1,113 research outputs found
Bond formation and slow heterogeneous dynamics in adhesive spheres with long--ranged repulsion: Quantitative test of Mode Coupling Theory
A colloidal system of spheres interacting with both a deep and narrow
attractive potential and a shallow long-ranged barrier exhibits a prepeak in
the static structure factor. This peak can be related to an additional
mesoscopic length scale of clusters and/or voids in the system. Simulation
studies of this system have revealed that it vitrifies upon increasing the
attraction into a gel-like solid at intermediate densities. The dynamics at the
mesoscopic length scale corresponding to the prepeak represents the slowest
mode in the system. Using mode coupling theory with all input directly taken
from simulations, we reveal the mechanism for glassy arrest in the system at
40% packing fraction. The effects of the low-q peak and of polydispersity are
considered in detail. We demonstrate that the local formation of physical bonds
is the process whose slowing down causes arrest.
It remains largely unaffected by the large-scale heterogeneities, and sets
the clock for the slow cluster mode. Results from mode-coupling theory without
adjustable parameters agree semi-quantitatively with the local density
correlators but overestimate the lifetime of the mesoscopic structure (voids).Comment: 10 pages, 8 figure
Probability densities of a forced probe particle in glass: results from mode coupling theory and simulations of active microrheology
We investigate the displacements of a probe particle inside a glass, when a
strong external force is applied to the probe (active nonlinear microrheology).
Calculations within mode coupling theory are presented for glasses of hard
spheres and compared to Langevin and Brownian dynamics simulations. Under not
too strong forces where the probe remains trapped, the probe density
distribution becomes anisotropic. It is shifted towards the direction of the
force, develops an enhanced tail in that direction (signalled by a positive
skewness), and exhibits different variances along and perpendicular to the
force direction. A simple model of an harmonically trapped probe rationalizes
the low force limit, with strong strain softening setting in at forces of the
order of a few thermal energies per particle radius
Aperture-free star formation rate of SDSS star-forming galaxies
Large area surveys with a high number of galaxies observed have undoubtedly
marked a milestone in the understanding of several properties of galaxies, such
as star-formation history, morphology, and metallicity. However, in many cases,
these surveys provide fluxes from fixed small apertures (e.g. fibre), which
cover a scant fraction of the galaxy, compelling us to use aperture corrections
to study the global properties of galaxies. In this work, we derive the current
total star formation rate (SFR) of Sloan Digital Sky Survey (SDSS) star-forming
galaxies, using an empirically based aperture correction of the measured flux for the first time, thus minimising the uncertainties associated
with reduced apertures. All the fluxes have been
extinction-corrected using the ratio free from aperture
effects. The total SFR for 210,000 SDSS star-forming galaxies has been
derived applying pure empirical and aperture
corrections based on the Calar Alto Legacy Integral Field Area (CALIFA) survey.
We find that, on average, the aperture-corrected SFR is 0.65dex higher
than the SDSS fibre-based SFR. The relation between the SFR and stellar mass
for SDSS star-forming galaxies (SFR--) has been obtained, together
with its dependence on extinction and equivalent width. We
compare our results with those obtained in previous works and examine the
behaviour of the derived SFR in six redshift bins, over the redshift range . The SFR-- sequence derived here is in
agreement with selected observational studies based on integral field
spectroscopy of individual galaxies as well as with the predictions of recent
theoretical models of disc galaxies
Spatially resolved integral field spectroscopy of the ionized gas in IZw18
We present a detailed 2D study of the ionized ISM of IZw18 using new PMAS-IFU
optical observations. IZw18 is a high-ionization galaxy which is among the most
metal-poor starbursts in the local Universe. This makes IZw18 a local benchmark
for understanding the properties most closely resembling those prevailing at
distant starbursts. Our IFU-aperture (~ 1.4 kpc x 1.4 kpc) samples the entire
IZw18 main body and an extended region of its ionized gas. Maps of relevant
emission lines and emission line ratios show that higher-excitation gas is
preferentially located close to the NW knot and thereabouts. We detect a
Wolf-Rayet feature near the NW knot. We derive spatially resolved and
integrated physical-chemical properties for the ionized gas in IZw18. We find
no dependence between the metallicity-indicator R23 and the ionization
parameter (as traced by [OIII]/[OII]) across IZw18. Over ~ 0.30 kpc^2, using
the [OIII]4363 line, we compute Te[OIII] values (~ 15000 - 25000 K), and oxygen
abundances are derived from the direct determinations of Te[OIII]. More than
70% of the higher-Te[OIII] (> 22000 K) spaxels are HeII4686-emitting spaxels
too. From a statistical analysis, we study the presence of variations in the
ISM physical-chemical properties. A galaxy-wide homogeneity, across hundreds of
parsecs, is seen in O/H. Based on spaxel-by-spaxel measurements, the
error-weighted mean of 12 + log(O/H) = 7.11 +/- 0.01 is taken as the
representative O/H for IZw18. Aperture effects on the derivation of O/H are
discussed. Using our IFU data we obtain, for the first time, the IZw18
integrated spectrum.Comment: Accepted for publication in MNRAS, 13 pages, 10 figures, 4 table
Structure and dynamics of colloidal depletion gels: coincidence of transitions and heterogeneity
Transitions in structural heterogeneity of colloidal depletion gels formed
through short-range attractive interactions are correlated with their dynamical
arrest. The system is a density and refractive index matched suspension of 0.20
volume fraction poly(methyl methacyrlate) colloids with the non-adsorbing
depletant polystyrene added at a size ratio of depletant to colloid of 0.043.
As the strength of the short-range attractive interaction is increased,
clusters become increasingly structurally heterogeneous, as characterized by
number-density fluctuations, and dynamically immobilized, as characterized by
the single-particle mean-squared displacement. The number of free colloids in
the suspension also progressively declines. As an immobile cluster to gel
transition is traversed, structural heterogeneity abruptly decreases.
Simultaneously, the mean single-particle dynamics saturates at a localization
length on the order of the short-range attractive potential range. Both
immobile cluster and gel regimes show dynamical heterogeneity. Non-Gaussian
distributions of single particle displacements reveal enhanced populations of
dynamical trajectories localized on two different length scales. Similar
dependencies of number density fluctuations, free particle number and dynamical
length scales on the order of the range of short-range attraction suggests a
collective structural origin of dynamic heterogeneity in colloidal gels.Comment: 14 pages, 10 figure
Theory and simulation of gelation, arrest and yielding in attracting colloids
We present some recent theory and simulation results addressing the phenomena
of colloidal gelation at both high and low volume fractions, in the presence of
short-range attractive interactions. We discuss the ability of mode-coupling
theory and its adaptations to address situations with strong heterogeneity in
density and/or dynamics. We include a discussion of the effect of attractions
on the shear-thinning and yield behaviour under flow.Comment: 17 pages, 6 figure
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