arXiv subjects

Rudolf Podgornik

Publications and source records attributed to Rudolf Podgornik.

At least 55 records · Page 3Linked to original sources

arXiv2020

Critical behavior of charge-regulated macro-ions

Based on a collective description of electrolytes composed of charge-regulated macro-ions and simple salt ions, we analyze their equilibrium charge state in the bulk and their behavior in the vicinity of an external electrified surface. The mean-field formulation of mobile macro-ions in an electrolyte bathing solution is extended to include interactions between association/dissociation sites. We demonstrate that above a critical concentration of salt, and similar to the critical micelle concentration, a non-trivial distribution of charge states sets in. Such a charge state can eventually lead to a liquid-liquid phase separation based on charge regulation.

cond-mat.soft↗

arXiv2020

Interactions between Zwitterionic Membranes in Complex Electrolytes

We investigate the electrostatic interactions of zwitterionic membranes immersed in mixed electrolytes composed of mono- and multivalent ions. We show that the presence of monovalent salt is a necessary condition for the existence of a finite electrostatic force on the membrane. As a result, the mean-field membrane pressure originating from the surface dipoles exhibits a non-uniform salt dependence, characterized by an enhancement for dilute salt conditions and a decrease at intermediate salt concentrations. Upon addition of multivalent cations to the submolar salt solution, the separate interactions of these cations with the opposite charges of the surface dipoles makes the intermembrane pressure more repulsive at low membrane separation distances and strongly attractive at intermediate distances, resulting in a discontinuous like-charge binding transition followed by the membrane binding transition. By extending our formalism to account for correlation corrections associated with large salt concentrations, we show that membranes of high surface dipole density immersed in molar salt solutions may undergo a membrane binding transition even without the multivalent cations. Hence, the tuning of the surface polarization forces by membrane engineering can be an efficient way to adjust the equilibrium configuration of dipolar membranes in concentrated salt solutions.

cond-mat.soft↗

arXiv2020

Thermal Casimir interactions for higher derivative field Lagrangians: generalized Brazovskii models

We examine the Casimir effect for free statistical field theories which have Hamiltonians with second order derivative terms. Examples of such Hamiltonians arise from models of non-local electrostatics, membranes with non-zero bending rigidities and field theories of the Brazovskii type that arise for polymer systems. The presence of a second derivative term means that new types of boundary conditions can be imposed, leading to a richer phenomenology of interaction phenomena. In addition zero modes can be generated that are not present in standard first derivative models, and it is these zero modes which give rise to long range Casimir forces. Two physically distinct cases are considered: (i) unconfined fields, usually considered for finite size embedded inclusions in an infinite fluctuating medium, here in a two plate geometry the fluctuating field exists both inside and outside the plates, (ii) confined fields, where the field is absent outside the slab confined between the two plates. We show how these two physically distinct cases are mathematically related and discuss a wide range of commonly applied boundary conditions. We concentrate our analysis to the critical region where the underlying bulk Hamiltonian has zero modes and show that very exotic Casimir forces can arise, characterised by very long range effects and oscillatory behavior that can lead to strong metastability in the system.

cond-mat.stat-mech↗

arXiv2019

Charge regulation radically modifies electrostatics in membrane stacks

Motivated by biological membrane-containing organelles in plants and photosynthetic bacteria, we study charge regulation in a model membrane stack. Considering (de)protonation as the simplest mechanism of charge equilibration between the membranes and with the bathing environment, we uncover a symmetry-broken charge state in the stack with a quasiperiodic effective charge sequence. In the case of a monovalent bathing salt solution our model predicts complex, inhomogeneous charge equilibria depending on the strength of the (de)protonation reaction, salt concentration, intermembrane separation, and their number in the stack. Our results shed light on the basic reorganization mechanism of thylakoid membrane stacks.

cond-mat.soft↗

arXiv2019

Surface alignment disorder and pseudo-Casimir forces in smectic-A liquid crystalline films

Random (disordered) components in the surface anchoring of the smectic-A liquid crystalline film in general modify the thermal pseudo-Casimir interaction. Anchoring disorder of the quenched type is in general decoupled from the thermal pseudo-Casimir force and gives rise to an additional disorder-generated interaction, in distinction to the annealed disorder, whose effect on the pseudo-Casimir force is non-additive. We consider the effects of the surface anchoring disorder by assuming that one of the substrates of the film is contaminated by a disorder source, resulting in a Gaussian-weighted distribution of the preferred molecular anchoring orientation (easy axes) on that substrate, having a finite mean and variance or, more generally, a homogeneous in-plane, two-point correlation function. We show that the presence of disorder, either of the quenched or annealed type, leads to a significant reduction in the magnitude of the net thermal fluctuation force between the confining substrates of the film. In the quenched case this is a direct consequence of an additive free energy dependent on the variance of the disorder, while in the annealed case, the suppression of the interaction force can be understood based on a disorder-renormalized, effective anchoring strength.

cond-mat.soft↗

arXiv2019

Orientational transition and complexation of DNA with anionic membranes: weak and intermediate electrostatic coupling

We characterize the role of charge correlations in the adsorption of a short, rod-like anionic polyelectrolyte onto a similarly charged membrane. Our theory reveals two different mechanisms driving the like-charge polyelectrolyte-membrane complexation: in weakly charged membranes, repulsive polyelectrolyte-membrane interactions lead to the interfacial depletion and a parallel orientation of the polyelectrolyte with respect to the membrane; while in the intermediate membrane charge regime, the interfacial counterion excess gives rise to an attractive "salt-induced" image force. This furthermore results in an orientational transition from a parallel to a perpendicular configuration and a subsequent short-ranged like-charge adsorption of the polyelectrolyte to the substrate. A further increase of the membrane charge engenders a charge inversion, originating from surface-induced ionic correlations, that act as a separate mechanism capable of triggering the like-charge polyelectrolyte-membrane complexation over an extended distance interval from the membrane surface. The emerging picture of this complexation phenomenon identifies the interfacial "salt-induced" image forces as a powerful control mechanism in polyelectrolyte-membrane complexation.

cond-mat.soft↗

arXiv2019

Like-charge polymer-membrane complexation mediated by multivalent cations: one-loop-dressed strong coupling theory

We probe the electrostatic mechanism driving adsorption of polyelectrolytes onto like-charged membranes upon the addition of tri- and tetravalent counterions to a bathing monovalent salt solution. We develop a one-loop-dressed strong coupling theory that treats the monovalent salt at the electrostatic one-loop level and the multivalent counterions within a strong-coupling approach. It is shown that the adhesive force of the multivalent counterions mediating the like-charge adsorption arises from their strong condensation at the charged membrane. The resulting interfacial counterion excess locally maximizes the screening ability of the electrolyte and minimizes the electrostatic polymer grand potential. This translates into an attractive force that pulls the polymer to the similarly charged membrane. We show that the high counterion valency enables this adsorption transition even at weakly charged membranes. Additionally, strongly charged membranes give rise to salt-induced correlations and intensify the interfacial multivalent counterion condensation, strenghtening the complexation of the polymer with the like-charged membrane, as well as triggering the orientational transition of the molecule prior to its adsorption. Finally, our theory provides two additional key features as evidenced by previous adsorption experiments: first, the critical counterion concentration for polymer adsorption decreases with the rise of the counterion valency, and second, the addition of monovalent salt enhances the screening of the membrane charges and suppresses salt correlations. This weakens the interfacial multivalent counterion condensation and results in the desorption of the polymer from the substrate.

cond-mat.soft↗

arXiv2018

Density--nematic coupling in isotropic linear polymers

Linear polymers and other connected "line liquids" exhibit a coupling between density and equilibrium nematic order on the macroscopic level that gives rise to a Meyer-de Gennes vectorial conservation law. Nevertheless, isotropic linear polymer melts/solutions exhibit fluctuations of the density and of the nematic order that are not coupled by this vectorial constraint, just like for isotropic liquids composed of disconnected non-spherical particles. It takes the proper tensorial description of the nematic order in linear polymer liquids, leading to a tensorial conservation law connecting density and orientational order, that finally implicates coupled density and nematic order fluctuations, already in the isotropic system and not subject to the existence of an orientational phase transition. This coupling implies that a spatial variation of density or a local concentration gradient will induce nematic order and thereby an acoustic or osmotic optical birefringence even in an otherwise isotropic polymer melt/solution. We validate the theoretical conceptions by performing detailed Monte Carlo simulations of isotropic melts of "soft" worm-like chains with variable length and flexibility, and comparing the numerically determined orientation correlation functions with predictions of the macroscopic theory. The methodology drawn sets forth a means of determining the macroscopic parameters by microscopic simulations to yield realistic continuum models of specific polymeric materials.

cond-mat.soft↗

arXiv2018

Pseudo-Casimir interactions and surface anchoring duality in bookshelf geometry of smectic-A liquid crystals

We analyze the transverse intersubstrate pseudo-Casimir force, arising as a result of thermal fluctuations of the liquid crystalline layers of a smectic-A film confined between two planar substrates in a bookshelf geometry, in which the equidistant smectic layers are placed perpendicular to the bounding surfaces. We discuss the variation of the interaction force as a function of the intersubstrate separation in the presence of surface anchoring to the substrates, showing that the force induced by confined fluctuations is attractive and depends on the penetration length as well as the layer spacing. The strongest effect occurs for tightly confined fluctuations, in which the surface anchoring energy is set to infinity, where the force per area scales linearly with the thermal energy and inversely with the fourth power of the intersubstrate separation. By reducing the strength of the surface anchoring energy, the force first becomes weaker in magnitude but then increases in magnitude as the surface anchoring strength is further reduced down to zero, in which case the force tends to that obtained in the limit of strong anchoring.

cond-mat.soft↗

arXiv2018

Splay--density coupling in semiflexible main-chain nematic polymers with hairpins

We establish a macroscopic description of the splay--density coupling in semiflexible main-chain nematic polymers with hairpins, using a vectorial continuity constraint for the "recovered" polar order of the chain tangents and introducing chain backfolds (hairpins) as its new type of sources besides chain ends. We treat both types of sources on a unified basis as a mixture of two ideal gases with fixed composition. Performing detailed Monte Carlo simulations of nematic monodomain melts of "soft" worm-like chains with variable length and flexibility, we show via their structure factors that the chain backfolding weakens the splay--density coupling, and demonstrate how this weakening can be consistently quantified on the macroscopic level. We also probe and discuss the deviations from the noninteracting gas idealization of the chain ends and backfolds.

cond-mat.soft↗

arXiv2018

On the voltage-controlled assembly of NP arrays at electrochemical solid/liquid interfaces

Research in the field of nanoplasmonic metamaterials is moving towards more and more interesting and, potentially useful, applications. The present work tackles the problem of nanoparticle self-assembly at an electrochemical solid-liquid interface from a purely theoretical perspective. We perform a simplified, comprehensive analysis of the stability of a nanoparticle arrays under different conditions and assembly. From the Poisson-Bjerrum model of electrostatic interactions between a metallic nanoparticle and the electrode and between the nanoparticles at the electrode, as well the Hamaker-Lifshitz model of the corresponding van der Waals interactions, we reach some conclusions regarding the possibility to build arrays of charged nanoparticles on electrodes and disassemble them, subject to variation of applied voltage. Since system of this type have been shown, recently, to provide nontrivial electrotuneable optical response, such analysis is crucial for answering the question whether such scenarios of electrochemical plasmonics are feasible.

physics.chem-ph↗

arXiv2018

Rodlike counterions at heterogeneously charged surfaces

We study the spatial and orientational distribution of rodlike counterions (such as mobile nanorods) as well as the effective interaction mediated by them between two plane-parallel surfaces that carry fixed (quenched) heterogeneous charge distributions. The rodlike counterions are assumed to have an internal charge distribution, specified by a multivalent monopolar moment and a finite quadrupolar moment, and the quenched surface charge is assumed to be randomly distributed with equal mean and variance on the two surfaces. While equally charged surfaces are known to repel within the traditional mean-field theories, the presence of multivalent counterions has been shown to cause attractive interactions between uniformly charged surfaces due to the prevalence of strong electrostatic couplings that grow rapidly with the counterion valency. We show that the combined effects due to electrostatic correlations (caused by the coupling between the mean surface field and the multivalent, monopolar, charge valency of counterions) as well as the disorder-induced interactions (caused by the coupling between the surface disorder field and the quadrupolar moment of counterions) lead to much stronger attractive interactions between two randomly charged surfaces. The interaction profile turns out to be a nonmonotonic function of the intersurface separation, displaying an attractive minimum at relatively small separations, where the ensuing attraction can exceed the maximum strong-coupling attraction (produced by multivalent monopolar counterions between uniformly charged surfaces) by more than an order of magnitude.

cond-mat.soft↗

arXiv2018

Charge Regulation with Fixed and Mobile Charges

Uncompensated charges do not occur in Nature and any local charge should be a result of charge separation. Dissociable chemical groups at interfaces in contact with ions in solution, whose chemical equilibrium depends both on short-range non-electrostatic and long-range electrostatic interactions, are the physical basis of this charge separation, known as charge regulation phenomena. The charged groups can be either fixed and immobile, as in the case of solvent-exposed solid substrate and soft bounding surfaces, (e.g., molecularly smooth mica surfaces and soft phospholipid membranes), or free and mobile, as in the case of charged macro-ions, (e.g., protein or other biomolecules). Here, we review the mean-field formalism used to describe both cases, with a focus on recent advances in the modeling of mobile charge-regulated macro-ions in an ionic solution. The general form of the screening length in such a solution is derived, and is shown to combine the concept of intrinsic capacitance, introduced by Lund and Jönsson, and bulk capacitance, resulting from the mobility of small ions and macro-ions. The advantages and disadvantages of different formulations, such as the cell model vs. the collective approach, are discussed, along with several suggestions for future experiments and modeling.

cond-mat.soft↗

arXiv2018

Role of metallic core for the stability of virus-like particles in strongly coupled electrostatics

We investigate the osmotic (electrostatic) pressure acting on the proteinaceous shell of a generic model of virus-like particles (VLPs), comprising a charged outer shell and a metallic nanoparticle core, coated by a charged layer and bathed in an aqueous electrolyte (salt) solution. Motivated by the recent studies accentuating the role of multivalent ions for the stability of VLPs, we focus on the effects of multivalent cations and anions in an otherwise monovalent ionic bathing solution. We perform extensive Monte-Carlo simulations based on appropriate Coulombic interactions that consistently take into account the effects of salt screening, the dielectric polarization of the metallic core, and the strong-coupling electrostatics due to the presence of multivalent ions. We specifically study the intricate roles these factors play in the electrostatic stability of the model VLPs. It is shown that while the insertion of a metallic nanoparticle by itself can produce negative, inward-directed, pressure on the outer shell, addition of only a small amount of multivalent counterions can robustly engender negative pressures, enhancing the VLP stability across a wide range of values for the system parameters.

cond-mat.soft↗

arXiv2018

Hydrodynamic stress correlations in fluid films driven by stochastic surface forcing

We study hydrodynamic fluctuations in a compressible and viscous fluid film confined between two rigid, no-slip, parallel plates, where one of the plates is kept fixed, while the other one is driven in small-amplitude, translational, displacements around its reference position. This jiggling motion is assumed to be driven by a stochastic, external, surface forcing of zero mean and finite variance. Thus, while the transverse (shear) and longitudinal (compressional) hydrodynamic stresses produced in the film vanish on average on either of the plates, these stresses exhibit fluctuations that can be quantified through their equal-time, two-point, correlation functions. For transverse stresses, we show that the correlation functions of the stresses acting on the same plate (self-correlators) as well as the correlation function of the stresses acting on different plates (cross-correlators) exhibit universal, decaying, power-law behaviors as functions of the inter-plate separation. At small separations, the exponents are given by -1, while at large separations, the exponents are found as -2 (self-correlator on the fixed plate), -4 (excess self-correlator on the mobile plate) and -3 (cross-correlator). For longitudinal stresses, we find much weaker power-law decays in the large separation regime, with exponents -3/2 (excess self-correlator on the mobile plate) and -1 (cross-correlator). The self-correlator on the fixed plate increases and levels off upon increasing the inter-plate separation, reflecting the non-decaying nature of the longitudinal forces acting on the fixed plate.

cond-mat.soft↗

arXiv2018

General theory of charge regulation and surface capacitance

A generalization of the mean-field approach will be derived that will take into account the ion-ion as well as ion-surface non-electrostatic effects on an equal footing, being based on the bulk and surface equations of state in the absence of electrostatic interactions. This approach will be applied to the calculation of the differential capacitance for several models of the specific ion-surface non-electrostatic interactions providing a thermodynamic insight into the characteristics of the differential capacitance. The packing considerations due to surface adsorbed ions will be shown to be more relevant then the bulk packing constraints for ions vicinal to the surface, and to set in prior to the conditions where the bulk packing constraints would become relevant.

cond-mat.soft↗

arXiv2018

Anomalous Multipole Expansion: Charge Regulation of Patchy, Inhomogeneously Charged Spherical Particles

Charge regulation is an important aspect of electrostatics in biological and colloidal systems, where the charges are generally not fixed, but depend on the environmental variables. Here, we analyze the charge regulation mechanism in patchy, inhomogeneously charged spherical particles, such as globular proteins, colloids, or viruses. Together with the multipole expansion of inhomogeneously charged spherical surfaces, the charge regulation mechanism on the level of linear approximation is shown to lead to a mixing between different multipole moments depending on their capacitance -- the response function of the charge distribution with respect to the electrostatic potential. This presents an additional anomalous feature of molecular electrostatics in the presence of ionic screening. We demonstrate the influence of charge regulation on several examples of inhomogeneously charged spherical particles, showing that it leads to significant changes in their multipole moments.

cond-mat.soft↗

arXiv2018

Mean-field theory of active electrolytes: dynamic adsorption and overscreening

We investigate active electrolytes within the mean-field level of description. The focus is on how the double-layer structure of passive, thermalized charges is affected by active dynamics of all constituting ions. One feature of active dynamics is that particles adhere to hard-surfaces, regardless of chemical properties of a surface and specifically in complete absence of any chemipsorption or physisorption. To carry out the mean-field analysis of the system that is out of equilibrium, we develop the "mean-field simulation" technique, where the simulated system consists of charged parallel sheets moving on a line and obeying active dynamics, with the interaction strength rescaled by the number of sheets. The mean-field limit becomes exact in the limit of an infinite number of movable sheets.

cond-mat.soft↗