952 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
Collective Molecular Dynamics in Proteins and Membranes
The understanding of dynamics and functioning of biological membranes and in
particular of membrane embedded proteins is one of the most fundamental
problems and challenges in modern biology and biophysics. In particular the
impact of membrane composition and properties and of structure and dynamics of
the surrounding hydration water on protein function is an upcoming hot topic,
which can be addressed by modern experimental and computational techniques.
Correlated molecular motions might play a crucial role for the understanding
of, for instance, transport processes and elastic properties, and might be
relevant for protein function. Experimentally that involves determining
dispersion relations for the different molecular components, i.e., the length
scale dependent excitation frequencies and relaxation rates. Only very few
experimental techniques can access dynamical properties in biological materials
on the nanometer scale, and resolve dynamics of lipid molecules, hydration
water molecules and proteins and the interaction between them. In this context,
inelastic neutron scattering turned out to be a very powerful tool to study
dynamics and interactions in biomolecular materials up to relevant nanosecond
time scales and down to the nanometer length scale. We review and discuss
inelastic neutron scattering experiments to study membrane elasticity and
protein-protein interactions of membrane embedded proteins
Stochastic simulations of cargo transport by processive molecular motors
We use stochastic computer simulations to study the transport of a spherical
cargo particle along a microtubule-like track on a planar substrate by several
kinesin-like processive motors. Our newly developed adhesive motor dynamics
algorithm combines the numerical integration of a Langevin equation for the
motion of a sphere with kinetic rules for the molecular motors. The Langevin
part includes diffusive motion, the action of the pulling motors, and
hydrodynamic interactions between sphere and wall. The kinetic rules for the
motors include binding to and unbinding from the filament as well as active
motor steps. We find that the simulated mean transport length increases
exponentially with the number of bound motors, in good agreement with earlier
results. The number of motors in binding range to the motor track fluctuates in
time with a Poissonian distribution, both for springs and cables being used as
models for the linker mechanics. Cooperativity in the sense of equal load
sharing only occurs for high values for viscosity and attachment time.Comment: 40 pages, Revtex with 13 figures, to appear in Journal of Chemical
Physic
Cooperative Cargo Transport by Several Molecular Motors
The transport of cargo particles which are pulled by several molecular motors
in a cooperative manner is studied theoretically. The transport properties
depend primarily on the maximal number, , of motor molecules that may pull
simultaneously on the cargo particle. Since each motor must unbind from the
filament after a finite number of steps but can also rebind to it again, the
actual number of pulling motors is not constant but varies with time between
zero and . An increase in the maximal number leads to a strong increase
of the average walking distance (or run length) of the cargo particle. If the
cargo is pulled by up to kinesin motors, e.g., the walking distance is
estimated to be micrometers which implies that seven or eight
kinesin molecules are sufficient to attain an average walking distance in the
centimeter range. If the cargo particle is pulled against an external load
force, this force is shared between the motors which provides a nontrivial
motor-motor coupling and a generic mechanism for nonlinear force-velocity
relationships. With increasing load force, the probability distribution of the
instantenous velocity is shifted towards smaller values, becomes broader, and
develops several peaks. Our theory is consistent with available experimental
data and makes quantitative predictions that are accessible to systematic in
vitro experiments.Comment: 24 pages, latex, 6 figures, includes Supporting Tex
Phase transitions in systems with two species of molecular motors
Systems with two species of active molecular motors moving on (cytoskeletal)
filaments into opposite directions are studied theoretically using driven
lattice gas models. The motors can unbind from and rebind to the filaments. Two
motors are more likely to bind on adjacent filament sites if they belong to the
same species. These systems exhibit (i) Continuous phase transitions towards
states with spontaneously broken symmetry, where one motor species is largely
excluded from the filament, (ii) Hysteresis of the total current upon varying
the relative concentrations of the two motor species, and (iii) Coexistence of
traffic lanes with opposite directionality in multi-filament systems. These
theoretical predictions should be experimentally accessible.Comment: 7 pages, 4 figures, epl style (.cls-file included), to appear in
Europhys. Lett. (http://www.edpsciences.org/epl
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
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
- …