569 research outputs found
Pulling adsorbed polymers from surfaces with the AFM: stick versus slip, peeling versus gliding
We consider the response of an adsorbed polymer that is pulled by an AFM
within a simple geometric framework. We separately consider the cases of i)
fixed polymer-surface contact point, ii) sticky case where the polymer is
peeled off from the substrate, and iii) slippery case where the polymer glides
over the surface. The resultant behavior depends on the value of the surface
friction coefficient and the adsorption strength. Our resultant force profiles
in principle allow to extract both from non-equilibrium force-spectroscopic
data.Comment: 6 pages, 3 figures; accepted for publication in Europhys. Lett.,
http://www.edpsciences.org/journal/index.cfm?edpsname=ep
Thermodynamic and Dynamic Anomalies for Dumbbell Molecules Interacting with a Repulsive Ramp-Like Potential
Using collision driven discrete molecular dynamics (DMD), we investigate the
thermodynamics and dynamics of systems of 500 dumbbell molecules interacting by
a purely repulsive ramp-like discretized potential, consisting of steps of
equal size. We compare the behavior of the two systems, with and steps. Each system exhibits both thermodynamic and dynamic anomalies, a
density maximum and the translational and rotational mobilities show anomalous
behavior. Starting with very dense systems and decreasing the density, both
mobilities first increase, reache a maximum, then decrease, reache a minimum,
and finally increase; this behavior is similar to the behavior of SPC/E water.
The regions in the pressure-temperature plane of translational and rotational
mobility anomalies depend strongly on . The product of the translational
diffusion coefficient and the orientational correlation time increases with
temperature, in contrast with the behavior of most liquids
IBI: Targeting cumulative coordination within an iterative protocol to derive coarse-grained models of (multi-component) complex fluids
We present a coarse-graining strategy that we test for aqueous mixtures. The
method uses pair-wise cumulative coordination as a target function within an
iterative Boltzmann inversion (IBI) like protocol. We name this method
coordination iterative Boltzmann inversion (IBI). While the
underlying coarse-grained model is still structure based and, thus, preserves
pair-wise solution structure, our method also reproduces solvation
thermodynamics of binary and/or ternary mixtures. Additionally, we observe much
faster convergence within IBI compared to IBI. To validate the
robustness, we apply IBI to study test cases of solvation
thermodynamics of aqueous urea and a triglycine solvation in aqueous urea
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
Surface states in nearly modulated systems
A Landau model is used to study the phase behavior of the surface layer for
magnetic and cholesteric liquid crystal systems that are at or near a Lifshitz
point marking the boundary between modulated and homogeneous bulk phases. The
model incorporates surface and bulk fields and includes a term in the free
energy proportional to the square of the second derivative of the order
parameter in addition to the usual term involving the square of the first
derivative. In the limit of vanishing bulk field, three distinct types of
surface ordering are possible: a wetting layer, a non-wet layer having a small
deviation from bulk order, and a different non-wet layer with a large deviation
from bulk order which decays non-monotonically as distance from the wall
increases. In particular the large deviation non-wet layer is a feature of
systems at the Lifshitz point and also those having only homogeneous bulk
phases.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
The interplay between screening properties and colloid anisotropy: towards a reliable pair potential for disc-like charged particles
The electrostatic potential of a highly charged disc (clay platelet) in an
electrolyte is investigated in detail. The corresponding non-linear
Poisson-Boltzmann (PB) equation is solved numerically, and we show that the
far-field behaviour (relevant for colloidal interactions in dilute suspensions)
is exactly that obtained within linearized PB theory, with the surface boundary
condition of a uniform potential. The latter linear problem is solved by a new
semi-analytical procedure and both the potential amplitude (quantified by an
effective charge) and potential anisotropy coincide closely within PB and
linearized PB, provided the disc bare charge is high enough. This anisotropy
remains at all scales; it is encoded in a function that may vary over several
orders of magnitude depending on the azimuthal angle under which the disc is
seen. The results allow to construct a pair potential for discs interaction,
that is strongly orientation dependent.Comment: 13 pages, 19 figure
Weak Long-Ranged Casimir Attraction in Colloidal Crystals
We investigate the influence of geometric confinement on the free energy of
an idealized model for charge-stabilized colloidal suspensions. The mean-field
Poisson-Boltzmann formulation for this system predicts pure repulsion among
macroionic colloidal spheres. Fluctuations in the simple ions' distribution
provide a mechanism for the macroions to attract each other at large
separations. Although this Casimir interaction is long-ranged, it is too weak
to influence colloidal crystals' dynamics.Comment: 5 pages 2 figures ReVTe
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