2,480 research outputs found
Quenched central limit theorem for the stochastic heat equation in weak disorder
We continue with the study of the mollified stochastic heat equation in
given by with spatially
smoothened cylindrical Wiener process , whose (renormalized) Feynman-Kac
solution describes the partition function of the continuous directed polymer.
In an earlier work (\cite{MSZ16}), a phase transition was obtained, depending
on the value of in the limiting object of the smoothened solution
as the smoothing parameter This partition function
naturally defines a quenched polymer path measure and we prove that as long as
stays small enough while converges to a strictly
positive non-degenerate random variable, the distribution of the diffusively
rescaled Brownian path converges under the aforementioned polymer path measure
to standard Gaussian distribution.Comment: Minor revisio
Dimer diffusion in a washboard potential
The transport of a dimer, consisting of two Brownian particles bounded by a
harmonic potential, moving on a periodic substrate is investigated both
numerically and analytically. The mobility and diffusion of the dimer center of
mass present distinct properties when compared with those of a monomer under
the same transport conditions. Both the average current and the diffusion
coefficient are found to be complicated non-monotonic functions of the driving
force. The influence of dimer equilibrium length, coupling strength and damping
constant on the dimer transport properties are also examined in detail.Comment: Final revised version. 7 pages, 6 figure
Probing molecular free energy landscapes by periodic loading
Single molecule pulling experiments provide information about interactions in
biomolecules that cannot be obtained by any other method. However, the
reconstruction of the molecule's free energy profile from the experimental data
is still a challenge, in particular for the unstable barrier regions. We
propose a new method for obtaining the full profile by introducing a periodic
ramp and using Jarzynski's identity for obtaining equilibrium quantities from
non-equilibrium data. Our simulated experiments show that this method delivers
significant more accurate data than previous methods, under the constraint of
equal experimental effort.Comment: 4 pages, 3 figure
The Continuum Directed Random Polymer
Motivated by discrete directed polymers in one space and one time dimension,
we construct a continuum directed random polymer that is modeled by a
continuous path interacting with a space-time white noise. The strength of the
interaction is determined by an inverse temperature parameter beta, and for a
given beta and realization of the noise the path evolves in a Markovian way.
The transition probabilities are determined by solutions to the one-dimensional
stochastic heat equation. We show that for all beta > 0 and for almost all
realizations of the white noise the path measure has the same Holder continuity
and quadratic variation properties as Brownian motion, but that it is actually
singular with respect to the standard Wiener measure on C([0,1]).Comment: 21 page
Automation and robotics considerations for a lunar base
An envisioned lunar outpost shares with other NASA missions many of the same criteria that have prompted the development of intelligent automation techniques with NASA. Because of increased radiation hazards, crew surface activities will probably be even more restricted than current extravehicular activity in low Earth orbit. Crew availability for routine and repetitive tasks will be at least as limited as that envisioned for the space station, particularly in the early phases of lunar development. Certain tasks are better suited to the untiring watchfulness of computers, such as the monitoring and diagnosis of multiple complex systems, and the perception and analysis of slowly developing faults in such systems. In addition, mounting costs and constrained budgets require that human resource requirements for ground control be minimized. This paper provides a glimpse of certain lunar base tasks as seen through the lens of automation and robotic (A&R) considerations. This can allow a more efficient focusing of research and development not only in A&R, but also in those technologies that will depend on A&R in the lunar environment
Undulation Instability of Epithelial Tissues
Treating the epithelium as an incompressible fluid adjacent to a viscoelastic
stroma, we find a novel hydrodynamic instability that leads to the formation of
protrusions of the epithelium into the stroma. This instability is a candidate
for epithelial fingering observed in vivo. It occurs for sufficiently large
viscosity, cell-division rate and thickness of the dividing region in the
epithelium. Our work provides physical insight into a potential mechanism by
which interfaces between epithelia and stromas undulate, and potentially by
which tissue dysplasia leads to cancerous invasion.Comment: 4 pages, 3 figure
Sequence-dependent thermodynamics of a coarse-grained DNA model
We introduce a sequence-dependent parametrization for a coarse-grained DNA
model [T. E. Ouldridge, A. A. Louis, and J. P. K. Doye, J. Chem. Phys. 134,
085101 (2011)] originally designed to reproduce the properties of DNA molecules
with average sequences. The new parametrization introduces sequence-dependent
stacking and base-pairing interaction strengths chosen to reproduce the melting
temperatures of short duplexes. By developing a histogram reweighting
technique, we are able to fit our parameters to the melting temperatures of
thousands of sequences. To demonstrate the flexibility of the model, we study
the effects of sequence on: (a) the heterogeneous stacking transition of single
strands, (b) the tendency of a duplex to fray at its melting point, (c) the
effects of stacking strength in the loop on the melting temperature of
hairpins, (d) the force-extension properties of single strands and (e) the
structure of a kissing-loop complex. Where possible we compare our results with
experimental data and find a good agreement. A simulation code called oxDNA,
implementing our model, is available as free software.Comment: 15 page
Diffusion-controlled generation of a proton-motive force across a biomembrane
Respiration in bacteria involves a sequence of energetically-coupled electron
and proton transfers creating an electrochemical gradient of protons (a
proton-motive force) across the inner bacterial membrane. With a simple kinetic
model we analyze a redox loop mechanism of proton-motive force generation
mediated by a molecular shuttle diffusing inside the membrane. This model,
which includes six electron-binding and two proton-binding sites, reflects the
main features of nitrate respiration in E. coli bacteria. We describe the time
evolution of the proton translocation process. We find that the electron-proton
electrostatic coupling on the shuttle plays a significant role in the process
of energy conversion between electron and proton components. We determine the
conditions where the redox loop mechanism is able to translocate protons
against the transmembrane voltage gradient above 200 mV with a thermodynamic
efficiency of about 37%, in the physiologically important range of temperatures
from 250 to 350 K.Comment: 26 pages, 4 figures. A similar model is used in arXiv:0806.3233 for a
different biological system. Minor changes in the Acknowledgements sectio
Fluctuation-Induced Interactions between Rods on Membranes and Interfaces
We consider the interaction between two rods embedded in a fluctuating
surface which is governed by either surface tension or rigidity. The
modification of fluctuations by the rods leads to an attractive long-range
interaction that falls off as with their separation. The orientational
dependence of the resulting interaction is non-trivial and may lead to
interesting patterns of rod-like objects on such surfaces.Comment: Revtex, 10 pages, one figur
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