7,652 research outputs found
Optimisation of a Brownian dynamics algorithm for semidilute polymer solutions
Simulating the static and dynamic properties of semidilute polymer solutions
with Brownian dynamics (BD) requires the computation of a large system of
polymer chains coupled to one another through excluded-volume and hydrodynamic
interactions. In the presence of periodic boundary conditions, long-ranged
hydrodynamic interactions are frequently summed with the Ewald summation
technique. By performing detailed simulations that shed light on the influence
of several tuning parameters involved both in the Ewald summation method, and
in the efficient treatment of Brownian forces, we develop a BD algorithm in
which the computational cost scales as O(N^{1.8}), where N is the number of
monomers in the simulation box. We show that Beenakker's original
implementation of the Ewald sum, which is only valid for systems without bead
overlap, can be modified so that \theta-solutions can be simulated by switching
off excluded-volume interactions. A comparison of the predictions of the radius
of gyration, the end-to-end vector, and the self-diffusion coefficient by BD,
at a range of concentrations, with the hybrid Lattice Boltzmann/Molecular
Dynamics (LB/MD) method shows excellent agreement between the two methods. In
contrast to the situation for dilute solutions, the LB/MD method is shown to be
significantly more computationally efficient than the current implementation of
BD for simulating semidilute solutions. We argue however that further
optimisations should be possible.Comment: 17 pages, 8 figures, revised version to appear in Physical Review E
(2012
Equilibrium binding energies from fluctuation theorems and force spectroscopy simulations
Brownian dynamics simulations are used to study the detachment of a particle
from a substrate. Although the model is simple and generic, we attempt to map
its energy, length and time scales onto a specific experimental system, namely
a bead that is weakly bound to a cell and then removed by an optical tweezer.
The external driving force arises from the combined optical tweezer and
substrate potentials, and thermal fluctuations are taken into account by a
Brownian force. The Jarzynski equality and Crooks' fluctuation theorem are
applied to obtain the equilibrium free energy difference between the final and
initial states. To this end, we sample non--equilibrium work trajectories for
various tweezer pulling rates. We argue that this methodology should also be
feasible experimentally for the envisioned system. Furthermore, we outline how
the measurement of a whole free energy profile would allow the experimentalist
to retrieve the unknown substrate potential by means of a suitable
deconvolution. The influence of the pulling rate on the accuracy of the results
is investigated, and umbrella sampling is used to obtain the equilibrium
probability of particle escape for a variety of trap potentials.Comment: 21 pages, 11 figures, To appear in Soft Matte
Universality of the collapse transition of sticky polymers
The universality of the swelling of the radius of gyration of a homopolymer
relative to its value in the state, independent of polymer-solvent
chemistry, in the crossover regime between and athermal solvent
conditions, is well known. Here we study, by Brownian dynamics, a polymer model
where a subset of monomers is labelled as "stickers". The mutual interaction of
the stickers is more attractive than those of the other ("backbone") monomers,
and has the additional important characteristic of "functionality" ,
i.e., the maximum number of stickers that can locally bind to a given sticker.
A saturated bond formed in this manner remains bound until it breaks due to
thermal fluctuations, a requirement which can be viewed as an additional
Boolean degree of freedom that describes the bonding. This, in turn, makes the
question of the order of the collapse transition a non-trivial one.
Nevertheless, for the parameters that we have studied (in particular,
), we find a standard second-order collapse, using a
renormalised solvent quality parameter that takes into account the increased
average attraction due to the presence of stickers. We examine the swelling of
the radius of gyration of such a sticky polymer relative to its value in the
altered state, using a novel potential to model the various excluded
volume interactions that occur between the monomers on the chain. We find that
the swelling of such sticky polymers is identical to the universal swelling of
homopolymers in the thermal crossover regime. Additionally, for our model, the
Kuhn segment length under conditions is found to be the same for
chains with and without stickers.Comment: 13 pages, 10 figures, supplementary material (see ancillary
directory), to appear in Soft Matte
Brownian dynamics simulations of planar mixed flows of polymer solutions at finite concentrations
Periodic boundary conditions for planar mixed flows are implemented in the
context of a multi-chain Brownian dynamics simulation algorithm. The effect of
shear rate , and extension rate , on the size of
polymer chains, \left, and on the polymer contribution to
viscosity, , is examined for solutions of FENE dumbbells at finite
concentrations, with excluded volume interactions between the beads taken into
account. The influence of the mixedness parameter, , and flow strength,
, on \left and , is also examined, where
corresponds to pure shear flow, and
corresponds to pure extensional flow. It is shown that there exists a critical
value, , such that the flow is shear dominated for , and extension dominated for .Comment: 18 pages, 12 figures, to appear in Chemical Engineering Scienc
Compositional changes on GaN surfaces under low-energy ion bombardment studied by synchrotron-based spectroscopies
We have investigated compositional changes on GaNsurfaces under Ar-ion bombardment using synchrotron-based high-resolution x-rayphotoemission (PES) and near-edge x-ray absorption fine structure(NEXAFS)spectroscopy. The low-energy ion bombardment of GaN produces a Ga-rich surface layer which transforms into a metallic Ga layer at higher bombarding energies. At the same time, the photoemissionspectra around N 1s core levels reveal the presence of both uncoordinated nitrogen and nitrogen interstitials, which we have analyzed in more details by x-rayabsorption measurements at N K edge. We have demonstrated that PES and NEXAFS provide a powerful combination for studying the compositional changes on GaNsurfaces. A mechanism for the relocation and loss of nitrogen during ion bombardment in agreement with some recent experimental and theoretical studies of defect formation in GaN has been proposed.P.N.K.D. is grateful for the financial
support of the Australian Research Council
Rouse Chains with Excluded Volume Interactions: Linear Viscoelasticity
Linear viscoelastic properties for a dilute polymer solution are predicted by
modeling the solution as a suspension of non-interacting bead-spring chains.
The present model, unlike the Rouse model, can describe the solution's
rheological behavior even when the solvent quality is good, since excluded
volume effects are explicitly taken into account through a narrow Gaussian
repulsive potential between pairs of beads in a bead-spring chain. The use of
the narrow Gaussian potential, which tends to the more commonly used
delta-function repulsive potential in the limit of a width parameter "d" going
to zero, enables the performance of Brownian dynamics simulations. The
simulations results, which describe the exact behavior of the model, indicate
that for chains of arbitrary but finite length, a delta-function potential
leads to equilibrium and zero shear rate properties which are identical to the
predictions of the Rouse model. On the other hand, a non-zero value of "d"
gives rise to a prediction of swelling at equilibrium, and an increase in zero
shear rate properties relative to their Rouse model values. The use of a
delta-function potential appears to be justified in the limit of infinite chain
length. The exact simulation results are compared with those obtained with an
approximate solution which is based on the assumption that the non-equilibrium
configurational distribution function is Gaussian. The Gaussian approximation
is shown to be exact to first order in the strength of excluded volume
interaction, and is found to be accurate above a threshold value of "d", for
given values of chain length and strength of excluded volume interaction.Comment: Revised version. Long chain limit analysis has been deleted. An
improved and corrected examination of the long chain limit will appear as a
separate posting. 32 pages, 9 postscript figures, LaTe
Shear thinning in dilute and semidilute solutions of polystyrene and DNA
The viscosity of dilute and semidilute unentangled DNA solutions, in steady
simple shear flow, has been measured across a range of temperatures and
concentrations. For polystyrene solutions, measurements of viscosity have been
carried out in the semidilute unentangled regime, while results of prior
experimental measurements in the dilute regime have been used for the purpose
of data analysis, and for comparison with the behaviour of DNA solutions.
Interpretation of the shear rate dependence of viscosity in terms of suitably
defined non-dimensional variables, is shown to lead to master plots,
independent of temperature and concentration, in each of the two concentration
regimes. In the case of semidilute unentangled solutions, defining the
Weissenberg number in terms of a concentration dependent large scale relaxation
time is found not to lead to data collapse across different concentrations. On
the other hand, the use of an alternative relaxation time, with the
concentration dependence of a single correlation blob, suggests the existence
of universal shear thinning behaviour at large shear rates.Comment: 24 pages, 13 figures, supplementary material (see ancillary
directory), to appear in Journal of Rheolog
The common and uncommon cestodal infestation encountered in routine histopathological practice from a semi-urban population in south India and their public health importance.
Parasites are encountered uncommonly in routine histopathologic practice. Among them, cestodes form a major bulk. Cysticercosis heads the list forming the bulk of cases followed by Hydatidosis and Sparganosis. Microscopic identification of inflammation with surrounding reactions along with other morphological features forms the mainstay of diagnosis of parasitic diseases on histopathology. Identification of the parasites on histopathological examination would reduce the cost-diagnosis ratio avoiding expensive serological investigation
MHD Mixed Convective Flow of Viscoelastic and Viscous Fluids in a Vertical Porous Channel
In this paper, we analyze the problem of steady, mixed convective, laminar flow of two incompressible, electrically conducting and heat absorbing immiscible fluids in a vertical porous channel filled with viscoelastic fluid in one region and viscous fluid in the other region. A uniform magnetic field is applied in the transverse direction, the fluids rise in the channel driven by thermal buoyancy forces associated with thermal radiation. The equations are modeled using the fully developed flow conditions. An exact solution is obtained for the velocity, temperature, skin friction and Nusselt number distributions. The physical interpretation to these expressions is examined through graphs and table for the shear stress and rate of heat transfer coefficients at the channel walls
Universal scaling and characterisation of gelation in associative polymer solutions
A Brownian dynamics algorithm is used to describe the static behaviour of
associative polymer solutions. Predictions for the fractions of stickers bound
by intra-chain and inter-chain association, as a function of system parameters,
such as the number of stickers, the number of monomers between stickers, the
solvent quality, and concentration are obtained. A systematic comparison with
the scaling relations predicted by the mean-field theory of Dobrynin
(Macromolecules, 37, 3881, 2004) is carried out. Different regimes of scaling
behaviour are identified depending on the monomer concentration, the density of
stickers on a chain, and the solvent quality for backbone monomers. Simulation
results validate the predictions of the mean-field theory across a wide range
of parameter values in all the scaling regimes. The value of the des Cloizeaux
exponent proposed by Dobrynin for sticky polymer solutions, is shown to lead to
a collapse of simulation data for all the scaling relations considered here.
Three different signatures for the characterisation of gelation are identified,
with each leading to a different value of the concentration at the sol-gel
transition. The modified Flory-Stockmayer expression is found to be validated
by simulations for all three gelation signatures. Simulation results confirm
the prediction of scaling theory for the gelation line that separates sol and
gel phases, when the modified Flory-Stockmayer expression is used. Phase
separation is found to occur with increasing concentration for systems in which
the backbone monomers are under theta-solvent conditions, and is shown to
coincide with a breakdown in the predictions of scaling theory.Comment: 34 pages, 22 figures, includes Supplemental Material, accepted
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