Biomedical subjects
Publications and source records attributed to R van Roij.
Quantitative predictions are presented of a depletion-induced torque and force acting on a single colloidal hard rod immersed in a solvent of hard spheres close to a planar hard wall. This torque and force, which are entirely of entropic origin, may play an important role for the key-lock principle, according to which a biological macromolecule (the key) is functional only in a particular orientation with respect to a cavity (the lock).
Within nonlinear Poisson-Boltzmann theory we calculate the pair and triplet interactions between charged colloidal spheres, specifically in the nonlinear regime of low salt concentrations and high charges. We find repulsive pair interactions and attractive triplet interactions. Within a van der Waals-like mean-field theory we estimate in which parameter regime a gas-liquid coexistence is to be expected.
We present results of a simulation study of a fluid of hard spherocylinders with a length-to-diameter ratio of 15 in contact with a planar hard wall and confined by two parallel hard walls. A Monte Carlo method is developed for simulating fluids in contact with a single wall. Using this method, we find a transition from a uniaxial to a biaxial surface phase, followed, at larger bulk densities, by the formation of a thick nematic film, with the director parallel to the wall, at the wall-isotropic fluid interface. As the density far from the wall cb approaches the value at bulk isotropic-nematic coexistence cI, the thickness of the nematic film appears to increase as -ln(cI-cb). For a fluid confined by two parallel hard walls, a first-order capillary nematization transition is found. The phase equilibria are determined by Gibbs ensemble Monte Carlo simulations for several wall separations. The difference in the coexisting densities of the capillary condensed nematic and isotropic phases becomes smaller upon decreasing the wall separation, and no capillary nematization transition is found when the wall separation is smaller than about twice the length of the spherocylinders. These features imply that the capillary nematization transition ends in a capillary critical point at a critical wall separation. Our simulation results are fully consistent with the findings of our recent theoretical study of the Zwanzig model for a hard-rod fluid.
We study the phase behavior and structure of highly asymmetric binary hard-sphere mixtures. By first integrating out the degrees of freedom of the small spheres in the partition function we derive a formal expression for the effective Hamiltonian of the large spheres. Then using an explicit pairwise (depletion) potential approximation to this effective Hamiltonian in computer simulations, we determine fluid-solid coexistence for size ratios q=0.033, 0.05, 0.1, 0.2, and 1.0. The resulting two-phase region becomes very broad in packing fractions of the large spheres as q becomes very small. We find a stable, isostructural solid-solid transition for q< or =0.05 and a fluid-fluid transition for q< or =0.10. However, the latter remains metastable with respect to the fluid-solid transition for all size ratios we investigate. In the limit q-->0 the phase diagram mimics that of the sticky-sphere system. As expected, the radial distribution function g(r) and the structure factor S(k) of the effective one-component system show no sharp signature of the onset of the freezing transition and we find that at most points on the fluid-solid boundary the value of S(k) at its first peak is much lower than the value given by the Hansen-Verlet freezing criterion. Direct simulations of the true binary mixture of hard spheres were performed for q > or =0.05 in order to test the predictions from the effective Hamiltonian. For those packing fractions of the small spheres where direct simulations are possible, we find remarkably good agreement between the phase boundaries calculated from the two approaches-even up to the symmetric limit q=1 and for very high packings of the large spheres, where the solid-solid transition occurs. In both limits one might expect that an approximation which neglects higher-body terms should fail, but our results support the notion that the main features of the phase equilibria of asymmetric binary hard-sphere mixtures are accounted for by the effective pairwise depletion potential description. We also compare our results with those of other theoretical treatments and experiments on colloidal hard-sphere mixtures.