arXiv subjects

Rudolf Podgornik

Publications and source records attributed to Rudolf Podgornik.

139 records · Page 8Linked to original sources

arXiv2009

Dispersion interactions in stratified anisotropic and optically active media at all separations

We propose a method to calculate dispersion interactions in a system composed of a one dimensional layering of finite thickness anisotropic and optically active slabs. The result is expressed within the algebra of 4x4 matrices and is demonstrated to be equivalent to the known limits of isotropic, nonretarded and uniaxial dispersion interactions. The method is also capable of handling dielectric media with smoothly varying anisotropy axes.

cond-mat.mes-hall↗

arXiv2009

One-dimensional counterion gas between charged surfaces: Exact results compared with weak- and strong-coupling analysis

We evaluate exactly the statistical integral for an inhomogeneous one-dimensional counterion-only Coulomb gas between two charged boundaries and from this compute the effective interaction, or disjoining pressure, between the bounding surfaces. Our exact results are compared with the limiting cases of weak and strong coupling which are the same for 1D and 3D systems. For systems with a large number of counterions it is found that the weak coupling (mean-field) approximation for the disjoining pressure works perfectly and that fluctuations around the mean-field in 1D are much smaller than in 3D. In the case of few counterions it works less well and strong coupling approximation performs much better as it takes into account properly the discreteness of the counterion charges.

cond-mat.stat-mech↗

arXiv2008

Scaling relations in the collapse of elastic icosadeltahedral shells under uniform external pressure

We discuss collapse of icosadeltahedral shells subjected to uniform external pressure. We demonstrate that there is a universal collapse pressure curve. The parameter that uniquely determines the collapse pressure is shown to be the Foppl-von Karman number of the non-pressurized shells. Numerical results are interpreted in terms of buckling instabilities of spheres and cylinders under hydrostatic pressure.

physics.bio-ph↗

arXiv2008

Influence of salt and viral protein charge distribution on encapsidation of single-stranded viral RNA molecules

We examine the limits on viral composition that are set by the electrostatic interactions effected by the charge on the viral proteins, the single-stranded viral RNA molecule and monovalent salt ions in the solution. Within the mean-field model of viral energetics we demonstrate the prime importance of the salt concentration for the assembly of a virus. We find that the encapsidation of the viral RNA molecule is thermodynamically suppressed in solutions with high concentrations of monovalent salt. This effect is significantly less important in viruses with proteins whose charge distribution protrudes into the interior of the capsid, leading to an increase in the stability of such viruses in solutions with high salt concentrations. The delocalization of positive charge on the capsid protein arms thus profoundly increases reliability of viral assembly in high-salt solutions.

physics.bio-ph↗

arXiv2007

Ionic Cloud Distribution close to a Charged Surface in the Presence of Salt

Despite its importance, the understanding of ionic cloud distribution close to a charged macroion under physiological salt conditions has remained very limited especially for strongly coupled systems with, for instance, multivalent counterions. Here we present a formalism that predicts both counterion and coion distributions in the vicinity of a charged macroion for an arbitrary amount of added salt and in both limits of mean field and strong coupling. The distribution functions are calculated explicitly for ions next to an infinite planar charged wall. We present a schematic phase diagram identifying different physical regimes in terms of electrostatic coupling parameter and bulk salt concentration.

physics.bio-ph↗

arXiv2007

Confined nanorods: jamming due to helical buckling

We investigate a longitudinally loaded elastic nanorod inside a cylindrical channel and show within the context of classical elasticity theory that the Euler buckling instability leads to a helical postbuckling form of the rod within the channel. The local pitch of the confined helix changes along the channel and so does the longitudinal force transmitted along the rod, diminishing away from the loaded end. This creates a possibility of jamming of the nanorod within the channel.

cond-mat.soft↗

arXiv2007

Role of electrostatic interactions in the assembly of empty spherical viral capsids

We examine the role of electrostatic interactions in the assembly of empty spherical viral capsids. The charges on the protein subunits that make the viral capsid mutually interact and are expected to yield electrostatic repulsion acting against the assembly of capsids. Thus, attractive protein-protein interactions of non-electrostatic origin must act to enable the capsid formation. We investigate whether the interplay of repulsive electrostatic and attractive interactions between the protein subunits can result in the formation of spherical viral capsids of a preferred radius. For this to be the case, we find that the attractive interactions must depend on the angle between the neighboring protein subunits (i.e. on the mean curvature of the viral capsid) so that a particular angle(s) is (are) preferred energywise. Our results for the electrostatic contributions to energetics of viral capsids nicely correlate with recent experimental determinations of the energetics of protein-protein contacts in Hepatitis B virus [P. Ceres and A. Zlotnick, Biochemistry {\bf 41}, 11525 (2002).

physics.bio-ph↗

arXiv2007

The boundary element approach to Van der Waals interactions

We develop a boundary element method to calculate Van der Waals interactions for systems composed of domains of spatially constant dielectric response. We achieve this by rewriting the interaction energy expression exclusively in terms of surface integrals of surface operators. We validate this approach in the Lifshitz case and give numerical results for the interaction of two spheres as well as the van der Waals self-interaction of a uniaxial ellipsoid. Our method is simple to implement and is particularly suitable for a full, non-perturbative numerical evaluation of non-retarded van der Waals interactions between objects of a completely general shape.

cond-mat.soft↗

arXiv2006

Comparison of the DFT and field approaches to Van der Waals interactions in planparallel geometry

We establish a general equivalence between van der Waals interaction energies within the formalism of the non-local van der Waals functional of the density functional theory and within the formalism of the field approach based on the secular determinants of the electromagnetic field modes. We then compare the two methods explicitly in the case of a planparallel geometry with a continuously varying dielectric response function and show that their respective numerical implementations are not equivalent. This allows us to discuss the merits of the two approaches and possible advantages of either method in a simple model calculation.

cond-mat.mtrl-sci↗

arXiv2006

On the connected-charges Thomson problem

We investigate the modifications brought about by the linear connectivity among charges in the classical Thomson problem. Instead of packing with local hexagonal order intersperced with topological defects, we find charge distributions with helical symmetry wound around the surface of the sphere. This finding should have repercussions in the viral packing and macroion adsorption theories.

cond-mat.soft↗

arXiv2006

Electrostatic Disorder-Induced Interactions in Inhomogeneous Dielectrics

We investigate the effect of quenched surface charge disorder on electrostatic interactions between two charged surfaces in the presence of dielectric inhomogeneities and added salt. We show that in the linear weak-coupling regime (i.e., by including mean-field and Gaussian-fluctuations contributions), the image-charge effects lead to a non-zero disorder-induced interaction free energy between two surfaces of equal mean charge that can be repulsive or attractive depending on the dielectric mismatch across the bounding surfaces and the exact location of the disordered charge distribution.

cond-mat.soft↗

arXiv2005

Quenched Charge Disorder and Coulomb Interactions

We develop a general formalism to investigate the effect of quenched fixed charge disorder on effective electrostatic interactions between charged surfaces in a one-component (counterion-only) Coulomb fluid. Analytical results are explicitly derived for two asymptotic and complementary cases: i) mean-field or Poisson-Boltzmann limit (including Gaussian-fluctuations correction), which is valid for small electrostatic coupling, and ii) strong-coupling limit, where electrostatic correlations mediated by counterions become significantly large as, for instance, realized in systems with high-valency counterions. In the particular case of two apposed and ideally polarizable planar surfaces with equal mean surface charge, we find that the effect of the disorder is nil on the mean-field level and thus the plates repel. In the strong-coupling limit, however, the effect of charge disorder turns out to be additive in the free energy and leads to an enhanced long-range attraction between the two surfaces. We show that the equilibrium inter-plate distance between the surfaces decreases for elevated disorder strength (i.e. for increasing mean-square deviation around the mean surface charge), and eventually tends to zero, suggesting a disorder-driven collapse transition.

cond-mat.soft↗

arXiv1996

Isotropic - Nematic Transition of Surface Embedded Polymers and the Associated Tubulization Transition of the Embedding Surface

A self-interacting polymer can undergo an orientational ordering transition, depending on the magnitude of the nematic interaction. The effect of embedding such a polymer into a flexible surface on this transition is studied on the mean-field level. Renormalized values of the elastic constants of the ``dressed'' surface are derived as functions of the orientational order parameter of the polymer chain. In the disordered state the surface tension and curvature modulus remain scalars but depend on the surface coverage of the embedded polymer. In the nematic state there is a symmetry breaking transition leading to anisotropic elastic constants. At a sufficiently large nematic order parameter the effective surface tension in the direction perpendicular to the nematic axis can become negative, leading to tubulization of the embedding surface.

cond-mat↗