1,387 research outputs found
Bundles of Interacting Strings in Two Dimensions
Bundles of strings which interact via short-ranged pair potentials are
studied in two dimensions. The corresponding transfer matrix problem is solved
analytically for arbitrary string number N by Bethe ansatz methods. Bundles
consisting of N identical strings exhibit a unique unbinding transition. If the
string bundle interacts with a hard wall, the bundle may unbind from the wall
via a unique transition or a sequence of N successive transitions. In all
cases, the critical exponents are independent of N and the density profile of
the strings exhibits a scaling form that approaches a mean-field profile in the
limit of large N.Comment: 8 pages (latex) with two figure
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
Membrane adhesion and domain formation
We review theoretical results for the adhesion-induced phase behavior of
biomembranes. The focus is on models in which the membranes are represented as
discretized elastic sheets with embedded adhesion molecules. We present several
mechanism that lead to the formation of domains during adhesion, and discuss
the time-dependent evolution of domain patterns obtained in Monte-Carlo
simulations. The simulated pattern dynamics has striking similarities to the
pattern evolution observed during T cell adhesion.Comment: 68 pages, 29 figure
Critical behavior of interacting surfaces with tension
Wetting phenomena, molecular protrusions of lipid bilayers and membrane
stacks under lateral tension provide physical examples for interacting surfaces
with tension. Such surfaces are studied theoretically using functional
renormalization and Monte Carlo simulations. The critical behavior arising from
thermally-excited shape fluctuations is determined both for global quantities
such as the mean separation of these surfaces and for local quantities such as
the probabilities for local contacts.Comment: 13 pages, 17 figures; accepted for publication in The European
Physical Journa
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
Remodeling of membrane shape and topology by curvature elasticity and membrane tension
Cellular membranes exhibit a fascinating variety of different morphologies, which are continuously remodeled by transformations of membrane shape and topology. This remodeling is essential for important biological processes (cell division, intracellular vesicle trafficking, endocytosis) and can be elucidated in a systematic and quantitative manner using synthetic membrane systems. Here, recent insights obtained from such synthetic systems are reviewed, integrating experimental observations and molecular dynamics simulations with the theory of membrane elasticity. The study starts from the polymorphism of biomembranes as observed for giant vesicles by optical microscopy and small nanovesicles in simulations. This polymorphism reflects the unusual elasticity of fluid membranes and includes the formation of membrane necks or fluid 'worm holes'. The proliferation of membrane necks generates stable multi-spherical shapes, which can form tubules and tubular junctions. Membrane necks are also essential for the remodeling of membrane topology via membrane fission and fusion. Neck fission can be induced by fine-tuning of membrane curvature, which leads to the controlled division of giant vesicles, and by adhesion-induced membrane tension as observed for small nanovesicles. Challenges for future research include the interplay of curvature elasticity and membrane tension during membrane fusion and the localization of fission and fusion processes within intramembrane domains
Multispherical shapes of vesicles highlight the curvature elasticity of biomembranes
Giant lipid vesicles form unusual multispherical or “multi-balloon” shapes consisting of several spheres that are connected by membrane necks. Such multispherical shapes have been recently observed when the two sides of the membranes were exposed to different sugar solutions. This sugar asymmetry induced a spontaneous curvature, the sign of which could be reversed by swapping the interior with the exterior solution. Here, previous studies of multispherical shapes are reviewed and extended to develop a comprehensive theory for these shapes. Each multisphere consists of large and small spheres, characterized by two radii, the large-sphere radius, Rl, and the small-sphere radius, Rs. For positive spontaneous curvature, the multisphere can be built up from variable numbers Nl and Ns of large and small spheres. In addition, multispheres consisting of N*=Nl+Ns equally sized spheres are also possible and provide examples for constant-mean-curvature surfaces. For negative spontaneous curvature, all multispheres consist of one large sphere that encloses a variable number Ns of small spheres. These general features of multispheres arise from two basic properties of curvature elasticity: the local shape equation for spherical membrane segments and the stability conditions for closed membrane necks. In addition, the (Nl+Ns)-multispheres can form several (Nl+Ns)-patterns that differ in the way, in which the spheres are mutually connected. These patterns may involve multispherical junctions consisting of individual spheres that are connected to more than two neighboring spheres. The geometry of the multispheres is governed by two polynomial equations which imply that (Nl+Ns)-multispheres can only be formed within a certain restricted range of vesicle volumes. Each (Nl+Ns)-pattern can be characterized by a certain stability regime that depends both on the stability of the closed necks and on the multispherical geometry. Interesting and challenging topics for future studies include the response of multispheres to locally applied external forces, membrane fusion between spheres to create multispherical shapes of higher-genus topology, and the enlarged morphological complexity of multispheres arising from lipid phase separation and intramembrane domains
Segregation of receptor-ligand complexes in cell adhesion zones: Phase diagrams and role of thermal membrane roughness
The adhesion zone of immune cells, the 'immunological synapse', exhibits
characteristic domains of receptor-ligand complexes. The domain formation is
likely caused by a length difference of the receptor-ligand complexes, and has
been investigated in experiments in which T cells adhere to supported membranes
with anchored ligands. For supported membranes with two types of anchored
ligands, MHCp and ICAM1, that bind to the receptors TCR and LFA1 in the cell
membrane, the coexistence of domains of TCR-MHCp and LFA1-ICAM1 complexes in
the cell adhesion zone has been observed for a wide range of ligand
concentrations and affinities. For supported membranes with long and short
ligands that bind to the same cell receptor CD2, in contrast, domain
coexistence has been observed for a rather narrow ratio of ligand
concentrations. In this article, we determine detailed phase diagrams for cells
adhering to supported membranes with a statistical-physical model of cell
adhesion. We find a characteristic difference between the adhesion scenarios in
which two types of ligands in a supported membrane bind (i) to the same cell
receptor or (ii) to two different cell receptors, which helps to explain the
experimental observations. Our phase diagrams fully include thermal shape
fluctuations of the cell membranes on nanometer scales, which lead to a
critical point for the domain formation and to a cooperative binding of the
receptors and ligands.Comment: 23 pages, 6 figure
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