H. R. Ma

Effective interaction of nonuniformly charged colloid spheres in a bulk electrolyte

The interaction energy between two like-charged colloid spheres with nonuniformly distributed surface charges immersed in a bulk electrolyte is calculated under the linearized Poisson-Boltzmann equation. The calculated results are sensitive to the relative orientations of the spheres due to the anisotropy of the distribution of charges on the sphere surface. The effective repulsive interaction after thermal average is weaker than the case when charges are uniformly distributed on the spheres, and with the increase of the nonuniformity, an attractive interaction between the two spheres emerges.

Depletion force and torque on an ellipsoid

The depletion force and torque acting on a hard rotational ellipsoid near a hard wall or two hard walls, induced by a small hard-sphere fluid, are calculated by Monte Carlo simulations. The depletion potential is determined by the acceptance ratio method, while the force and the torque on the ellipsoid are obtained by numerical differentiation of the differentiation of the depletion potential.

Topological defects of tetratic liquid-crystal order on a soft spherical surface

We model the orientational and positional order of tetratically shaped molecules, each having four-fold structural symmetry, confined on a spherical surface. Our Monte Carlo simulation shows that at a high molecular density, a tetratic orientational order develops in the system, accompanied by eight disclinations arranged in an anticube configuration form on the hard spherical surface. We also consider an elastic-energy model, which consists of both Helfrich and Frank energies for a soft surface; the solution confirms the Monte Carlo study and further predicts the tetratic morphology that can be realized on the surface of a soft vesicle. Assuming that the induced interaction between these disclinations is repulsive, we demonstrate that the anticube structure has a lower free energy than, for example, the cubic structure.

Defect-free states and disclinations in toroidal nematics

We investigate the structure of the nematic director field on a toroidal surface, on the basis of two different free-energy models. The two models treat the variation of the nematic field differently; one in full-derivative form and the other in covariant-derivative form. Through solving the Euler–Lagrange equation used to minimize the energy model and conducting a simulated annealing Monte Carlo simulation, we confirm that both models produce a trivial solution as a defect-free state. In the first model, however, there exists a second-order phase transition, beyond which the energy bifurcates into a non-trivial solution as the ground state, as the torus ring radius grows. Using the simulated annealing technique on both models, we also trap the system in excited, metastable states that display nematic-field disclinations.

Reentrant wetting transition on surfactant solution surfaces

The wetting of polydimethylsiloxane oil drops on the surfaces of anionic surfactant sodium dodecylsulfate solutions is studied systematically by changing the bulk surfactant concentration. The wetting state changes from complete wetting to pseudopartial wetting at 0.3 cmc (critical micelle concentration) surfactant concentration and there is a reentrant transition back to complete wetting at 1.4 cmc. The measured free energy is consistent with the prediction of the wetting theory. The interaction potential minimum of the two surfaces of the oil film disappears at the reentrant point, which is speculated to be an effect of micelle formation in the solution.

Entropic interactions on a colloidal sphere near the edge of a terrace

Abstract.We calculated the entropic interactions between a colloidal sphere and the hard edge of a terrace by means of Monte Carlo simulation with the acceptance ratio method. Comparison with the Asakura-Oosawa approximation indicates that the AO approximation is more accurate in this model than other known models. Our simulation results are also in qualitative agreement with the experimental results obtained by A.D. Dinsmore et al.

Depletion interaction between two ellipsoids

The depletion interactions between two ellipsoids in three configurations were studied by both Monte Carlo simulation with the Wang-Landau algorithm and the density functional theory in the curvature expansion approximation. Common features of the depletion interactions were found and the results were as expected. By comparing the results of the two methods, it is concluded that density functional theory under the curvature expansion approximation gave very good results to the depletion forces.

The density distributions of the counterions and the coions confined in two similarly charged plates.

By using the field-theoretic method, we established a unified systematic formulation of a model of counterions and coions confined in two similarly charged plates, and calculated the density distributions of counterions and coions with various coupling parameters by the two methods: Poisson-Boltzmann (PB) approach and the strong coupling (SC) theory, respectively. We also performed Monte Carlo simulations, and obtained the density distributions of counterions and coions with several different coupling parameters. Comparing our theoretical results with those from Monte Carlo simulation, we find that the PB approach is valid when the coupling parameter Xi is smaller than 1, but, as Xi > or = 1, the results by the PB approach deviate from the corresponding Monte Carlo simulation data, and the deviation gets larger with the coupling parameter increasing. This shows that the PB approach is completely invalid when the coupling parameter is equal to 1 or larger than 1. For the latter case, the development trend of the distribution curve calculated by SC theory agrees with that from Monte Carlo simulation as the coupling parameter increases. This demonstrates that the SC theory can give a qualitative available explanation on the density distribution of the counterions in the system in which the coupling parameters are strictly confined.

Column size effects of DER fluids

Abstract:The static yield stress of dielectric electrorheological (DER) fluids of infinite column state and chain state are calculated from the first principle method. The results indicate that the column surface contributions to ER effects is very small and both states will give correct results to the real DER fluids.