260 research outputs found
Responses to Conflicting Stimuli in a Simple Stimulus–Response Pathway
The “local bend response” of the medicinal leech (Hirudo verbana) is a stimulus–response pathway that enables the animal to bend away from a pressure stimulus applied anywhere along its body. The neuronal circuitry that supports this behavior has been well described, and its responses to individual stimuli are understood in quantitative detail. We probed the local bend system with pairs of electrical stimuli to sensory neurons that could not logically be interpreted as a single touch to the body wall and used multiple suction electrodes to record simultaneously the responses in large numbers of motor neurons. In all cases, responses lasted much longer than the stimuli that triggered them, implying the presence of some form of positive feedback loop to sustain the response. When stimuli were delivered simultaneously, the resulting motor neuron output could be described as an evenly weighted linear combination of the responses to the constituent stimuli. However, when stimuli were delivered sequentially, the second stimulus had greater impact on the motor neuron output, implying that the positive feedback in the system is not strong enough to render it immune to further input
Percolation of Immobile Domains in Supercooled Thin Polymeric Films
We present an analysis of heterogeneous dynamics in molecular dynamics
simulations of a thin polymeric film, supported by an absorbing structured
surface. Near the glass transition "immobile" domains occur throughout the
film, yet the probability of their occurrence decreasing with larger distance
from the surface. Still, enough immobile domains are located near the free
surface to cause them to percolate in the direction perpendicular to surface,
at a temperature near the glass transition temperature. This result is in
agreement with a recent theoretical model of glass transition
The Aggregation Kinetics of a Simulated Telechelic Polymer
We investigate the aggregation kinetics of a simulated telechelic polymer
gel. In the hybrid Molecular Dynamics (MD) / Monte Carlo (MC) algorithm,
aggregates of associating end groups form and break according to MC rules,
while the position of the polymers in space is dictated by MD. As a result, the
aggregate sizes change every time step. In order to describe this aggregation
process, we employ master equations. They define changes in the number of
aggregates of a certain size in terms of reaction rates. These reaction rates
indicate the likelihood that two aggregates combine to form a large one, or
that a large aggregate splits into two smaller parts. The reaction rates are
obtained from the simulations for a range of temperatures.
Our results indicate that the rates are not only temperature dependent, but
also a function of the sizes of the aggregates involved in the reaction. Using
the measured rates, solutions to the master equations are shown to be stable
and in agreement with the aggregate size distribution, as obtained directly
from simulation data. Furthermore, we show how temperature induced variations
in these rates give rise to the observed changes in the aggregate distribution
that characterizes the sol-gel transition.Comment: 9 pages, 10 figure
Individual Entanglements in a Simulated Polymer Melt
We examine entanglements using monomer contacts between pairs of chains in a
Brownian-dynamics simulation of a polymer melt. A map of contact positions with
respect to the contacting monomer numbers (i,j) shows clustering in small
regions of (i,j) which persists in time, as expected for entanglements. Using
the ``space''-time correlation function of the aforementioned contacts, we show
that a pair of entangled chains exhibits a qualitatively different behavior
than a pair of distant chains when brought together. Quantitatively, about 50%
of the contacts between entangled chains are persistent contacts not present in
independently moving chains. In addition, we account for several observed
scaling properties of the contact correlation function.Comment: latex, 12 pages, 7 figures, postscript file available at
http://arnold.uchicago.edu/~ebn
Growth, microstructure, and failure of crazes in glassy polymers
We report on an extensive study of craze formation in glassy polymers.
Molecular dynamics simulations of a coarse-grained bead-spring model were
employed to investigate the molecular level processes during craze nucleation,
widening, and breakdown for a wide range of temperature, polymer chain length
, entanglement length and strength of adhesive interactions between
polymer chains. Craze widening proceeds via a fibril-drawing process at
constant drawing stress. The extension ratio is determined by the entanglement
length, and the characteristic length of stretched chain segments in the
polymer craze is . In the craze, tension is mostly carried by the
covalent backbone bonds, and the force distribution develops an exponential
tail at large tensile forces. The failure mode of crazes changes from
disentanglement to scission for , and breakdown through scission
is governed by large stress fluctuations. The simulations also reveal
inconsistencies with previous theoretical models of craze widening that were
based on continuum level hydrodynamics
Tensile Forces and Shape Entropy Explain Observed Crista Structure in Mitochondria
A model is presented from which the observed morphology of the inner
mitochondrial membrane can be inferred as minimizing the system's free energy.
Besides the usual energetic terms for bending, surface area, and pressure
difference, our free energy includes terms for tension that we believe to be
exerted by proteins and for an entropic contribution due to many dimensions
worth of shapes available at a given energy.
In order to test the model, we measured the structural features of
mitochondria in HeLa cells and mouse embryonic fibroblasts using 3D electron
tomography. Such tomograms reveal that the inner membrane self-assembles into a
complex structure that contains both tubular and flat lamellar crista
components. This structure, which contains one matrix compartment, is believed
to be essential to the proper functioning of mitochondria as the powerhouse of
the cell. We find that tensile forces of the order of 10 pN are required to
stabilize a stress-induced coexistence of tubular and flat lamellar cristae
phases. The model also predicts \Deltap = -0.036 \pm 0.004 atm and \sigma=0.09
\pm 0.04 pN/nm
Why is Understanding Glassy Polymer Mechanics So Difficult?
In this Perspective, I describe recent work on systems in which the
traditional distinctions between (i) unentangled vs. well-entangled systems and
(ii) melts vs. glasses seem least useful, and argue for the broader use in
glassy polymer mechanics of two more dichotomies: systems which possess (iii)
unary vs. binary and (iv) cooperative vs. nonccoperative relaxation dynamics. I
discuss the applicability of (iii-iv) to understanding the functional form of
glassy strain hardening. Results from molecular dynamics simulations show that
the "dramatic" strain hardening observed in densely entangled systems is
associated with a crossover from unary, noncooperative to binary, cooperative
relaxation as strain increases; chains stretch between entanglement points,
altering the character of local plasticity. Promising approaches for future
research along these lines are discussed.Comment: Results and conclusions same but manuscript extensively edited for
clarity. Accepted for publication in J. Polym. Sci. - Polym. Phy
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