258 research outputs found
Collective force generated by multiple biofilaments can exceed the sum of forces due to individual ones
Collective dynamics and force generation by cytoskeletal filaments are
crucial in many cellular processes. Investigating growth dynamics of a bundle
of N independent cytoskeletal filaments pushing against a wall, we show that
chemical switching (ATP/GTP hydrolysis) leads to a collective phenomenon that
is currently unknown. Obtaining force-velocity relations for different models
that capture chemical switching, we show, analytically and numerically, that
the collective stall force of N filaments is greater than N times the stall
force of a single filament. Employing an exactly solvable toy model, we
analytically prove the above result for N=2. We, further, numerically show the
existence of this collective phenomenon, for N>=2, in realistic models (with
random and sequential hydrolysis) that simulate actin and microtubule bundle
growth. We make quantitative predictions for the excess forces, and argue that
this collective effect is related to the non-equilibrium nature of chemical
switching.Comment: New J. Phys., 201
Giant number fluctuations in microbial ecologies
Statistical fluctuations in population sizes of microbes may be quite large
depending on the nature of their underlying stochastic dynamics. For example,
the variance of the population size of a microbe undergoing a pure birth
process with unlimited resources is proportional to the square of its mean. We
refer to such large fluctuations, with the variance growing as square of the
mean, as Giant Number Fluctuations (GNF). Luria and Delbruck showed that
spontaneous mutation processes in microbial populations exhibit GNF. We explore
whether GNF can arise in other microbial ecologies. We study certain simple
ecological models evolving via stochastic processes: (i) bi-directional
mutation, (ii) lysis-lysogeny of bacteria by bacteriophage, and (iii)
horizontal gene transfer (HGT). For the case of bi-directional mutation
process, we show analytically exactly that the GNF relationship holds at large
times. For the ecological model of bacteria undergoing lysis or lysogeny under
viral infection, we show that if the viral population can be experimentally
manipulated to stay quasi-stationary, the process of lysogeny maps essentially
to one-way mutation process and hence the GNF property of the lysogens follows.
Finally, we show that even the process of HGT may map to the mutation process
at large times, and thereby exhibits GNF.Comment: 18 pages, 5 figure
Inhomogeneous Cooling of the Rough Granular Gas in Two Dimensions
We study the inhomogeneous clustered regime of a freely cooling granular gas
of rough particles in two dimensions using large-scale event driven simulations
and scaling arguments. During collisions, rough particles dissipate energy in
both the normal and tangential directions of collision. In the inhomogeneous
regime, translational kinetic energy and the rotational energy decay with time
as power-laws and . We numerically determine
and , independent of the
coefficients of restitution. The inhomogeneous regime of the granular gas has
been argued to be describable by the ballistic aggregation problem, where
particles coalesce on contact. Using scaling arguments, we predict
and for ballistic aggregation, being different from
that obtained for the rough granular gas. Simulations of ballistic aggregation
with rotational degrees of freedom are consistent with these exponents.Comment: 6 pages, 5 figure
Persistence of a Rouse polymer chain under transverse shear flow
We consider a single Rouse polymer chain in two dimensions in presence of a
transverse shear flow along the direction and calculate the persistence
probability that the coordinate of a bead in the bulk of the chain
does not return to its initial position up to time . We show that the
persistence decays at late times as a power law, with
a nontrivial exponent . The analytical estimate of
obtained using an independent interval approximation is in excellent agreement
with the numerical value .Comment: 6 page
Critical Dynamics of Dimers: Implications for the Glass Transition
The Adam-Gibbs view of the glass transition relates the relaxation time to
the configurational entropy, which goes continuously to zero at the so-called
Kauzmann temperature. We examine this scenario in the context of a dimer model
with an entropy vanishing phase transition, and stochastic loop dynamics. We
propose a coarse-grained master equation for the order parameter dynamics which
is used to compute the time-dependent autocorrelation function and the
associated relaxation time. Using a combination of exact results, scaling
arguments and numerical diagonalizations of the master equation, we find
non-exponential relaxation and a Vogel-Fulcher divergence of the relaxation
time in the vicinity of the phase transition. Since in the dimer model the
entropy stays finite all the way to the phase transition point, and then jumps
discontinuously to zero, we demonstrate a clear departure from the Adam-Gibbs
scenario. Dimer coverings are the "inherent structures" of the canonical
frustrated system, the triangular Ising antiferromagnet. Therefore, our results
provide a new scenario for the glass transition in supercooled liquids in terms
of inherent structure dynamics
Spatial Structures and Giant Number Fluctuations in Models of Active Matter
The large scale fluctuations of the ordered state in active matter systems
are usually characterised by studying the "giant number fluctuations" of
particles in any finite volume, as compared to the expectations from the
central limit theorem. However, in ordering systems, the fluctuations in
density ordering are often captured through their structure functions deviating
from Porod law. In this paper we study the relationship between giant number
fluctuations and structure functions, for different models of active matter as
well as other non-equilibrium systems. A unified picture emerges, with
different models falling in four distinct classes depending on the nature of
their structure functions. For one class, we show that experimentalists may
find Porod law violation, by measuring subleading corrections to the number
fluctuations.Comment: 5 pages, 3 figure
Violation of Porod law in a freely cooling granular gas in one dimension
We study a model of freely cooling inelastic granular gas in one dimension,
with a restitution coefficient which approaches the elastic limit below a
relative velocity scale v. While at early times (t << 1/v) the gas behaves as a
completely inelastic sticky gas conforming to predictions of earlier studies,
at late times (t >> 1/v) it exhibits a new fluctuation dominated phase ordering
state. We find distinct scaling behavior for the (i) density distribution
function, (ii) occupied and empty gap distribution functions, (iii) the density
structure function and (iv) the velocity structure function, as compared to the
completely inelastic sticky gas. The spatial structure functions (iii) and (iv)
violate the Porod law. Within a mean-field approximation, the exponents
describing the structure functions are related to those describing the spatial
gap distribution functions.Comment: 4 pages, 5 figure
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