1,774 research outputs found
Role of cross-links in bundle formation, phase separation and gelation of long filaments
We predict the thermodynamic and structural behavior of solutions of long
cross-linked filaments. We find that at the mean field level, the entropy of
self-assembled junctions induces an effective attraction between the filaments
that can result in a phase separation into a connected network, in equilibrium
with a dilute phase. A connected network can also be formed in a
non-thermodynamic transition upon increase of the chain, or cross link density,
or with decreasing temperature. For rigid rods, at low temperatures, we predict
a transition from an isotropic network, to anisotropic bundles of rods tightly
bound by cross links, that is triggered by the interplay between the
configurational entropy of the cross-link distribution among the rods, and the
rotational and translational entropy of the rods.Comment: typos and graphics corrected; 6 pages 1 figur
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
Molecular dynamics study of nanoparticle stability at liquid interfaces : effect of nanoparticle-solvent interaction and capillary waves
While the interaction of colloidal particles (sizes in excess of 100 nm) with liquid interfaces may be understood in terms of continuum models, which are grounded in macroscopic properties such as surface and line tensions, the behaviour of nanoparticles at liquid interfaces may be more complex. Recent simulations [D. L. Cheung and S. A. F. Bon, Phys. Rev. Lett. 102, 066103 (2009)] of nanoparticles at an idealised liquid-liquid interface showed that the nanoparticle-interface interaction range was larger than expected due, in part, to the action of thermal capillary waves. In this paper, molecular dynamics simulations of a Lennard-Jones nanoparticle in a binary Lennard-Jones mixture are used to confirm that these previous results hold for more realistic models. Furthermore by including attractive interactions between the nanoparticle and the solvent, it is found that the detachment energy decreases as the nanoparticle-solvent attraction increases. Comparison between the simulation results and recent theoretical predictions [H. Lehle and M. Oettel, J. Phys. Condens. Matter 20, 404224 (2008)] shows that for small particles the incorporation of capillary waves into the predicted effective nanoparticle-interface interaction improves agreement between simulation and theory
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
Long-Range Interaction between Heterogeneously Charged Membranes
Despite their neutrality, surfaces or membranes with equal amounts of positive and negative charge can exhibit long-range electrostatic interactions if the surface charge is heterogeneous; this can happen when the surface charges form finite-size domain structures. These domains can be formed in lipid membranes where the balance of the different ranges of strong but short-ranged hydrophobic interactions and longer-ranged electrostatic repulsion result in a finite, stable domain size. If the domain size is large enough, oppositely charged domains in two opposing surfaces or membranes can be strongly correlated by the elecrostatic interactions; these correlations give rise to an attractive interaction of the two membranes or surfaces over separations on the order of the domain size. We use numerical simulations to demonstrate the existence of strong attractions at separations of tens of nanometers. Large line tensions result in larger domains but also increase the charge density within the domain. This promotes correlations and, as a result, increases the intermembrane attraction. On the other hand, increasing the salt concentration increases both the domain size and degree of domain anticorrelation, but the interactions are ultimately reduced due to increased screening. The result is a decrease in the net attraction as salt concentration is increased
Inter-filament Attractions Narrow the Length Distribution of Actin Filaments
We show that the exponential length distribution that is typical of actin
filaments under physiological conditions dramatically narrows in the presence
of (i) crosslinker proteins (ii) polyvalent counterions or (iii) depletion
mediated attractions. A simple theoretical model shows that in equilibrium,
short-range attractions enhance the tendency of filaments to align parallel to
each other, eventually leading to an increase in the average filament length
and a decrease in the relative width of the distribution of filament lengths.Comment: 5 pages, 4 figure
Non-monotonic fluctuation spectra of membranes pinned or tethered discretely to a substrate
The thermal fluctuation spectrum of a fluid membrane coupled harmonically to
a solid support by an array of tethers is calculated. For strong tethers, this
spectrum exhibits non-monotonic, anisotropic behavior with a relative maximum
at a wavelength about twice the tether distance. The root mean square
displacement is evaluated to estimate typical membrane displacements. Possible
applications cover pillar-supported or polymer-tethered membranes.Comment: 4 pages, 5 figure
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