1,852 research outputs found
Competition between glass transition and liquid-gas separation in attracting colloids
We present simulation results addressing the phenomena of colloidal gelation
induced by attractive interactions. The liquid-gas transition is prevented by
the glass arrest at high enough attraction strength, resulting in a colloidal
gel. The dynamics of the system is controlled by the glass, with little effect
of the liquid-gas transition. When the system separates in a liquid and vapor
phases, even if the denser phase enters the non-ergodic region, the vapor phase
enables the structural relaxation of the system as a whole.Comment: Proceedings of the glass conference in Pisa (September 06
Dynamical heterogeneities in an attraction driven colloidal glass
The dynamical heterogeneities (DH) in non-ergodic states of an attractive
colloidal glass are studied, as a function of the waiting time. Whereas the
fluid states close to vitrify showed strong DH, the distribution of squared
displacements of the glassy states studied here only present a tail of
particles with increased mobility for the lower attraction strength at short
waiting times. These particles are in the surface of the percolating cluster
that comprises all of the particles, reminiscent of the fastest particles in
the fluid. The quench deeper into the attractive glass is dynamically more
homogeneous, in agreement with repulsive glasses (i.e. Lennard-Jones glass).Comment: Proceedings of 5th IDMRCS - Lille 200
Tagged-particle dynamics in a hard-sphere system: mode-coupling theory analysis
The predictions of the mode-coupling theory of the glass transition (MCT) for
the tagged-particle density-correlation functions and the mean-squared
displacement curves are compared quantitatively and in detail to results from
Newtonian- and Brownian-dynamics simulations of a polydisperse
quasi-hard-sphere system close to the glass transition. After correcting for a
17% error in the dynamical length scale and for a smaller error in the
transition density, good agreement is found over a wide range of wave numbers
and up to five orders of magnitude in time. Deviations are found at the highest
densities studied, and for small wave vectors and the mean-squared
displacement. Possible error sources not related to MCT are discussed in
detail, thereby identifying more clearly the issues arising from the MCT
approximation itself. The range of applicability of MCT for the different types
of short-time dynamics is established through asymptotic analyses of the
relaxation curves, examining the wave-number and density-dependent
characteristic parameters. Approximations made in the description of the
equilibrium static structure are shown to have a remarkable effect on the
predicted numerical value for the glass-transition density. Effects of small
polydispersity are also investigated, and shown to be negligible.Comment: 20 pages, 23 figure
Structural relaxation of polydisperse hard spheres: comparison of the mode-coupling theory to a Langevin dynamics simulation
We analyze the slow, glassy structural relaxation as measured through
collective and tagged-particle density correlation functions obtained from
Brownian dynamics simulations for a polydisperse system of quasi-hard spheres
in the framework of the mode-coupling theory of the glass transition (MCT).
Asymptotic analyses show good agreement for the collective dynamics when
polydispersity effects are taken into account in a multi-component calculation,
but qualitative disagreement at small when the system is treated as
effectively monodisperse. The origin of the different small- behaviour is
attributed to the interplay between interdiffusion processes and structural
relaxation. Numerical solutions of the MCT equations are obtained taking
properly binned partial static structure factors from the simulations as input.
Accounting for a shift in the critical density, the collective density
correlation functions are well described by the theory at all densities
investigated in the simulations, with quantitative agreement best around the
maxima of the static structure factor, and worst around its minima. A
parameter-free comparison of the tagged-particle dynamics however reveals large
quantiative errors for small wave numbers that are connected to the well-known
decoupling of self-diffusion from structural relaxation and to dynamical
heterogeneities. While deviations from MCT behaviour are clearly seen in the
tagged-particle quantities for densities close to and on the liquid side of the
MCT glass transition, no such deviations are seen in the collective dynamics.Comment: 23 pages, 26 figure
Aging in attraction-driven colloidal glasses
Aging in an attraction-driven colloidal glass is studied by computer
simulations. The system is equilibrated without attraction and instantaneously
``quenched'', at constant colloid volume fraction, to one of two states beyond
the glass transition; one is close to the transition, and the other one deep in
the glass. The evolution of structural properties shows that bonds form in the
system, increasing the local density, creating density deficits (holes)
elsewhere. This process slows down with the time elapsed since the quench. As a
consequence of bond formation, there is a slowing down of the dynamics, as
measured by the mean squared displacement and the density, bond, and
environment correlation functions. The density correlations can be
time-rescaled to collapse their long time (structural) decay. The time scale
for structural relaxation shows for both quenches a super-linear dependence on
waiting time; it grows faster than the bond lifetime, showing the collective
origin of the transition. At long waiting times and high attraction strength,
we observe {\rem completely} arrested dynamics for more than three decades in
time, although individual bonds are not permanent on this time scale. The
localization length decreases as the state moves deeper in the glass; the
non-ergodicity parameter oscillates in phase with the structure factor. Our
main results are obtained for systems with a barrier in the pair potential that
inhibits phase separation. However, when this barrier is removed for the case
of a deep quench, we find changes in the static structure but almost none in
the dynamics. Hence our results for the aging behavior remain relevant to
experiments in which the glass transition competes with phase separation.Comment: 12 pages, 15 figure
The mass and environmental dependence on the secular processes of AGN in terms of morphology, colour, and specific star-formation rate
Galaxy mass and environment play a major role in the evolution of galaxies.
In the transition from star-forming to quenched galaxies, Active galactic
nuclei (AGN) have also a principal action. However, the connections between
these three actors are still uncertain. In this work we investigate the effects
of stellar mass and the large-scale environment (LSS), on the fraction of
optical nuclear activity in a population of isolated galaxies, where AGN would
not be triggered by recent galaxy interactions or mergers. As a continuation of
a previous work, we focus on isolated galaxies to study the effect of stellar
mass and the LSS in terms of morphology (early- and late-type), colour (red and
blue), and specific star formation rate (quenched and star-forming). To explore
where AGN activity is affected by the LSS we fix the stellar mass into low- and
high-mass galaxies. We use the tidal strength parameter to quantify their
effects. We found that AGN is strongly affected by stellar mass in 'active'
galaxies (namely late-type, blue, and star-forming), however it has no
influence for 'quiescent' galaxies (namely early-type, red, and quenched), at
least for masses down to . In relation to the LSS, we
found an increment on the fraction of SFN with denser LSS in low-mass star
forming and red isolated galaxies. Regarding AGN, we find a clear increment of
the fraction of AGN with denser environment in quenched and red isolated
galaxies, independently of the stellar mass. AGN activity would be 'mass
triggered' in 'active' isolated galaxies. This means that AGN is independent of
the intrinsic property of the galaxies, but on its stellar mass. On the other
hand, AGN would be 'environment triggered' in 'quiescent' isolated galaxies,
where the fraction of AGN in terms of sSFR and colour increases from void
regions to denser LSS, independently of its stellar mass.Comment: 14 pages, 9 figures (11 pages and 6 figures without appendix),
accepted for publication in Astronomy & Astrophysic
Mode Coupling and Dynamical Heterogeneity in Colloidal Gelation: A Simulation Study
We present simulation results addressing the dynamics of a colloidal system
with attractive interactions close to gelation. Our interaction also has a
soft, long range repulsive barrier which suppresses liquid-gas type phase
separation at long wavelengths. The new results presented here lend further
weight to an intriguing picture emerging from our previous simulation work on
the same system. Whereas mode coupling theory (MCT) offers quantitatively good
results for the decay of correlators, closer inspection of the dynamics reveals
a bimodal population of fast and slow particles with a very long exchange
timescale. This population split represents a particular form of dynamic
heterogeneity (DH). Although DH is usually associated with activated hopping
and/or facilitated dynamics in glasses, the form of DH observed here may be
more collective in character and associated with static (i.e., structural)
heterogeneity.Comment: 12 pages, 12 figure
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
- …