383 research outputs found
Bundle formation in parallel aligned polymers with competing interactions
Aggregation of like-charged polymers is widely observed in biological and
soft matter systems. In many systems, bundles are formed when a short-range
attraction of diverse physical origin like charge-bridging, hydrogen-bonding or
hydrophobic interaction, overcomes the longer- range charge repulsion. In this
Letter, we present a general mechanism of bundle formation in these systems as
the breaking of the translational invariance in parallel aligned polymers with
competing interactions of this type. We derive a criterion for finite-sized
bundle formation as well as for macroscopic phase separation (formation of
infinite bundles).Comment: accepted for publication in Europhys Let
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
Temperature Dependent Mean Free Path Spectra of Thermal Phonons Along the c-axis of Graphite
Heat conduction in graphite has been studied for decades because of its
exceptionally large thermal anisotropy. While the bulk thermal conductivities
along the in-plane and cross-plane directions are well known, less understood
are the microscopic properties of the thermal phonons responsible for heat
conduction. In particular, recent experimental and computational works indicate
that the average phonon mean free path (MFP) along the c-axis is considerably
larger than that estimated by kinetic theory, but the distribution of MFPs
remains unknown. Here, we report the first quantitative measurements of c-axis
phonon MFP spectra in graphite at a variety of temperatures using time-domain
thermoreflectance measurements of graphite flakes with variable thickness. Our
results indicate that c-axis phonon MFPs have values of a few hundred
nanometers at room temperature and a much narrower distribution than in
isotropic crystals. At low temperatures, phonon scattering is dominated by
grain boundaries separating crystalline regions of different rotational
orientation. Our study provides important new insights into heat transport and
phonon scattering mechanisms in graphite and other anisotropic van der Waals
solids
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