R.H.H.G. van Roij

Isotropic-nematic interface in suspensions of hard rods: mean-field properties and capillary waves.

We present a study of the isotropic-nematic interface in a system of hard spherocylinders. First we compare results from Monte Carlo simulations and Onsager density functional theory for the interfacial profiles of the orientational order parameter and the density. Those interfacial properties that are not affected by capillary waves are in good agreement, despite the fact that Onsager theory overestimates the coexistence densities. Then we show results of a Monte Carlo study of the capillary waves of the interface. In agreement with recent theoretical investigations [Elgeti and Schmid, Eur. Phys. J. E 18, 407 (2005)] we find a strongly anisotropic capillary wave spectrum. For the wave numbers accessed in our simulations, the spectrum is quadratic, i.e., elasticity does not play a role. We conjecture that this effect is due to the strong bending rigidity of the director field in suspensions of spherocylinders.

The electric double layer at high surface potentials : the influence of excess ion polarizability

By including the excess ion polarizability into the Poisson-Boltzmann theory, we show that the decrease in differential capacitance with voltage, observed for metal electrodes above a threshold potential, can be understood in terms of thickening of the double layer due to ion-induced polarizability holes in water. We identify a new length which controls the role of excess ion polarizability in the double layer, and show that when this is comparable to the size of the effective Debye layer, ion polarizability can significantly influence the properties of the double layer.

Interfaces, wetting, and capillary nematization of a hard-rod fluid: Theory for the Zwanzig model

We investigate interfacial and capillary phenomena in a simple model for a fluid of hard rods, viz. the Zwanzig model, in which the orientations of rectangular blocks are restricted to three orthogonal directions. The theory, which is based on an Onsager-like free energy functional, predicts local biaxial ordering at the “free” interface between the coexisting isotropic and nematic phases. For an isotropic bulk fluid in contact with a single planar hard wall, we find a continuous surface phase transition from uniaxial to biaxial local symmetry, followed by complete wetting of the wall–isotropic fluid interface by a nematic film with director parallel to the wall, as the reservoir density approaches its value at bulk coexistence. For a fluid confined by two parallel hard walls we determine a first-order capillary nematization transition at large wall separation, which terminates in a capillary critical point when the wall separation is about twice the length of the rods. This transition is the analog of th...

Charge Fluctuations in Nanoscale Capacitors

The fluctuations of the charge on an electrode contain information on the microscopic correlations within the adjacent fluid and their effect on the electronic properties of the interface. We investigate these fluctuations using molecular dynamics simulations in a constant-potential ensemble with histogram reweighting techniques. This approach offers, in particular, an efficient, accurate, and physically insightful route to the differential capacitance that is broadly applicable. We demonstrate these methods with three different capacitors: pure water between platinum electrodes and a pure as well as a solvent-based organic electrolyte each between graphite electrodes. The total charge distributions with the pure solvent and solvent-based electrolytes are remarkably Gaussian, while in the pure ionic liquid the total charge distribution displays distinct non-Gaussian features, suggesting significant potential-driven changes in the organization of the interfacial fluid.

Liquid-liquid interfacial tension of electrolyte solutions

It is theoretically shown that the excess liquid-liquid interfacial tension between two electrolyte solutions as a function of the ionic strength I behaves asymptotically as O(-sqrt[I]) for small I and as O(+/-I) for large I. The former regime is dominated by the electrostatic potential due to an unequal partitioning of ions between the two liquids whereas the latter regime is related to a finite interfacial thickness. The crossover between the two asymptotic regimes depends sensitively on material parameters suggesting that the experimentally accessible range of ionic strengths can correspond to either the small or the large ionic strength regime. In the limiting case of a liquid-gas surface where ion partitioning is absent, the image charge interaction can dominate the surface tension for small ionic strength I such that an Onsager-Samaras limiting law O(-Iln(I)) is expected.

Extended sedimentation profiles in charged colloids: the gravitational length, entropy, and electrostatics

We have measured equilibrium sedimentation profiles in a colloidal model system with confocal microscopy. By tuning the interactions, we have determined the gravitational length in the limit of hard-sphere-like interactions, and using the same particles tested a recent theory (van Roij 2003 J. Phys.: Condens. Matter 15 S3569), which predicts a significantly extended sedimentation profile in the case of charged colloids with long-ranged repulsions, due to a spontaneously formed macroscopic electric field. For the hard-sphere-like system we find the gravitational length matches that expected. By tuning the buoyancy of the colloidal particles we have shown that a mean field hydrostatic equilibrium description even appears to hold in the case where the colloid volume fraction changes significantly on the length scale of the particle size. The extended sedimentation profiles of the colloids with long-ranged repulsions are well described by theory. Surprisingly, the theory even seems to hold at concentrations where interactions between the colloids, which are not modelled explicitly, play a considerable role.

Translocation of DNA Molecules through Nanopores with Salt Gradients: The Role of Osmotic Flow

Recent experiments of translocation of double-stranded DNA through nanopores [M. Wanunu et al., Nature Nanotech. 5, 160 (2009)] reveal that the DNA capture rate can be significantly influenced by a salt gradient across the pore. We show that osmotic flow combined with electrophoretic effects can quantitatively explain the experimental data on the salt-gradient dependence of the capture rate.

Interfacial tension and wetting in colloid–polymer mixtures

We calculate the interfacial tension and the wetting behavior in phase separated colloid-polymer mixtures both for ideal and excluded volume interacting polymers. Within the recently developed extension of the free volume theory to include polymer interactions the interfacial tension of the free interface is calculated by adding a van der Waals squared gradient term. The wetting behavior at a hard wall is calculated following a Cahn-Fisher-Nakanishi approach taking the one- and two-body colloid-wall interactions into account. Comparing results for interacting polymers with those for ideal polymers we find that for interacting polymers the interfacial tension does not increase as steeply as a function of the gas-liquid colloid density difference. Furthermore, the wetting transition shifts to higher polymer concentrations, even to above the triple line. The predictions for both the interfacial tension and the wetting are compared to recent experiments.

Density functional theory for chiral nematic liquid crystals.

Even though chiral nematic phases were the first liquid crystals experimentally observed more than a century ago, the origin of the thermodynamic stability of cholesteric states is still unclear. In this Rapid Communication we address the problem by means of a density functional theory for the equilibrium pitch of chiral particles. When applied to right-handed hard helices, our theory predicts an entropy-driven cholesteric phase, which can be either right or left handed, depending not only on the particle shape but also on the thermodynamic state. We explain the origin of the chiral ordering as an interplay between local nematic alignment and excluded-volume differences between left- and right-handed particle pairs.

Phase Diagram and Effective Shape of Semiflexible Colloidal Rods and Biopolymers

We study suspensions of semiflexible colloidal rods and biopolymers using an Onsager-type second-virial functional for a segmented-chain model. For mixtures of thin and thick fd virus particles, we calculate full phase diagrams, finding quantitative agreement with experimental observations. We show that flexibility, which renders the rods effectively shorter and thicker depending on the state point, is crucial to understanding the topologies of the phase diagrams. We also calculate the stretching of wormlike micelles in a host fd virus suspension, finding agreement with experiments.