7,693 research outputs found
Characterizing Granular Networks Using Topological Metrics
We carry out a direct comparison of experimental and numerical realizations
of the exact same granular system as it undergoes shear jamming. We adjust the
numerical methods used to optimally represent the experimental settings and
outcomes up to microscopic contact force dynamics. Measures presented here
range form microscopic, through mesoscopic to system-wide characteristics of
the system. Topological properties of the mesoscopic force networks provide a
key link between micro and macro scales. We report two main findings: the
number of particles in the packing that have at least two contacts is a good
predictor for the mechanical state of the system, regardless of strain history
and packing density. All measures explored in both experiments and numerics,
including stress tensor derived measures and contact numbers depend in a
universal manner on the fraction of non-rattler particles, . The force
network topology also tends to show this universality, yet the shape of the
master curve depends much more on the details of the numerical simulations. In
particular we show that adding force noise to the numerical data set can
significantly alter the topological features in the data. We conclude that both
and topological metrics are useful measures to consider when
quantifying the state of a granular system.Comment: 8 pages, 8 figure
Exploring the Free Energy Landscape: From Dynamics to Networks and Back
The knowledge of the Free Energy Landscape topology is the essential key to
understand many biochemical processes. The determination of the conformers of a
protein and their basins of attraction takes a central role for studying
molecular isomerization reactions. In this work, we present a novel framework
to unveil the features of a Free Energy Landscape answering questions such as
how many meta-stable conformers are, how the hierarchical relationship among
them is, or what the structure and kinetics of the transition paths are.
Exploring the landscape by molecular dynamics simulations, the microscopic data
of the trajectory are encoded into a Conformational Markov Network. The
structure of this graph reveals the regions of the conformational space
corresponding to the basins of attraction. In addition, handling the
Conformational Markov Network, relevant kinetic magnitudes as dwell times or
rate constants, and the hierarchical relationship among basins, complete the
global picture of the landscape. We show the power of the analysis studying a
toy model of a funnel-like potential and computing efficiently the conformers
of a short peptide, the dialanine, paving the way to a systematic study of the
Free Energy Landscape in large peptides.Comment: PLoS Computational Biology (in press
Entanglement reduction induced by geometrical confinement in polymer thin films
We report simulation results on melts of entangled linear polymers confined
in a free-standing thin film. We study how the geometric constraints imposed by
the confinement alter the entanglement state of the system compared to the
equivalent bulk system using various observables. We find that the confinement
compresses the chain conformation uniaxially, decreasing the volume pervaded by
the chain, which in turn reduces the number of the accessible inter-chain
contact that could lead to entanglements. This local and non-uniform effect
depends on the position of the chain within the film. We also test a recently
presented theory that predicts how the number of entanglements decreases with
geometrical confinement.Comment: 28 pages, 10 figure
Variational Methods for Biomolecular Modeling
Structure, function and dynamics of many biomolecular systems can be
characterized by the energetic variational principle and the corresponding
systems of partial differential equations (PDEs). This principle allows us to
focus on the identification of essential energetic components, the optimal
parametrization of energies, and the efficient computational implementation of
energy variation or minimization. Given the fact that complex biomolecular
systems are structurally non-uniform and their interactions occur through
contact interfaces, their free energies are associated with various interfaces
as well, such as solute-solvent interface, molecular binding interface, lipid
domain interface, and membrane surfaces. This fact motivates the inclusion of
interface geometry, particular its curvatures, to the parametrization of free
energies. Applications of such interface geometry based energetic variational
principles are illustrated through three concrete topics: the multiscale
modeling of biomolecular electrostatics and solvation that includes the
curvature energy of the molecular surface, the formation of microdomains on
lipid membrane due to the geometric and molecular mechanics at the lipid
interface, and the mean curvature driven protein localization on membrane
surfaces. By further implicitly representing the interface using a phase field
function over the entire domain, one can simulate the dynamics of the interface
and the corresponding energy variation by evolving the phase field function,
achieving significant reduction of the number of degrees of freedom and
computational complexity. Strategies for improving the efficiency of
computational implementations and for extending applications to coarse-graining
or multiscale molecular simulations are outlined.Comment: 36 page
Flow rate of transport network controls uniform metabolite supply to tissue
Life and functioning of higher organisms depends on the continuous supply of
metabolites to tissues and organs. What are the requirements on the transport
network pervading a tissue to provide a uniform supply of nutrients, minerals,
or hormones? To theoretically answer this question, we present an analytical
scaling argument and numerical simulations on how flow dynamics and network
architecture control active spread and uniform supply of metabolites by
studying the example of xylem vessels in plants. We identify the fluid inflow
rate as the key factor for uniform supply. While at low inflow rates
metabolites are already exhausted close to flow inlets, too high inflow flushes
metabolites through the network and deprives tissue close to inlets of supply.
In between these two regimes, there exists an optimal inflow rate that yields a
uniform supply of metabolites. We determine this optimal inflow analytically in
quantitative agreement with numerical results. Optimizing network architecture
by reducing the supply variance over all network tubes, we identify patterns of
tube dilation or contraction that compensate sub-optimal supply for the case of
too low or too high inflow rate.Comment: 11 pages, 4 figures, 8 pages supplemen
Phase behavior and material properties of hollow nanoparticles
Effective pair potentials for hollow nanoparticles like the ones made from
carbon (fullerenes) or metal dichalcogenides (inorganic fullerenes) consist of
a hard core repulsion and a deep, but short-ranged, van der Waals attraction.
We investigate them for single- and multi-walled nanoparticles and show that in
both cases, in the limit of large radii the interaction range scales inversely
with the radius, , while the well depth scales linearly with . We predict
the values of the radius and the wall thickness at which the gas-liquid
coexistence disappears from the phase diagram. We also discuss unusual material
properties of the solid, which include a large heat of sublimation and a small
surface energy.Comment: Revtex, 13 pages with 8 Postscript files included, submitted to Phys.
Rev.
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