22 research outputs found
Heirarchical and synergistic self-assembly in composites of model Wormlike micellar-polymers and nanoparticles results in nanostructures with diverse morphologies
Using Monte Carlo simulations, we investigate the self-assembly of model
nanoparticles inside a matrix of model equilibrium polymers (or matrix of
Wormlike micelles) as a function of the polymeric matrix density and the
excluded volume parameter between polymers and nanoparticles. In this paper, we
show morphological transitions in the system architecture via synergistic
self-assembly of nanoparticles and the equilibrium polymers. In a synergistic
self-assembly, the resulting morphology of the system is a result of the
interaction between both nanoparticles and the polymers, unlike the polymer
templating method. We report the morphological transition of nanoparticle
aggregates from percolating network-like structures to non-percolating clusters
as a result of the change in the excluded volume parameter between
nanoparticles and polymeric chains. In parallel with the change in the
self-assembled structures of nanoparticles, the matrix of equilibrium polymers
also shows a transition from a dispersed state to a percolating network-like
structure formed by the clusters of polymeric chains. We show that the shape
anisotropy of the nanoparticle clusters formed is governed by the polymeric
density resulting in rod-like, sheet-like or other anisotropic nanoclusters. It
is also shown that the pore shape and the pore size of the porous network of
nanoparticles can be changed by changing the minimum approaching distance
between nanoparticles and polymers. We provide a theoretical understanding of
why various nanostructures with very different morphologies are obtained.Comment: 24 pages, 23 figure
Origin of spatial organization of DNA-polymer in bacterial chromosomes
In-vivo DNA organization at large length scales () is highly
debated and polymer models have proved useful to understand the principle of
DNA-organization. Here, we show that % cross-links at specific points in a
ring polymer can lead to a distinct spatial organization of the polymer. The
specific pairs of cross-linked monomers were extracted from contact maps of
bacterial DNA. We are able to predict the structure of 2 DNAs using Monte Carlo
simulations of the bead-spring polymer with cross-links at these special
positions. Simulations with cross-links at random positions along the chain
show that the organization of the polymer is different in nature from the
previous case.Comment: arXiv admin note: text overlap with arXiv:1701.0506
Entropy mediated organization of E.coli chromosome in fast growth conditions
Recent experiments have been able to visualise chromosome organization in
fast-growing E.coli cells. However, the mechanism underlying the
spatio-temporal organization remains poorly understood. We propose that the DNA
adopts a specific polymer topology as it goes through its cell cycle. We
establish that the emergent entropic forces between polymer segments of the
DNA-polymer with modified topology, leads to chromosome organization as seen
in-vivo. We employ computer simulations of a replicating bead spring model of a
polymer in a cylinder to investigate the problem. Our simulation of the
overlapping cell cycles not only show successful segregation, but also
reproduces the evolution of the spatial organization of the chromosomes as
observed in experiments. This manuscript in addition to our previous work on
slowly growing bacterial cells, shows that our topology-based model can explain
the organization of chromosomes in all growth conditions
Combining Molecular Dynamics with Lattice-Boltzmann: A Hybrid Method for the Simulation of (Charged) Colloidal Systems
We present a hybrid method for the simulation of colloidal systems, that
combines molecular dynamics (MD) with the Lattice-Boltzmann (LB) scheme. The LB
method is used as a model for the solvent in order to take into account the
hydrodynamic mass and momentum transport through the solvent. The colloidal
particles are propagated via MD and they are coupled to the LB fluid by viscous
forces. With respect to the LB fluid, the colloids are represented by uniformly
distributed points on a sphere. Each such point (with a velocity V(r) at any
off-lattice position r is interacting with the neighboring eight LB nodes by a
frictional force F=\xi_0(V(r)-u(r)) with \xi_0 being a friction force and u(r)
being the velocity of the fluid at the position r. Thermal fluctuations are
introduced in the framework of fluctuating hydrodynamics. This coupling scheme
has been proposed recently for polymer systems by Ahlrichs and D"unweg [J.
Chem. Phys. 111, 8225 (1999)]. We investigate several properties of a single
colloidal particle in a LB fluid, namely the effective Stokes friction and long
time tails in the autocorrelation functions for the translational and
rotational velocity. Moreover, a charged colloidal system is considered
consisting of a macroion, counterions and coions that are coupled to a LB
fluid. We study the behavior of the ions in a constant electric field. In
particular, an estimate of the effective charge of the macroion is yielded from
the number of counterions that move with the macroion in the direction of the
electric field.Comment: 37 pages, 12 figure
Electrophoretic Properties of Highly Charged Colloids: A Hybrid MD/LB Simulation Study
Using computer simulations, the electrophoretic motion of a positively
charged colloid (macroion) in an electrolyte solution is studied in the
framework of the primitive model. Hydrodynamic interactions are fully taken
into account by applying a hybrid simulation scheme, where the charged ions
(i.e. macroion and electrolyte), propagated via molecular dynamics (MD), are
coupled to a Lattice Boltzmann (LB) fluid. In a recent experiment it was shown
that, for multivalent salt ions, the mobility initially increases with
charge density , reaches a maximum and then decreases with further
increase of . The aim of the present work is to elucidate the behaviour
of at high values of . Even for the case of monovalent microions,
we find a decrease of with . A dynamic Stern layer is defined
that includes all the counterions that move with the macroion while subject to
an external electrical field. The number of counterions in the Stern layer,
, is a crucial parameter for the behavior of at high values of
. In this case, the mobility depends primarily on the ratio
(with the valency of the macroion). The previous contention that
the increase in the distortion of the electric double layer (EDL) with
increasing leads to the lowering of does not hold for high
. In fact, we show that the deformation of the EDL decreases with
increase of . The role of hydrodynamic interactions is inferred from
direct comparisons to Langevin simulations where the coupling to the LB fluid
is switched off. Moreover, systems with divalent counterions are considered. In
this case, at high values of the phenomenon of charge inversion is
found.Comment: accepted in J. Chem Phys., 10 pages, 9 figure