655 research outputs found
Lateral phase separation of confined membranes
We consider membranes interacting via short, intermediate and long stickers.
The effects of the intermediate stickers on the lateral phase separation of the
membranes are studied via mean-field approximation. The critical potential
depth of the stickers increases in the presence of the intermediate sticker.
The lateral phase separation of the membrane thus suppressed by the
intermediate stickers. Considering membranes interacting with short and long
stickers, the effect of confinement on the phase behavior of the membranes is
also investigated analytically
Probing the elastic response of lipid bilayers and nanovesicles to leaflet tensions via volume per lipid
Biological and biomimetic membranes are based on lipid bilayers, consisting of two monolayers or leaflets. One important but challenging physical parameter of these membranes is their tension. For a long time, this tension was explicitly or implicitly taken to be the bilayer tension, acting on the whole bilayer membrane. More recently, it has been realized that it is useful to decompose the bilayer tension into two leaflet tensions and that these tensions are accessible to molecular dynamics simulations. To divide the bilayer up into two leaflets, it is necessary to introduce a midsurface that defines the spatial extent of the two leaflets. In previous studies, this midsurface was obtained from the density profiles across the bilayer and was then used to compute the molecular area per lipid. Here, we develop an alternative approach based on three-dimensional Voronoi tessellation and molecular volume per lipid. Using this volume-based approach, we determine the reference states with tensionless leaflets as well as the optimal volumes and areas per lipid. The optimal lipid volumes have practically the same value in both leaflets, irrespective of the size and curvature of the nanovesicles, whereas the optimal lipid areas are different for the two leaflets and depend on the vesicle size. In addition, we introduce lateral volume compressibilities to describe the elastic response of the lipid volume to the leaflet tensions. We show that the outer leaflet of a nanovesicle is more densely packed and less compressible than the inner leaflet and that this difference becomes more pronounced for smaller vesicles.publishedVersio
Coupling of actin hydrolysis and polymerization: Reduced description with two nucleotide states
The polymerization of actin filaments is coupled to the hydrolysis of
adenosine triphosphate (ATP), which involves both the cleavage of ATP and the
release of inorganic phosphate. We describe hydrolysis by a reduced two-state
model with a cooperative cleavage mechanism, where the cleavage rate depends on
the state of the neighboring actin protomer in a filament. We obtain
theoretical predictions of experimentally accessible steady state quantities
such as the size of the ATP-actin cap, the size distribution of ATP-actin
islands, and the cleavage flux for cooperative cleavage mechanisms.Comment: 6 page
Traffic by multiple species of molecular motors
We study the traffic of two types of molecular motors using the two-species
symmetric simple exclusion process (ASEP) with periodic boundary conditions and
with attachment and detachment of particles. We determine characteristic
properties such as motor densities and currents by simulations and analytical
calculations. For motors with different unbinding probabilities, mean field
theory gives the correct bound density and total current of the motors, as
shown by numerical simulations. For motors differing in their stepping
probabilities, the particle-hole symmetry of the current-density relationship
is broken and mean field theory fails drastically. The total motor current
exhibits exponential finite-size scaling, which we use to extrapolate the total
current to the thermodynamic limit. Finally, we also study the motion of a
single motor in the background of many non-moving motors.Comment: 23 pages, 6 figures, late
Finite size effects in nonequilibrium wetting
Models with a nonequilibrium wetting transition display a transition also in
finite systems. This is different from nonequilibrium phase transitions into an
absorbing state, where the stationary state is the absorbing one for any value
of the control parameter in a finite system. In this paper, we study what kind
of transition takes place in finite systems of nonequilibrium wetting models.
By solving exactly a microscopic model with three and four sites and performing
numerical simulations we show that the phase transition taking place in a
finite system is characterized by the average interface height performing a
random walk at criticality and does not discriminate between the bounded-KPZ
classes and the bounded-EW class. We also study the finite size scaling of the
bKPZ universality classes, showing that it presents peculiar features in
comparison with other universality classes of nonequilibrium phase transitions.Comment: 14 pages, 6figures, major change
Boundary and Bulk Phase Transitions in the Two Dimensional Q > 4 State Potts Model
The surface and bulk properties of the two-dimensional Q > 4 state Potts
model in the vicinity of the first order bulk transition point have been
studied by exact calculations and by density matrix renormalization group
techniques. For the surface transition the complete analytical solution of the
problem is presented in the limit, including the critical and
tricritical exponents, magnetization profiles and scaling functions. According
to the accurate numerical results the universality class of the surface
transition is independent of the value of Q > 4. For the bulk transition we
have numerically calculated the latent heat and the magnetization discontinuity
and we have shown that the correlation lengths in the ordered and in the
disordered phases are identical at the transition point.Comment: 11 pages, RevTeX, 6 PostScript figures included. Manuscript
substantially extended, details on the analytical and numerical calculations
added. To appear in Phys. Rev.
Nonequilibrium wetting transitions with short range forces
We analyze within mean-field theory as well as numerically a KPZ equation
that describes nonequilibrium wetting. Both complete and critical wettitng
transitions were found and characterized in detail. For one-dimensional
substrates the critical wetting temperature is depressed by fluctuations. In
addition, we have investigated a region in the space of parameters (temperature
and chemical potential) where the wet and nonwet phases coexist. Finite-size
scaling analysis of the interfacial detaching times indicates that the finite
coexistence region survives in the thermodynamic limit. Within this region we
have observed (stable or very long-lived) structures related to spatio-temporal
intermittency in other systems. In the interfacial representation these
structures exhibit perfect triangular (pyramidal) patterns in one (two
dimensions), that are characterized by their slope and size distribution.Comment: 11 pages, 5 figures. To appear in Physical Review
Surface induced disorder in body-centered cubic alloys
We present Monte Carlo simulations of surface induced disordering in a model
of a binary alloy on a bcc lattice which undergoes a first order bulk
transition from the ordered DO3 phase to the disordered A2 phase. The data are
analyzed in terms of an effective interface Hamiltonian for a system with
several order parameters in the framework of the linear renormalization
approach due to Brezin, Halperin and Leibler. We show that the model provides a
good description of the system in the vicinity of the interface. In particular,
we recover the logarithmic divergence of the thickness of the disordered layer
as the bulk transition is approached, we calculate the critical behavior of the
maxima of the layer susceptibilities, and demonstrate that it is in reasonable
agreement with the simulation data. Directly at the (110) surface, the theory
predicts that all order parameters vanish continuously at the surface with a
nonuniversal, but common critical exponent. However, we find different
exponents for the order parameter of the DO3 phase and the order parameter of
the B2 phase. Using the effective interface model, we derive the finite size
scaling function for the surface order parameter and show that the theory
accounts well for the finite size behavior of the DO3 ordering but not for that
of B2 ordering. The situation is even more complicated in the neighborhood of
the (100) surface, due to the presence of an ordering field which couples to
the B2 order.Comment: To appear in Physical Review
Single polymer adsorption in shear: flattening versus hydrodynamic lift and corrugation effects
The adsorption of a single polymer to a flat surface in shear is investigated
using Brownian hydrodynamics simulations and scaling arguments. Competing
effects are disentangled: in the absence of hydrodynamic interactions, shear
drag flattens the chain and thus enhances adsorption. Hydrodynamic lift on the
other hand gives rise to long-ranged repulsion from the surface which preempts
the surface-adsorbed state via a discontinuous desorption transition, in
agreement with theoretical arguments. Chain flattening is dominated by
hydrodynamic lift, so overall, shear flow weakens the adsorption of flexible
polymers. Surface friction due to small-wavelength surface potential
corrugations is argued to weaken the surface attraction as well.Comment: 6 pages, 4 figure
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