282 research outputs found
Structural relaxation in orthoterphenyl: a schematic mode coupling theory model analysis
Depolarized light scattering spectra of orthoterphenyl showing the emergence
of the structural relaxation below the oscillatory microscopic excitations are
described by solutions of a schematic mode--coupling--theory model
Anomalous transport in the crowded world of biological cells
A ubiquitous observation in cell biology is that diffusion of macromolecules
and organelles is anomalous, and a description simply based on the conventional
diffusion equation with diffusion constants measured in dilute solution fails.
This is commonly attributed to macromolecular crowding in the interior of cells
and in cellular membranes, summarising their densely packed and heterogeneous
structures. The most familiar phenomenon is a power-law increase of the MSD,
but there are other manifestations like strongly reduced and time-dependent
diffusion coefficients, persistent correlations, non-gaussian distributions of
the displacements, heterogeneous diffusion, and immobile particles. After a
general introduction to the statistical description of slow, anomalous
transport, we summarise some widely used theoretical models: gaussian models
like FBM and Langevin equations for visco-elastic media, the CTRW model, and
the Lorentz model describing obstructed transport in a heterogeneous
environment. Emphasis is put on the spatio-temporal properties of the transport
in terms of 2-point correlation functions, dynamic scaling behaviour, and how
the models are distinguished by their propagators even for identical MSDs.
Then, we review the theory underlying common experimental techniques in the
presence of anomalous transport: single-particle tracking, FCS, and FRAP. We
report on the large body of recent experimental evidence for anomalous
transport in crowded biological media: in cyto- and nucleoplasm as well as in
cellular membranes, complemented by in vitro experiments where model systems
mimic physiological crowding conditions. Finally, computer simulations play an
important role in testing the theoretical models and corroborating the
experimental findings. The review is completed by a synthesis of the
theoretical and experimental progress identifying open questions for future
investigation.Comment: review article, to appear in Rep. Prog. Phy
Structure and structure relaxation
A discrete--dynamics model, which is specified solely in terms of the
system's equilibrium structure, is defined for the density correlators of a
simple fluid. This model yields results for the evolution of glassy dynamics
which are identical with the ones obtained from the mode-coupling theory for
ideal liquid--glass transitions. The decay of density fluctuations outside the
transient regime is shown to be given by a superposition of Debye processes.
The concept of structural relaxation is given a precise meaning. It is proven
that the long-time part of the mode-coupling-theory solutions is structural
relaxation, while the transient motion merely determines an overall time scale
for the glassy dynamics
Driven lattice gas of dimers coupled to a bulk reservoir
We investigate the non-equilibrium steady state of a one-dimensional (1D)
lattice gas of dimers. The dynamics is described by a totally asymmetric
exclusion process (TASEP) supplemented by attachment and detachment processes,
mimicking chemical equilibrium of the 1D driven transport with the bulk
reservoir. The steady-state phase diagram, current and density profiles are
calculated using both a refined mean-field theory and extensive stochastic
simulations. As a consequence of the on-off kinetics, a new phase coexistence
region arises intervening between low and high density phases such that the
discontinuous transition line of the TASEP splits into two continuous ones. The
results of the mean-field theory and simulations are found to coincide. We show
that the physical picture obtained in the corresponding lattice gas model with
monomers is robust, in the sense that the phase diagram changes quantitatively,
but the topology remains unaltered. The mechanism for phase separation is
identified as generic for a wide class of driven 1D lattice gases.Comment: 15 pages, 10 figures, 1tabl
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