11,835 research outputs found
Phase behavior and material properties of hollow nanoparticles
Effective pair potentials for hollow nanoparticles like the ones made from
carbon (fullerenes) or metal dichalcogenides (inorganic fullerenes) consist of
a hard core repulsion and a deep, but short-ranged, van der Waals attraction.
We investigate them for single- and multi-walled nanoparticles and show that in
both cases, in the limit of large radii the interaction range scales inversely
with the radius, , while the well depth scales linearly with . We predict
the values of the radius and the wall thickness at which the gas-liquid
coexistence disappears from the phase diagram. We also discuss unusual material
properties of the solid, which include a large heat of sublimation and a small
surface energy.Comment: Revtex, 13 pages with 8 Postscript files included, submitted to Phys.
Rev.
Elastic Interactions of Cells
Biological cells in soft materials can be modeled as anisotropic force
contraction dipoles. The corresponding elastic interaction potentials are
long-ranged ( with distance ) and depend sensitively on elastic
constants, geometry and cellular orientations. On elastic substrates, the
elastic interaction is similar to that of electric quadrupoles in two
dimensions and for dense systems leads to aggregation with herringbone order on
a cellular scale. Free and clamped surfaces of samples of finite size introduce
attractive and repulsive corrections, respectively, which vary on the
macroscopic scale. Our theory predicts cell reorientation on stretched elastic
substrates.Comment: Revtex, 6 pages, 2 Postscript files included, to appear in Phys. Rev.
Let
Stochastic dynamics of adhesion clusters under shared constant force and with rebinding
Single receptor-ligand bonds have finite lifetimes, so that biological
systems can dynamically react to changes in their environment. In cell
adhesion, adhesion bonds usually act cooperatively in adhesion clusters.
Outside the cellular context, adhesion clusters can be probed quantitatively by
attaching receptors and ligands to opposing surfaces. Here we present a
detailed theoretical analysis of the stochastic dynamics of a cluster of
parallel bonds under shared constant loading and with rebinding. Analytical
solutions for the appropriate one-step master equation are presented for
special cases, while the general case is treated with exact stochastic
simulations. If the completely dissociated state is modeled as an absorbing
boundary, mean cluster lifetime is finite and can be calculated exactly. We
also present a detailed analysis of fluctuation effects and discuss various
approximations to the full stochastic description.Comment: Revtex, 29 pages, 23 postscript figures included (some with reduced
image quality
Bending Frustration of Lipid-Water Mesophases Based on Cubic Minimal Surfaces
Inverse bicontinuous cubic phases are ubiquitous in lipid-water mixtures and
consist of a lipid bilayer forming a cubic minimal surface, thereby dividing
space into two cubic networks of water channels. For small hydrocarbon chain
lengths, the monolayers can be modeled as parallel surfaces to a minimal
midsurface. The bending energy of the cubic phases is determined by the
distribution of Gaussian curvature over the minimal midsurfaces which we
calculate for seven different structures (G, D, P, I-WP, C(P), S and F-RD). We
show that the free-energy densities of the structures G, D and P are
considerably lower than those of the other investigated structures due to their
narrow distribution of Gaussian curvature. The Bonnet transformation between G,
D, and P implies that these phases coexist along a triple line, which also
includes an excess water phase. Our model includes thermal membrane
undulations. Our qualitative predictions remain unchanged when higher order
terms in the curvature energy are included. Calculated phase diagrams agree
well with the experimental results for 2:1 lauric acid/dilauroyl
phosphatidylcholine and water.Comment: Revtex, 23 pages with 9 postscript figures included, to appear in
Langmui
Deformation and tribology of multi-walled hollow nanoparticles
Multi-walled hollow nanoparticles made from tungsten disulphide (WS) show
exceptional tribological performance as additives to liquid lubricants due to
effective transfer of low shear strength material onto the sliding surfaces.
Using a scaling approach based on continuum elasticity theory for shells and
pairwise summation of van der Waals interactions, we show that van der Waals
interactions cause strong adhesion to the substrate which favors release of
delaminated layers onto the surfaces. For large and thin nanoparticles, van der
Waals adhesion can cause considerable deformation and subsequent delamination.
For the thick WS nanoparticles, deformation due to van der Waals
interactions remains small and the main mechanism for delamination is pressure
which in fact leads to collapse beyond a critical value. We also discuss the
effect of shear flow on deformation and rolling on the substrate.Comment: Latex, 13 pages with 3 Postscript figures included, to appear in
Europhysics Letter
Collective effects in cellular structure formation mediated by compliant environments: a Monte Carlo study
Compliant environments can mediate interactions between mechanically active
cells like fibroblasts. Starting with a phenomenological model for the
behaviour of single cells, we use extensive Monte Carlo simulations to predict
non-trivial structure formation for cell communities on soft elastic substrates
as a function of elastic moduli, cell density, noise and cell position
geometry. In general, we find a disordered structure as well as ordered
string-like and ring-like structures. The transition between ordered and
disordered structures is controlled both by cell density and noise level, while
the transition between string- and ring-like ordered structures is controlled
by the Poisson ratio. Similar effects are observed in three dimensions. Our
results suggest that in regard to elastic effects, healthy connective tissue
usually is in a macroscopically disordered state, but can be switched to a
macroscopically ordered state by appropriate parameter variations, in a way
that is reminiscent of wound contraction or diseased states like contracture.Comment: 45 pages, 7 postscript figures included, revised version accepted for
publication in Acta Biomateriali
Effect of Poisson ratio on cellular structure formation
Mechanically active cells in soft media act as force dipoles. The resulting
elastic interactions are long-ranged and favor the formation of strings. We
show analytically that due to screening, the effective interaction between
strings decays exponentially, with a decay length determined only by geometry.
Both for disordered and ordered arrangements of cells, we predict novel phase
transitions from paraelastic to ferroelastic and anti-ferroelastic phases as a
function of Poisson ratio.Comment: 4 pages, Revtex, 4 Postscript figures include
Mean encounter times for cell adhesion in hydrodynamic flow: analytical progress by dimensional reduction
For a cell moving in hydrodynamic flow above a wall, translational and
rotational degrees of freedom are coupled by the Stokes equation. In addition,
there is a close coupling of convection and diffusion due to the
position-dependent mobility. These couplings render calculation of the mean
encounter time between cell surface receptors and ligands on the substrate very
difficult. Here we show for a two-dimensional model system how analytical
progress can be achieved by treating motion in the vertical direction by an
effective reaction term in the mean first passage time equation for the
rotational degree of freedom. The strength of this reaction term can either be
estimated from equilibrium considerations or used as a fit parameter. Our
analytical results are confirmed by computer simulations and allow to assess
the relative roles of convection and diffusion for different scaling regimes of
interest.Comment: Reftex, postscript figures include
Solitonic spin-liquid state due to the violation of the Lifshitz condition in FeTe
A combination of phenomenological analysis and M\"ossbauer spectroscopy
experiments on the tetragonal FeTe system indicates that the magnetic
ordering transition in compounds with higher Fe-excess, 0.11, is
unconventional. Experimentally, a liquid-like magnetic precursor with
quasi-static spin-order is found from significantly broadened M\"ossbauer
spectra at temperatures above the antiferromagnetic transition. The
incommensurate spin-density wave (SDW) order in FeTe is described by a
magnetic free energy that violates the weak Lifshitz condition in the Landau
theory of second-order transitions. The presence of multiple Lifshitz
invariants provides the mechanism to create multidimensional, twisted, and
modulated solitonic phases.Comment: 5 pages, 2 figure
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