559 research outputs found
Random networks of cross-linked directed polymers
We explore the effect of random permanent cross-links on a system of directed
polymers confined between two planes with their end-points free to slide on
them. We treat the cross-links as quenched disorder and we use a
semimicroscopic replica field theory to study the structure and elasticity of
this system. Upon increasing the cross-link density, we get a continuous
gelation transition signaled by the emergence of a finite in-plane localization
length. The distribution of localization length turns out to depend on the
height along the preferred direction of the directed polymers. The gelation
transition also gives rise to a finite in-plane shear modulus which we
calculate and turns out to be universal, i.e., independent of the energy and
length scales of the polymers and the cross-links. Using a symmetry argument,
we show that cross-links of negligible extent along the preferred axis of the
directed polymers do not cause any renormalization to the tilt modulus of the
uncross-linked system.Comment: 7 pages, 3 figure
Anisotropic Random Networks of Semiflexible Polymers
Motivated by the organization of crosslinked cytoskeletal biopolymers, we
present a semimicroscopic replica field theory for the formation of anisotropic
random networks of semiflexible polymers. The networks are formed by
introducing random permanent crosslinks which fix the orientations of the
corresponding polymer segments to align with one another. Upon increasing the
crosslink density, we obtain a continuous gelation transition from a fluid
phase to a gel where a finite fraction of the system gets localized at random
positions. For sufficiently stiff polymers, this positional localization is
accompanied by a {\em continuous} isotropic-to-nematic (IN) transition occuring
at the same crosslink density. As the polymer stiffness decreases, the IN
transition becomes first order, shifts to a higher crosslink density, and is
preceeded by an orientational glass (statistically isotropic amorphous solid)
where the average polymer orientations freeze in random directions.Comment: 5 pages, 2 figures; final version with expanded discussion to appear
in PR
Bimodality in the transverse fluctuations of a grafted semiflexible polymer and the diffusion-convection analogue: An effective-medium approach
Recent Monte Carlo simulations of a grafted semiflexible polymer in 1+1 dimensions have revealed a pronounced bimodal structure in the probability distribution of the transverse (bending) fluctuations of the free end, when the total contour length is of the order of the persistence length G. Lattanzi , Phys. Rev E 69, 021801 (2004)]. In this paper, we show that the emergence of bimodality is related to a similar behavior observed when a random walker is driven in the transverse direction by a certain type of shear flow. We adapt an effective-medium argument, which was first introduced in the context of the sheared random-walk problem E. Ben-Naim , Phys. Rev. A 45, 7207 (1992)], in order to obtain a simple analytic approximation of the probability distribution of the free-end fluctuations. We show that this approximation captures the bimodality and most of the qualitative features of the free-end fluctuations. We also predict that relaxing the local inextensibility constraint of the wormlike chain could lead to the disappearence of bimodality
Bundle formation in parallel aligned polymers with competing interactions
Aggregation of like-charged polymers is widely observed in biological and
soft matter systems. In many systems, bundles are formed when a short-range
attraction of diverse physical origin like charge-bridging, hydrogen-bonding or
hydrophobic interaction, overcomes the longer- range charge repulsion. In this
Letter, we present a general mechanism of bundle formation in these systems as
the breaking of the translational invariance in parallel aligned polymers with
competing interactions of this type. We derive a criterion for finite-sized
bundle formation as well as for macroscopic phase separation (formation of
infinite bundles).Comment: accepted for publication in Europhys Let
Bundling in brushes of directed and semiflexible polymers
We explore the effect of an attractive interaction between parallel-aligned
polymers, which are perpendicularly grafted on a substrate. Such an attractive
interaction could be due to, e.g., reversible cross-links. The competition
between permanent grafting favoring a homogeneous state of the polymer brush
and the attraction, which tends to induce in-plane collapse of the aligned
polymers, gives rise to an instability of the homogeneous phase to a bundled
state. In this latter state the in-plane translational symmetry is
spontaneously broken and the density is modulated with a finite wavelength,
which is set by the length scale of transverse fluctuations of the grafted
polymers. We analyze the instability for two models of aligned polymers:
directed polymers with a line tension and weakly bending chains with a bending
stiffness.Comment: 7 pages, 5 figures, final version as published in PR
Fully Integrated Hydrocarbon Reservoir Studies: Myth or Reality?
Abstract: Problem statement: In the petroleum industry and especially during reservoir studies, data coming from different disciplines must be combined in order to generate a model that is representative of the reservoir being studied and can be used for defining the most viable development strategy of the field from both an economic and technical standpoint. Each of these disciplines represents an independent piece of a puzzle that is solved by professionals from various scientific fields who have different educational backgrounds. Integration among geophysics, geology, fluid dynamics and geomechanics is truly essential, but requires specific approaches and procedures for generating and calibrating a reservoir model capable of dealing with all and each of these aspects. Approach: Independent workflows were examined for each of the disciplines involved so as to highlight unavoidable interdependencies between static, dynamic and geomechanical models, even when the goal is to tackle each issue separately. Then, the traditional working method was compared to the integrated approach that supports the generation and calibration of models based on data and interpretation results from all the disciplines involved in the entire project. Results: The Construction of a reservoir model should be regarded as a dynamic process, subject to repeated updates as new data is made available and by frequent modifications when inconsistencies are found between the understanding that different specialists have of the same system. This approach has exhibited great advantages in terms of improvement in the quality and flexibility of the model, reduction of working time and generation of a single final model that can be adapted or used for any kind of simulation problem. Conclusion: An integrated approach is necessary for reservoir modeling purposes. Modern reservoir studies should be designed accordingly to permit the full integration of static, dynamic and geomechanical data into a single reservoir model. Integration is always beneficial, even though there still remains a misconception that it is not needed at all times. For this reason, it is recommended that an effort is made to set up a model capable to handle all aspects of a reservoir study each time a new field study is undertaken, even when it is not envisioned that all aspects might be of interest in the futur
Mechanical properties of branched actin filaments
Cells moving on a two dimensional substrate generate motion by polymerizing
actin filament networks inside a flat membrane protrusion. New filaments are
generated by branching off existing ones, giving rise to branched network
structures. We investigate the force-extension relation of branched filaments,
grafted on an elastic structure at one end and pushing with the free ends
against the leading edge cell membrane. Single filaments are modeled as
worm-like chains, whose thermal bending fluctuations are restricted by the
leading edge cell membrane, resulting in an effective force. Branching can
increase the stiffness considerably; however the effect depends on branch point
position and filament orientation, being most pronounced for intermediate tilt
angles and intermediate branch point positions. We describe filament networks
without cross-linkers to focus on the effect of branching. We use randomly
positioned branch points, as generated in the process of treadmilling, and
orientation distributions as measured in lamellipodia. These networks reproduce
both the weak and strong force response of lamellipodia as measured in
force-velocity experiments. We compare properties of branched and unbranched
networks. The ratio of the network average of the force per branched filament
to the average force per unbranched filament depends on the orientation
distribution of the filaments. The ratio exhibits compression dependence and
may go up to about 4.5 in networks with a narrow orientation distribution. With
orientation distributions measured in lamellipodia, it is about two and
essentially independent from network compression, graft elasticity and filament
persistence length
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