of "unbiased decompositions", which may be of independent interest, and prove
it for a few special cases. Finally, we point towards a link with a notion of
non-classicality known as preparation contextuality.Comment: 3 pages, 1 figure. v2: Less typos in text and less punctuation in
titl
Properties of cage rearrangements observed near the colloidal glass transition
We use confocal microscopy to study the motions of particles in concentrated
colloidal systems. Near the glass transition, diffusive motion is inhibited, as
particles spend time trapped in transient ``cages'' formed by neighboring
particles. We measure the cage sizes and lifetimes, which respectively shrink
and grow as the glass transition approaches. Cage rearrangements are more
prevalent in regions with lower local concentrations and higher disorder.
Neighboring rearranging particles typically move in parallel directions,
although a nontrivial fraction move in anti-parallel directions, usually from
pairs of particles with initial separations corresponding to the local maxima
and minima of the pair correlation function , respectively.Comment: 5 pages, 4 figures; text & figures revised in v
Hard Spheres: Crystallization and Glass Formation
Motivated by old experiments on colloidal suspensions, we report molecular
dynamics simulations of assemblies of hard spheres, addressing crystallization
and glass formation. The simulations cover wide ranges of polydispersity s
(standard deviation of the particle size distribution divided by its mean) and
particle concentration. No crystallization is observed for s > 0.07. For 0.02 <
s < 0.07, we find that increasing the polydispersity at a given concentration
slows down crystal nucleation. The main effect here is that polydispersity
reduces the supersaturation since it tends to stabilise the fluid but to
destabilise the crystal. At a given polydispersity (< 0.07) we find three
regimes of nucleation: standard nucleation and growth at concentrations in and
slightly above the coexistence region; "spinodal nucleation", where the free
energy barrier to nucleation appears to be negligible, at intermediate
concentrations; and, at the highest concentrations, a new mechanism, still to
be fully understood, which only requires small re-arrangement of the particle
positions. The cross-over between the second and third regimes occurs at a
concentration, around 58% by volume, where the colloid experiments show a
marked change in the nature of the crystals formed and the particle dynamics
indicate an "ideal" glass transition
Short-time rotational diffusion in monodisperse charge-stabilized colloidal suspensions
We investigate the combined effects of electrostatic interactions and
hydrodynamic interactions on the short-time rotational self-diffusion
coefficient in charge-stabilized suspensions. We calculate this coefficient as
a function of volume fraction for various effective particle charges and
amounts of added electrolyte. The influence of the hydrodynamic interactions on
the rotational diffusion coefficient is less pronounced for charged particles
than for uncharged ones. Salt-free suspensions are weakly influenced by
hydrodynamic interactions. For these strongly correlated systems we obtain a
quadratic volume fraction-dependence of the diffusion coefficient, which is
well explained in terms of an effective hard sphere model.Comment: 21 pages, LaTeX, 7 Postscript figures included using epsf, to appear
in Physica
Crystal Nucleation of Colloidal Suspensions under Shear
We use Brownian Dynamics simulations in combination with the umbrella
sampling technique to study the effect of shear flow on homogeneous crystal
nucleation. We find that a homogeneous shear rate leads to a significant
suppression of the crystal nucleation rate and to an increase of the size of
the critical nucleus. A simple, phenomenological extension of classical
nucleation theory accounts for these observations. The orientation of the
crystal nucleus is tilted with respect to the shear direction.Comment: 4 pages, 3 figures, Submitted to Phys. Rev. Let
Self-diffusion coefficients of charged particles: Prediction of Nonlinear volume fraction dependence
We report on calculations of the translational and rotational short-time
self-diffusion coefficients and for suspensions of
charge-stabilized colloidal spheres. These diffusion coefficients are affected
by electrostatic forces and many-body hydrodynamic interactions (HI). Our
computations account for both two-body and three-body HI. For strongly charged
particles, we predict interesting nonlinear scaling relations and depending on volume fraction
, with essentially charge-independent parameters and . These
scaling relations are strikingly different from the corresponding results for
hard spheres. Our numerical results can be explained using a model of effective
hard spheres. Moreover, we perceptibly improve the known result for of
hard sphere suspensions.Comment: 8 pages, LaTeX, 3 Postscript figures included using eps
Crystallization of hard-sphere glasses
We study by molecular dynamics the interplay between arrest and
crystallization in hard spheres. For state points in the plane of volume
fraction () and polydispersity (), we delineate states that spontaneously crystallize from those that do
not. For noncrystallizing (or precrystallization) samples we find
isodiffusivity lines consistent with an ideal glass transition at , independent of . Despite this, for , crystallization
occurs at . This happens on time scales for which the system is
aging, and a diffusive regime in the mean square displacement is not reached;
by those criteria, the system is a glass. Hence, contrary to a widespread
assumption in the colloid literature, the occurrence of spontaneous
crystallization within a bulk amorphous state does not prove that this state
was an ergodic fluid rather than a glass.Comment: 4 pages, 3 figure
Radial vibration measurements directly from rotors using laser vibrometry: The effects of surface roughness, instrument misalignments and pseudo-vibration
Laser Doppler vibrometry (LDV) offers an attractive solution when radial vibration measurement directly from a rotor surface is required. Research to date has demonstrated application on polished-circular rotors and rotors coated with retro-reflective tape. In the latter case, however, a significant cross-sensitivity to the orthogonal radial vibration component occurs and post-processing is required to resolve individual radial vibration components. Until now, the fundamentally different behaviour observed between these cases has stood as an inconsistency in the published literature, symptomatic of the need to understand the effect of surface roughness. This paper offers the first consistent mathematical description of the polished-circular and rough rotor behaviours, combined with an experimental investigation of the relationship between surface roughness and cross-sensitivity. Rotors with surface roughness up to 10 nm satisfy the polished-circular rotor definition if vibration displacement is below 100% beam diameter, for a 90 μm beam, and below 40% beam diameter, for a 520 μm beam. On rotors with roughness between 10 nm and 50 nm, the polished-circular rotor definition is satisfied for vibration displacements up to 25% beam diameter, for a 90 μm beam, and up to 10% beam diameter, for a 520 μm beam. As roughness increases, cross-sensitivity increases but only rotors coated in retro-reflective tape satisfied the rough rotor definition fully. Consequently, when polished-circular surfaces are not available, rotor surfaces must be treated with retro-reflective tape and measurements post-processed to resolve individual vibration components. Through simulations, the value of the resolution and correction algorithms that form the post-processor has been demonstrated quantitatively. Simulations incorporating representative instrument misalignments and measurement noise have enabled quantification of likely error levels in radial vibration measurements. On a polished-circular rotor, errors around 0.2% for amplitude and 2 mrad for phase are likely, rising a little at the integer orders affected by pseudo-vibration. Higher pseudo-vibration levels and the need for resolution increase errors in the rough rotor measurements, especially around the synchronous frequency where errors reach 20% by amplitude and 100 mrad for phase. Outside a range of half an order either side of first order, errors are ten times lower and beyond fifth order errors are similar to those for the polished-circular rotor. Further simulations were performed to estimate sensitivities to axial vibration, speed variation and bending vibrations. © 2012 Elsevier Ltd
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