Biomedical subjects

H R Ma

Publications and source records attributed to H R Ma.

5 recordsLinked to original sources

PubMed2006

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.

Journal Article↗

PubMed2005

Depletion interactions between two spherocylinders.

The depletion interactions between two spherocylinders as functions of their separation and their relative orientation, induced by a small hard-sphere fluid, are calculated by Monte Carlo simulations using the acceptance ratio method (ARM). The torque on the spherocylinders is determined from the resulting potential. The calculation shows that the ARM is an effective way to obtain depletion interactions of spherocylinders. The depletion interaction under the Asakura-Oosawa (also excluded-volume) approximation is also calculated numerically.

Journal Article↗

PubMed2004

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.

Journal Article↗

PubMed2003

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

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.

Journal Article↗

PubMed2002

Depletion potential near curved surfaces.

We examine the depletion potential and force of a hard-sphere fluid on a single big hard sphere, located inside or outside of a hard spherical cavity, by Monte Carlo simulations to the hard-sphere fluid. The depletion potential is determined by the acceptance ratio method, while the force on the big sphere is obtained by two methods: numerical differentiation of the depletion potential and integration of the contact density of the fluid at the surface of the big sphere. The results are in excellent agreement with those obtained by density functional theory presented by Roth et al.

Biophysical Phenomena↗