115 research outputs found
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
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
Specific heat in two-dimensional melting
We report the specific heat around the melting transition(s) of
micrometer-sized superparamagnetic particles confined in two dimensions,
calculated from fluctuations of positions and internal energy, and
corresponding Monte Carlo simulations. Since colloidal systems provide single
particle resolution, they offer the unique possibility to compare the
experimental temperatures of peak position of and symmetry breaking,
respectively. While order parameter correlation functions confirm the
Kosterlitz-Thouless-Halperin-Nelson-Young melting scenario where translational
and orientational order symmetries are broken at different temperatures with an
intermediate so called hexatic phase, we observe a single peak of the specific
heat within the hexatic phase, with excellent agreement between experiment and
simulation. Thus, the peak is not associated with broken symmetries but can be
explained with the total defect density, which correlates with the maximum
increase of isolated dislocations. The absence of a latent heat strongly
supports the continuous character of both transitions
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
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
Viscoelasticity and Stokes-Einstein relation in repulsive and attractive colloidal glasses
We report a numerical investigation of the visco-elastic behavior in models
for steric repulsive and short-range attractive colloidal suspensions, along
different paths in the attraction-strength vs packing fraction plane. More
specifically, we study the behavior of the viscosity (and its frequency
dependence) on approaching the repulsive glass, the attractive glass and in the
re-entrant region where viscosity shows a non monotonic behavior on increasing
attraction strength. On approaching the glass lines, the increase of the
viscosity is consistent with a power-law divergence with the same exponent and
critical packing fraction previously obtained for the divergence of the density
fluctuations. Based on mode-coupling calculations, we associate the increase of
the viscosity with specific contributions from different length scales. We also
show that the results are independent on the microscopic dynamics by comparing
newtonian and brownian simulations for the same model. Finally we evaluate the
Stokes-Einstein relation approaching both glass transitions, finding a clear
breakdown which is particularly strong for the case of the attractive glass.Comment: 12 pages; sent to J. Chem. Phy
An Insight into the Viscoelasticity of Self-Assembling Smectic Liquid Crystals of Colloidal Rods from Active Microrheology Simulations
The rheology of colloidal suspensions is of utmost importance in an ample variety of interdisciplinary applications in formulation technology, determining equally interesting questions in fundamental science. This is especially intriguing when colloids exhibit a degree of long-range positional or orientational ordering, as in liquid crystals (LCs) of elongated particles. Along with standard methods, microrheology (MR) has emerged in recent years as a tool to assess the mechanical properties of materials at the microscopic level. In particular, by active MR one can infer the viscoelastic response of a soft material from the dynamics of a tracer particle being dragged through it by external forces. Although considerable efforts have been made to study the diffusion of guest particles in LCs, little is known on the combined effect of tracer size and directionality of the dragging force on the system’s viscoelastic response. By dynamic Monte Carlo simulations, we apply active MR to investigate the viscoelasticity of self-assembling smectic (Sm) LCs consisting of rod-like particles. In particular, we track the motion of a spherical tracer whose size is varied within a range of values matching the system’s characteristic length scales and being dragged by constant forces that are parallel, perpendicular or at 45◦ to the nematic director. Our results reveal a uniform value of the effective friction coefficient as probed by the tracer at small and large forces, whereas a nonlinear, force-thinning regime is observed at intermediate forces. However, at relatively weak forces the effective friction is strongly determined by correlations between the tracer size and the structure of the host fluid. Moreover, we also show that external forces forming an angle with the nematic director provide additional details that cannot be simply inferred from the mere analysis of parallel and perpendicular forces. Our results highlight the fundamental interplay between tracer size and force direction in assessing the MR of Sm LC fluids.F.A.G.D. was funded by the NextGenerationEU program of the European Union, the Plan de Recuperación, Transformación y Resiliencia, and the Ministerio de Universidades, as part of the “Maria Zambrano” grants for the requalification of the Spanish university system 2021-2023 called by the Pablo de Olavide University.F.A.G.D., A.M.P and A.P. acknowledge the International Exchanges Grant IES\R1\191066, awarded by The Royal SocietyA.P. is supported by a “Maria Zambrano Senior” researcher fellowship, financed by the European Union within the NextGenerationEU program and the Spanish Ministry of Universities.A.M.P. acknowledges financial support from project PID2021-127836NB-I00 (funded by MCIN/AEI/10.13039/501100011033/ FEDER “A way to make Europe”).A. C. acknowledges support from Consejería de Transformación Económica, Industria, Conocimiento y Universidades de la Junta de Andalucía/FEDER (project grant P20-00816), and from the Spanish Ministerio de Ciencia, Innovación y Universidades, and FEDER (project grant PPID2021-126121NB-I00).The authors acknowledge the C3UPO of the Pablo de Olavide University for the support with HPC facilities and the use of the Computational Shared Facility at The University of Mancheste
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