789 research outputs found
Wetting between structured surfaces: Liquid bridges and induced forces
Wetting phenomena are theoretically studied for a slab geometry
consisting of a wetting phase confined between two chemically
patterned substrates. Each of these is decorated by an array of
stripes whose composition alternates between two different surface
phases. For a single pair of opposing stripes, the wetting phase may
either form a bridge spanning from one surface to the other or it may
break up into two separate channels. The bridge state induces an
effective interaction between the two substrates. This leads to the
bridge itself having a preferred contact angle and the substrates
having a preferred separation. In the case of many stripes, one has a
whole sequence of morphological transitions with the number of bridges
decreasing as the surface separation grows
Contact angles on heterogeneous surfaces; a new look at Cassie's and Wenzel's laws
We consider a three dimensional liquid drop sitting on a rough and chemically
heterogeneous substrate. Using a novel minimization technique on the free
energy of this system, a generalized Young's equation for the contact angle is
found. In certain limits, the Cassie and Wenzel laws, and a new equivalent
rule, applicable in general, are derived. We also propose an equation in the
same spirit as these results but valid on a more `microscopic' level.
Throughout we work under the presence of gravity and keep account of line
tension terms.Comment: 10 pages RevTeX, 2 EPS figures. A few minor corrections mad
Barrier crossing of semiflexible polymers
We consider the motion of semiflexible polymers in double-well potentials. We
calculate shape, energy, and effective diffusion constant of kink excitations,
and in particular their dependence on the bending rigidity of the semiflexible
polymer. For symmetric potentials, the kink motion is purely diffusive whereas
kink motion becomes directed in the presence of a driving force on the polymer.
We determine the average velocity of the semiflexible polymer based on the kink
dynamics. The Kramers escape over the potential barriers proceeds by nucleation
and diffusive motion of kink-antikink pairs, the relaxation to the straight
configuration by annihilation of kink-antikink pairs. Our results apply to the
activated motion of biopolymers such as DNA and actin filaments or synthetic
polyelectrolytes on structured substrates.Comment: 7 pages, 3 figure
Domain Growth, Budding, and Fission in Phase Separating Self-Assembled Fluid Bilayers
A systematic investigation of the phase separation dynamics in self-assembled
multi-component bilayer fluid vesicles and open membranes is presented. We use
large-scale dissipative particle dynamics to explicitly account for solvent,
thereby allowing for numerical investigation of the effects of hydrodynamics
and area-to-volume constraints. In the case of asymmetric lipid composition, we
observed regimes corresponding to coalescence of flat patches, budding,
vesiculation and coalescence of caps. The area-to-volume constraint and
hydrodynamics have a strong influence on these regimes and the crossovers
between them. In the case of symmetric mixtures, irrespective of the
area-to-volume ratio, we observed a growth regime with an exponent of 1/2. The
same exponent is also found in the case of open membranes with symmetric
composition
Phase Transitions in Multicomponent String Model
We propose a one-dimensional model of a string decorated with adhesion
molecules (stickers) to mimic multicomponent membranes in restricted
geometries. The string is bounded by two parallel walls and it interacts with
one of them by short range attractive forces while the stickers are attracted
by the other wall. The exact solution of the model in the case of infinite wall
separation predicts both continuous and discontinuous transitions between
phases characterised by low and high concentration of stickers on the string.
Our model exhibits also coexistence of these two phases, similarly to models of
multicomponent membranes.Comment: letter, 8 pages, 3 figure
From supported membranes to tethered vesicles: lipid bilayers destabilisation at the main transition
We report results concerning the destabilisation of supported phospholipid
bilayers in a well-defined geometry. When heating up supported phospholipid
membranes deposited on highly hydrophilic glass slides from room temperature
(i.e. with lipids in the gel phase), unbinding was observed around the main gel
to fluid transition temperature of the lipids. It lead to the formation of
relatively monodisperse vesicles, of which most remained tethered to the
supported bilayer. We interpret these observations in terms of a sharp decrease
of the bending rigidity modulus in the transition region, combined
with a weak initial adhesion energy. On the basis of scaling arguments, we show
that our experimental findings are consistent with this hypothesis.Comment: 11 pages, 3 figure
Coarse-grained molecular model for the glycosylphosphatidylinositol anchor with and without protein
Glycosylphosphatidylinositol (GPI) anchors are a unique class of complex glycolipids that anchor a great variety of proteins to the extracellular leaflet of plasma membranes of eukaryotic cells. These anchors can exist either with or without an attached protein called GPI-anchored protein (GPI-AP) both in vitro and in vivo. Although GPIs are known to participate in a broad range of cellular functions, it is to a large extent unknown how these are related to GPI structure and composition. Their conformational flexibility and micro-heterogeneity makes it difficult to study them experimentally. Simplified atomistic models are amenable to all-atom computer simulations in small lipid bilayer patches, but not suitable for studying their partitioning and trafficking in complex and heterogeneous membranes. Here, we present a coarse-grained model of GPI anchor constructed with a modified version of MARTINI force-field that is suited for modeling carbohydrates, proteins and lipids in an aqueous environment using MARTINI's polarizable water. The non-bonded interactions for sugars were re-parameterized by calculating their partitioning free energies between polar and apolar phases. In addition, sugar-sugar interactions were optimized by adjusting the second virial coefficients of osmotic pressures for solutions of glucose, sucrose and trehalose to match with experimental data. With respect to the conformational dynamics of GPI-anchored green fluoresccent protein, the accessible timescales are now at least an order of magnitude larger than for the all-atom system. This is particularly important for fine-tuning the mutual interactions of lipids, carbohydrates, and amino-acids when comparing to experimental results. We discuss the prospective use of the coarse-grained GPI model for studying protein-sorting and trafficking in membrane models
Hydrodynamic lift on bound vesicles
Bound vesicles subject to lateral forces such as arising from shear flow are
investigated theoretically by combining a lubrication analysis of the bound
part with a scaling approach to the global motion. A minor inclination of the
bound part leads to significant lift due to the additive effects of lateral and
tank-treading motions. With increasing shear rate, the vesicle unbinds from the
substrate at a critical value. Estimates are in agreement with recent
experimental data.Comment: 9 pages, one figur
The unbinding transition of mixed fluid membranes
A phenomenological model for the unbinding transition of multi-component
fluid membranes is proposed, where the unbinding transition is described using
a theory analogous to Flory-Huggins theory for polymers. The coupling between
the lateral phase separation of inclusion molecules and the membrane-substrate
distance explains the phase coexistence between two unbound phases as observed
in recent experiments by Marx et al. [Phys. Rev. Lett. 88, 138102 (2002)].
Bellow a critical end-point temperature, we find that the unbinding transition
becomes first-order for multi-component membranes.Comment: 7 pages, 3 eps figure
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