980 research outputs found
Strong-Coupling Theory for Counter-Ion Distributions
The Poisson-Boltzmann approach gives asymptotically exact counter-ion density
profiles around charged objects in the weak-coupling limit of low valency and
high temperature. In this paper we derive, using field-theoretic methods, a
theory which becomes exact in the opposite limit of strong coupling. Formally,
it corresponds to a standard virial expansion. Long-range divergences, which
render the virial expansion intractable for homogeneous bulk systems, are shown
to be renormalizable for the case of inhomogeneous distribution functions by a
systematic expansion in inverse powers of the coupling parameter. For a planar
charged wall, our analytical results compare quantitatively with extensive
Monte-Carlo simulations.Comment: 7 pages, 3 figures; to appear in Europhys. Let
Multiple surface wave solutions on linear viscoelastic media
We study the generic dispersion relation of surface waves on a semi-infinite viscoelastic medium bounded by a 2D viscoelastic interface, including the effects of gravitation, surface tension and bending rigidity. The classical Rayleigh, capillary-gravity and Lucassen wave solutions result as limiting cases. We identify an additional solution that differs from all previously described waves in that gravitation, surface tension and bulk shear viscosity must simultaneously be nonzero, and which exists on a pure air-water interface. For a surfactant monolayer on water, the number of coexisting wave solutions switches between one and three, depending on interfacial compressibility and frequency
Discrete elastic model for stretching-induced flagellar polymorphs
Force-induced reversible transformations between coiled and normal polymorphs
of bacterial flagella have been observed in recent optical-tweezer experiment.
We introduce a discrete elastic rod model with two competing helical states
governed by a fluctuating spin-like variable that represents the underlying
conformational states of flagellin monomers. Using hybrid Brownian dynamics
Monte-Carlo simulations, we show that a helix undergoes shape transitions
dominated by domain wall nucleation and motion in response to externally
applied uniaxial tension. A scaling argument for the critical force is
presented in good agreement with experimental and simulation results.
Stretching rate-dependent elasticity including a buckling instability are
found, also consistent with the experiment
Non-equilibrium hydrodynamics of a rotating filament
The nonlinear dynamics of an elastic filament that is forced to rotate at its
base is studied by hydrodynamic simulation techniques; coupling between
stretch, bend, twist elasticity and thermal fluctuations is included. The
twirling-overwhirling transition is located and found to be strongly
discontinuous. For finite bend and twist persistence length, thermal
fluctuations lower the threshold rotational frequency, for infinite persistence
length the threshold agrees with previous analytical predictions
Cyclization dynamics of finite-length collapsed self-avoiding polymers
We study the end-point cyclization of ideal and interacting polymers as a function of chain length N. For the cyclization time �cyc of ideal chains we recover the known scaling �cyc � N2 for different backbone models, for a self-avoiding slightly collapsed chain we obtain from Langevin simulations and scaling theory a modified scaling �cyc � N5=3. By extracting the memory kernel
that governs the non-Markovian end-point kinetics, we demonstrate that the dynamics of a finite-length collapsed chain is dominated by the crossover between swollen and collapsed behavior
Collective exchange processes reveal an active site proton cage in bacteriorhodopsin
Proton translocation across membranes is vital to all kingdoms of life. Mechanistically, it relies on characteristic proton flows and modifications of hydrogen bonding patterns, termed protonation dynamics, which can be directly observed by fast magic angle spinning (MAS) NMR. Here, we demonstrate that reversible proton displacement in the active site of bacteriorhodopsin already takes place in its equilibrated dark-state, providing new information on the underlying hydrogen exchange processes. In particular, MAS NMR reveals proton exchange at D85 and the retinal Schiff base, suggesting a tautomeric equilibrium and thus partial ionization of D85. We provide evidence for a proton cage and detect a preformed proton path between D85 and the proton shuttle R82. The protons at D96 and D85 exchange with water, in line with ab initio molecular dynamics simulations. We propose that retinal isomerization makes the observed proton exchange processes irreversible and delivers a proton towards the extracellular release site
Instabilities and turbulence-like dynamics in an oppositely driven binary particle mixture
Using extensive particle-based simulations, we investigate out-of-equilibrium
pattern dynamics in an oppositely driven binary particle system in two
dimensions. A surprisingly rich dynamical behavior including lane formation,
jamming, oscillation and turbulence-like dynamics is found. The ratio of two
friction coefficients is a key parameter governing the stability of lane
formation. When the friction coefficient transverse to the external force
direction is sufficiently small compared to the longitudinal one, the lane
structure becomes unstable to shear-induced disturbances, and the system
eventually exhibits a dynamical transition into a novel turbulence-like phase
characterized by random convective flows. We numerically construct an
out-of-equilibrium phase diagram. Statistical analysis of complex
spatio-temporal dynamics of the fully nonlinear turbulence-like phase suggests
its apparent reminiscence to the swarming dynamics in certain active matter
systems.Comment: 6 pages, 6 figures, accepted for publication in EP
Counterions at Charged Cylinders: Criticality and universality beyond mean-field
The counterion-condensation transition at charged cylinders is studied using
Monte-Carlo simulation methods. Employing logarithmically rescaled radial
coordinates, large system sizes are tractable and the critical behavior is
determined by a combined finite-size and finite-ion-number analysis. Critical
counterion localization exponents are introduced and found to be in accord with
mean-field theory both in 2 and 3 dimensions. In 3D the heat capacity shows a
universal jump at the transition, while in 2D, it consists of discrete peaks
where single counterions successively condense.Comment: 4 pages, 3 figures; submitted to Phys. Rev. Lett. (2005
New Criticality of 1D Fermions
One-dimensional massive quantum particles (or 1+1-dimensional random walks)
with short-ranged multi-particle interactions are studied by exact
renormalization group methods. With repulsive pair forces, such particles are
known to scale as free fermions. With finite -body forces (m = 3,4,...), a
critical instability is found, indicating the transition to a fermionic bound
state. These unbinding transitions represent new universality classes of
interacting fermions relevant to polymer and membrane systems. Implications for
massless fermions, e.g. in the Hubbard model, are also noted. (to appear in
Phys. Rev. Lett.)Comment: 10 pages (latex), with 2 figures (not included
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