Macroscopic electrostatic potentials and interactions in self-assembled molecular bilayers: the case of Newton black films

We propose a very simple but 'realistic' model of amphiphilic bilayers,simple enough to be able to include a large number of molecules in the sample, but nevertheless detailed enough to include molecular charge distributions, flexible amphiphilic molecules and a reliable model of water. All these parameters are essential in a nanoscopic scale study of intermolecular and long range electrostatic interactions. We also propose a novel, simple and more accurate macroscopic electrostatic field for model bilayers. This model goes beyond the total dipole moment of the sample, which on a time average is zero for this type of symmetrical samples, i. e., it includes higher order moments of this macroscopic electric field. We show that by representing it with a superposition of gaussians it can be 'analytically' integrated, and therefore its calculation is easily implemented in a MD simulation (even in simulations of non-symmetrical bi- or multi-layers). In this paper we test our model by molecular dynamics simulations of Newton black films

Linear response functions of an electrolyte solution in a uniform flow

We study the steady state response of a dilute monovalent electrolyte solution to an external source with a constant relative velocity with respect to the fluid. The source is taken as a combination of three perturbations: an external force acting on the fluid, an externally imposed ionic chemical potential, and an external charge density. The linear response functions are obtained analytically and can be decoupled into three independent terms, corresponding to (i) fluid flow and pressure, (ii) total ionic number density and current, and (iii) charge density, electrostatic potential and electric current. It is shown how the uniform flow breaks the equilibrium radial symmetry of the response functions, leading to a distortion of the ionic cloud and electrostatic potential, which deviate from the standard Debye-H\"uckel result. The potential of a moving charge is under-screened in its direction of motion and over-screened in the opposite direction and normal plane. As a result, an unscreened dipolar electric field and electric currents are induced far from the charged source. We relate our general formalism to several experimental setups, such as colloidal sedimentation.Comment: 12 pages, 5 figure

Correlated disordered interactions on Potts models

Using a weak-disorder scheme and real-space renormalization-group techniques, we obtain analytical results for the critical behavior of various q-state Potts models with correlated disordered exchange interactions along d1 of d spatial dimensions on hierarchical (Migdal-Kadanoff) lattices. Our results indicate qualitative differences between the cases d-d1=1 (for which we find nonphysical random fixed points, suggesting the existence of nonperturbative fixed distributions) and d-d1>1 (for which we do find acceptable perturbartive random fixed points), in agreement with previous numerical calculations by Andelman and Aharony. We also rederive a criterion for relevance of correlated disorder, which generalizes the usual Harris criterion.Comment: 8 pages, 4 figures, to be published in Physical Review

Adsorption of polymers on a fluctuating surface

We study the adsorption of polymer chains on a fluctuating surface. Physical examples are provided by polymer adsorption at the rough interface between two non-miscible liquids, or on a membrane. In a mean-field approach, we find that the self--avoiding chains undergo an adsorption transition, accompanied by a stiffening of the fluctuating surface. In particular, adsorption of polymers on a membrane induces a surface tension and leads to a strong suppression of roughness.Comment: REVTEX, 9 pages, no figure