101 research outputs found
Nonlinear rheology of dense colloidal systems with short-ranged attraction: A mode-coupling theory analysis
The nonlinear rheology of glass-forming colloidal suspensions with
short-ranged attractions is discussed within the integration-through transients
framework combined with the mode-coupling theory of the glass transition
(ITT-MCT). Calculations are based on the square-well system (SWS), as a model
for colloid-polymer mixtures. The high-density regime featuring reentrant
melting of the glass upon increasing the attraction strength, and the crossover
from repulsive glasses formed at weak attraction to attractive glasses formed
at strong attraction, are discussed. Flow curves are found in qualitative
agreement with experimental data, featuring a strong increase in the yield
stress, and, for suitable interaction parameters, the crossover between two
yield stresses. The yield strain, defined as the position of the stress
overshoot under startup flow, is found to be proportional to the attraction
range for strong attraction. At weak and intermediate attraction strength, the
combined effects of hard-core caging and attraction-driven bonding result in a
richer dependence on the parameters. The first normal-stress difference
exhibits a weaker dependence on short-ranged attractions as the shear stress,
since the latter is more sensitive the short-wavelength features of the static
structure.Comment: 14 pages, 12 figure
How Glassy Relaxation Slows Down by Increasing Mobility
We investigate how structural relaxation in mixtures with strong dynamical
asymmetry is affected by the microscopic dynamics. Brownian and Newtonian
dynamics simulations of dense mixtures of fast and slow hard spheres reveal a
striking trend reversal. Below a critical density, increasing the mobility of
the fast particles fluidizes the system, yet, above that critical density, the
same increase in mobility strongly hinders the relaxation of the slow
particles. The critical density itself does not depend on the dynamical
asymmetry and can be identified with the glass-transition density of the
mode-coupling theory. The asymptotic dynamics close to the critical density is
universal, but strong pre-asymptotic effects prevail in mixtures with
additional size asymmetry. This observation reconciles earlier findings of a
strong dependence on kinetic parameters of glassy dynamics in colloid--polymer
mixtures with the paradigm that the glass transition is determined by the
properties of configuration space alone
Mode-Coupling Theory for Active Brownian Particles
We present a mode-coupling theory (MCT) for the high-density dynamics of
two-dimensional spherical active Brownian particles (ABP). The theory is based
on the integration-through-transients (ITT) formalism and hence provides a
starting point for the calculation of non-equilibrium averages in
active-Brownian particle systems. The ABP are characterized by a
self-propulsion velocity , and by their translational and rotational
diffusion coefficients, and . The theory treats both the
translational and the orientational degrees of freedom of ABP explicitly. This
allows to study the effect of self-propulsion of both weak and strong
persistence of the swimming direction, also at high densities where the
persistence length is large compared to the typical
interaction length scale. While the low-density dynamics of ABP is
characterized by a single P\'eclet number, , close to the
glass transition the dynamics is found to depend on and
separately. At fixed density, increasing the self-propulsion velocity causes
structural relaxatino to speed up, while decreasing the persistence length
slows down the relaxation. The theory predicts a non-trivial
idealized-glass-transition diagram in the three-dimensional parameter space of
density, self-propulsion velocity and rotational diffusivity. The active-MCT
glass is a nonergodic state where correlations of initial density fluctuations
never fully decay, but also an infinite memory of initial orientational
fluctuations is retained in the positions
Localization phenomena in models of ion-conducting glass formers
The mass transport in soft-sphere mixtures of small and big particles as well
as in the disordered Lorentz gas (LG) model is studied using molecular dynamics
(MD) computer simulations. The soft-sphere mixture shows anomalous
small-particle diffusion signifying a localization transition separate from the
big-particle glass transition. Switching off small-particle excluded volume
constraints slows down the small-particle dynamics, as indicated by incoherent
intermediate scattering functions. A comparison of logarithmic time derivatives
of the mean-squared displacements reveals qualitative similarities between the
localization transition in the soft-sphere mixture and its counterpart in the
LG. Nevertheless, qualitative differences emphasize the need for further
research elucidating the connection between both models.Comment: to appear in Eur. Phys. J. Special Topic
Qualitative Features at the Glass Crossover
We discuss some generic features of the dynamics of glass-forming liquids
close to the glass transition singularity of the idealized mode-coupling theory
(MCT). The analysis is based on a recent model by one of the authors for the
intermediate-time dynamics ( relaxation), derived by applying dynamical
field-theory techniques to the idealized MCT. Combined with the assumption of
time-temperature superposition for the slow structural () relaxation,
the model naturally explains three prominent features of the dynamical
crossover: the change from a power-law to exponential increase in the
structural relaxation time, the replacement of the Stokes-Einstein relation
between diffusion and viscosity by a fractional law, and two distinct growth
regimes of the thermal susceptibility that has been associated to dynamical
heterogeneities
Scaling equations for mode-coupling theories with multiple decay channels
Multiple relaxation channels often arise in the dynamics of liquids where the
momentum current associated to the particle-conservation law splits into
distinct contributions. Examples are strongly confined liquids for which the
currents in lateral and longitudinal direction to the walls are very different,
or fluids of nonspherical particles with distinct relaxation patterns for
translational and rotational degrees of freedom. Here, we perform an asymptotic
analysis of the slow structural relaxation close to kinetic arrest as described
by mode-coupling theory (MCT) with several relaxation channels. Compared to
standard MCT, the presence of multiple relaxation channels significantly
changes the structure of the underlying equations of motion and leads to
additional, non-trivial terms in the asymptotic solution. We show that the
solution can be rescaled, and thus prove that the well-known -scaling
equation of MCT remains valid even in the presence of multiple relaxation
channels. The asymptotic treatment is validated using a novel schematic model.
We demonstrate that the numerical solution of this schematic model can indeed
be described by the derived asymptotic scaling laws close to kinetic arrest.
Additionally, clear traces of the existence of two distinct decay channels are
found in the low-frequency susceptibility spectrum, suggesting that clear
footprints of the additional relaxation channels can in principle be detected
in simulations or experiments of confined or molecular liquids.Comment: Accepted by J. Stat. Mech.: Theory and Experiments,
https://iopscience.iop.org/journal/1742-546
- …