519 research outputs found
Self-consistent dynamics of wall slip
A simple molecular model is studied to explain wall slip in a polymer melt. We consider a tube model for tethered chains in which the most important relaxation mechanisms: convective constraint release and chain stretching (retraction), are incorporated. Furthermore, we take the interactions between tethered chains and bulk flow self-consistently into account. Numerical simulations show that our model exhibits an entanglement-disentanglement transition, leading to a jump in the slip velocity which increases with the number of entanglements and the grafting density. The wall shear stress is found to be a nonmonotonic function of the slip and plate velocity, yielding the possibility of hysteresis and spurt instabilities. In a simplified version of the model we show via an analytical approach that the stick-slip transition is asymmetrical: the transition from stick to slip is much faster than the slip to stick transition. Our analysis reveals the existence of a dimensionless parameter that determines the time scale of the dynamics for the slowing down of the bulk flow. The relative rate at which relaxation of the tethered chains and slowing down of the bulk take place, seems to be quintessential for the slip behavior of the melt
Understanding Nanopore Window Distortions in the Reversible Molecular Valve Zeolite RHO
Molecular valves are becoming popular for potential biomedical applications.
However, little is known concerning their performance in energy and
environmental areas. Zeolite RHO shows unique pore deformations upon changes in
hydration, cation siting, cation type, or temperature-pressure conditions. By
varying the level of distortion of double eight-rings, it is possible to
control the adsorption properties, which confer a molecular valve behavior to
this material. We have employed interatomic potentials-based simulations to
obtain a detailed atomistic view of the structural distortion mechanisms of
zeolite RHO, in contrast with the averaged and space group restricted
information provided by diffraction studies. We have modeled four
aluminosilicate structures, containing Li, Na, K, Ca, and
Sr cations. The distortions of the three different zeolite rings are
coupled, and the six- and eight-membered rings are largely flexible. A large
dependence on the polarizing power of the extra-framework cations and with the
loading of water has been found for the minimum aperture of the eight-membered
rings that control the nanovalve effect. The calculated energy barriers for
moving the cations across the eight-membered rings are very high, which
explains the experimentally observed slow kinetics of the phase transition as
well as the appearance of metastable phases
Polarizable Force Fields for CO<sub>2</sub> and CH<sub>4</sub> Adsorption in M-MOF-74
(Graph Presented) The family of M-MOF-74, with M = Co, Cr, Cu, Fe, Mg, Mn, Ni, Ti, V, and Zn, provides opportunities for numerous energy related gas separation applications. The pore structure of M-MOF-74 exhibits a high internal surface area and an exceptionally large adsorption capacity. The chemical environment of the adsorbate molecule in M-MOF-74 can be tuned by exchanging the metal ion incorporated in the structure. To optimize materials for a given separation process, insights into how the choice of the metal ion affects the interaction strength with adsorbate molecules and how to model these interactions are essential. Here, we quantitatively highlight the importance of polarization by comparing the proposed polarizable force field to orbital interaction energies from DFT calculations. Adsorption isotherms and heats of adsorption are computed for CO2, CH4, and their mixtures in M-MOF-74 with all 10 metal ions. The results are compared to experimental data, and to previous simulation results using nonpolarizable force fields derived from quantum mechanics. To the best of our knowledge, the developed polarizable force field is the only one so far trying to cover such a large set of possible metal ions. For the majority of metal ions, our simulations are in good agreement with experiments, demonstrating the effectiveness of our polarizable potential and the transferability of the adopted approach.</p
Forced translocation of a polymer: dynamical scaling vs. MD-simulation
We suggest a theoretical description of the force-induced translocation
dynamics of a polymer chain through a nanopore. Our consideration is based on
the tensile (Pincus) blob picture of a pulled chain and the notion of
propagating front of tensile force along the chain backbone, suggested recently
by T. Sakaue. The driving force is associated with a chemical potential
gradient that acts on each chain segment inside the pore. Depending on its
strength, different regimes of polymer motion (named after the typical chain
conformation, "trumpet", "stem-trumpet", etc.) occur. Assuming that the local
driving and drag forces are equal (i.e., in a quasi-static approximation), we
derive an equation of motion for the tensile front position . We show
that the scaling law for the average translocation time changes from
to (for the
free-draining case) as the dimensionless force (where , , , , are the Kuhn segment length, the chain
length, the Flory exponent, the driving force, and the temperature,
respectively) increases. These and other predictions are tested by Molecular
Dynamics (MD) simulation. Data from our computer experiment indicates indeed
that the translocation scaling exponent grows with the pulling force
) albeit the observed exponent stays
systematically smaller than the theoretically predicted value. This might be
associated with fluctuations which are neglected in the quasi-static
approximation.Comment: 17 pages, 8 figures; figure 5 is new; figures 4 and 6-8 are upgrade
Effect of External Noise Correlation in Optical Coherence Resonance
Coherence resonance occurring in semiconductor lasers with optical feedback
is studied via the Lang-Kobayashi model with external non-white noise in the
pumping current. The temporal correlation and the amplitude of the noise have a
highly relevant influence in the system, leading to an optimal coherent
response for suitable values of both the noise amplitude and correlation time.
This phenomenon is quantitatively characterized by means of several statistical
measures.Comment: RevTeX, 4 pages, 7 figure
Polymer translocation through a nanopore - a showcase of anomalous diffusion
The translocation dynamics of a polymer chain through a nanopore in the
absence of an external driving force is analyzed by means of scaling arguments,
fractional calculus, and computer simulations. The problem at hand is mapped on
a one dimensional {\em anomalous} diffusion process in terms of reaction
coordinate (i.e. the translocated number of segments at time ) and shown
to be governed by an universal exponent whose
value is nearly the same in two- and three-dimensions. The process is described
by a {\em fractional} diffusion equation which is solved exactly in the
interval with appropriate boundary and initial conditions. The
solution gives the probability distribution of translocation times as well as
the variation with time of the statistical moments: , and which provide full description of the diffusion process. The
comparison of the analytic results with data derived from extensive Monte Carlo
(MC) simulations reveals very good agreement and proves that the diffusion
dynamics of unbiased translocation through a nanopore is anomalous in its
nature.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev.
Driven polymer translocation through a nanopore: a manifestation of anomalous diffusion
We study the translocation dynamics of a polymer chain threaded through a
nanopore by an external force. By means of diverse methods (scaling arguments,
fractional calculus and Monte Carlo simulation) we show that the relevant
dynamic variable, the translocated number of segments , displays an {\em
anomalous} diffusive behavior even in the {\em presence} of an external force.
The anomalous dynamics of the translocation process is governed by the same
universal exponent , where is the Flory
exponent and - the surface exponent, which was established recently
for the case of non-driven polymer chain threading through a nanopore. A closed
analytic expression for the probability distribution function , which
follows from the relevant {\em fractional} Fokker - Planck equation, is derived
in terms of the polymer chain length and the applied drag force . It is
found that the average translocation time scales as . Also the corresponding time dependent
statistical moments, and reveal unambiguously the anomalous nature of the translocation
dynamics and permit direct measurement of in experiments. These
findings are tested and found to be in perfect agreement with extensive Monte
Carlo (MC) simulations.Comment: 6 pages, 4 figures, accepted to Europhys. Lett; some references were
supplemented; typos were correcte
- âŠ