119,916 research outputs found
Wetting and Capillary Condensation in Symmetric Polymer Blends: A comparison between Monte Carlo Simulations and Self-Consistent Field Calculations
We present a quantitative comparison between extensive Monte Carlo
simulations and self-consistent field calculations on the phase diagram and
wetting behavior of a symmetric, binary (AB) polymer blend confined into a
film. The flat walls attract one component via a short range interaction. The
critical point of the confined blend is shifted to lower temperatures and
higher concentrations of the component with the lower surface free energy. The
binodals close the the critical point are flattened compared to the bulk and
exhibit a convex curvature at intermediate temperatures -- a signature of the
wetting transition in the semi-infinite system. Investigating the spectrum of
capillary fluctuation of the interface bound to the wall, we find evidence for
a position dependence of the interfacial tension. This goes along with a
distortion of the interfacial profile from its bulk shape. Using an extended
ensemble in which the monomer-wall interaction is a stochastic variable, we
accurately measure the difference between the surface energies of the
components, and determine the location of the wetting transition via the Young
equation. The Flory-Huggins parameter at which the strong first order wetting
transition occurs is independent of chain length and grows quadratically with
the integrated wall-monomer interaction strength. We estimate the location of
the prewetting line. The prewetting manifests itself in a triple point in the
phase diagram of very thick films and causes spinodal dewetting of ultrathin
layers slightly above the wetting transition. We investigate the early stage of
dewetting via dynamic Monte Carlo simulations.Comment: to appear in Macromolecule
How ions in solution can change the sign of the critical Casimir potential
We show that hydrophilic ions present in a confined, near-critical aqueous
mixture can lead to an attraction between like charge surfaces with opposing
preferential adsorption of the two species of the mixture, even though the
corresponding Casimir potential in uncharged systems is repulsive. This
prediction agrees with recent experiment [Nellen {\it{et al.}}, Soft
Matter{\bf{80}}, 061143 (2011)]. We also show that oppositely charged
hydrophobic surfaces can repel each other, although the Casimir potential
between uncharged surfaces with like preferential adsorption (selectivity) is
attractive. This behavior is expected when the electrostatic screening length
is larger than the correlation length, and one of the confining surfaces is
strongly selective and weakly charged, whereas the other confining surface is
weakly selective and strongly charged. The Casimir potential can change sign
because the hydrophilic ions near the weakly hydrophobic surface can
overcompensate the effect of hydrophobicity, and this surface can act as a
hydrophilic one. We also predict a more attractive interaction between
hydrophilic surfaces and a more repulsive interaction between hydrophobic
surfaces than given by the sum of the Casimir and Deby-H\"uckel potentials. Our
theory is derived systematically from a microscopic approach, and combines the
Landau-type and Debye-H\"uckel theories with an additional contribution of an
entropic origin
Analysis of parametric biological models with non-linear dynamics
In this paper we present recent results on parametric analysis of biological
models. The underlying method is based on the algorithms for computing
trajectory sets of hybrid systems with polynomial dynamics. The method is then
applied to two case studies of biological systems: one is a cardiac cell model
for studying the conditions for cardiac abnormalities, and the second is a
model of insect nest-site choice.Comment: In Proceedings HSB 2012, arXiv:1208.315
Increased Concentration of Polyvalent Phospholipids in the Adsorption Domain of a Charged Protein
We studied the adsorption of a charged protein onto an oppositely charged
membrane, composed of mobile phospholipids of differing valence, using a
statistical-thermodynamical approach. A two-block model was employed, one block
corresponding to the protein-affected region on the membrane, referred to as
the adsorption domain, and the other to the unaffected remainder of the
membrane. We calculated the protein-induced lipid rearrangement in the
adsorption domain as arising from the interplay between the electrostatic
interactions in the system and the mixing entropy of the lipids. Equating the
electrochemical potentials of the lipids in the two blocks yields an expression
for the relations among the various lipid fractions in the adsorption domain,
indicating a sensitive dependence of lipid fraction on valence. This expression
is a result of the two-block picture but does not depend on further details of
the protein-membrane interaction. We subsequently calculated the lipid
fractions themselves using the Poisson-Boltzmann theory. We examined the
dependence of lipid enrichment, i.e., the ratio between the lipid fractions
inside and outside the adsorption domain, on various parameters such as ionic
strength and lipid valence. Maximum enrichment was found for lipid valence of
about (-3) to (-4) in physiological conditions. Our results are in qualitative
agreement with recent experimental studies on the interactions between peptides
having a domain of basic residues and membranes containing a small fraction of
the polyvalent phosphatidylinositol 4,5-bisphosphate (PIP2). This study
provides theoretical support for the suggestion that proteins adsorbed onto
membranes through a cluster of basic residues may sequester PIP2 and other
polyvalent lipids.Comment: 25 pages, 12 figure
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