Biomedical subjects

Yu-qiang Ma

Publications and source records attributed to Yu-qiang Ma.

8 recordsLinked to original sources

PubMed2006

Phase behavior in thin films of confined colloid-polymer mixtures.

Using self-consistent-field and density-functional theories, we first investigate colloidal self-assembly of colloid-polymer films confined between two soft surfaces grafted by polymers. With increasing colloidal concentrations, the film undergoes a series of transitions from disordered liquid --> sparse square --> hexagonal (or mixed square-hexagonal) --> dense square --> cylindrical structures in a plane, which results from the competition between the entropic elasticity of polymer brushes and the steric packing effect of colloidal particles. A phase diagram displays the stable regions of different in-layer ordering structures as the colloidal concentration is varied and layering transitions as the polymer-grafted density is decreased. Our results show a new control mechanism to stabilize the ordering of structures within the films.

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PubMed2005

Reentrant ordering transition of asymmetric copolymer solution film confined between polymer-grafted surfaces.

We study the equilibrium morphology of an asymmetric A-B diblock copolymer solution film confined between homopolymer-grafted substrates by using self-consistent-field calculations. We find that on decreasing the copolymer concentration, a reentrant structural transformation between hexagonal --> lamellar --> hexagonal phases occurs as a result of the competition between the wetting effect of the brush surface and the bulk phase behavior of the asymmetric copolymer driven by the A-B interfacial tension.

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PubMed2005

Interactions between colloidal particles induced by polymer brushes grafted onto the substrate.

We investigate the interaction energy between two colloidal particles on or immersed in nonadsorbing polymer brushes grafted onto the substrate as a function of the separation of the particles by the use of a self-consistent-field theory calculation. Depending on the colloidal size and the penetration depth, we demonstrate the existence of a repulsive energy barrier of several kBT, which can be interpreted by separating the interaction energy into three parts: colloid-polymer interfacial energy, entropic contribution due to "depletion zone" overlap of colloidal particles, and entropic elastic energy of grafted chains by the compression of particles. The existence of a repulsive barrier which is of entirely entropic origin can lead to kinetic stabilization of the mixture rather than depletion flocculation or phase separation. Therefore, the present result may suggest an approach for controlling the self-assembling behavior of colloids for the formation of target structures, by tuning the colloidal interaction on the grafting substrate under appropriate selection of colloidal size, effective gravity (influencing the penetration depth), and brush coverage density.

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PubMed2005

Ordering mechanism of asymmetric diblock copolymers confined between polymer-grafted surfaces.

Using self-consistent-field calculation, we study the equilibrium morphology of asymmetric block copolymer melts between two brush-covered surfaces. We discuss systematically the morphologies formed as a function of the grafting density of brushes and show a series of reentrant structural transformations between the lamellar and hexagonal phases. Such a selection of ordered microdomains has been attributed to the interplay among entropy effects of stretched brushes, the preferential interaction of the blocks with the grafted substrate, and the bulk microphase-separated behavior of asymmetric copolymers. Interestingly for the affinitive brush-copolymer interfaces, the bulk hexagonal phase may still be remained due to entropy elasticity of brushes, and the frustration can be relieved by deforming the shape of "soft" polymer-brush surface, in contrast to the hard-wall cases. The result demonstrates a simple way to realize molecular self-assembly for confined copolymer films with well-controlled thickness and interfacial environment.

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PubMed2003

Simulation study on kinetic temperatures of vibrated binary granular mixtures.

We study the unequal kinetic temperatures in two-dimensional binary granular mixtures heated by vertical vibrations under gravity by molecular dynamics simulations. By introducing several variable parameters describing particle and composition properties of mixtures, the coexistence of two different kinetic temperatures for the two components is exhibited. In dilute situations, our numerical results confirm the overall experimental measurements by Feitosa and Menon recently [Phys. Rev. Lett. 88, 198301 (2002)]. For example, the temperature ratio along the vertical direction is constant in the bulk of a system, and is insensitive to external driving or relative concentration of the two components, but mainly influenced by their masses. However, outside of the dilute situations, especially as the number density becomes high enough where the spatial and momentum correlations become salient, the ratio profiles show some different characteristics. Generally, the plateau shape along the vertical direction is broken, and instead, a saddlelike shape appears. Furthermore, the physical implications are discussed.

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PubMed2003

Effect of inert species in gas phase on oscillatory dynamics of oxidation system of CO on Pt(100).

We present a Monte Carlo simulation for the global oscillation of the CO catalytic oxidation system in the presence of inert species in gas phase, which can adsorb and desorb on the catalytic surface but cannot react with other species. It is found that the impurity has a dramatic effect on the oscillatory dynamics, although it does not involve in the reaction of CO oxidation. The simulation results show that with an increase in the fraction of impurity in gas phase, the periodic oscillation may change into an irregular oscillation and even can be inhibited completely. However, as the desorption rate of the impurity is increased, the regular oscillation will be recovered again.

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PubMed2003

Phase separation of a polymer blend driven by oscillating particles.

We study the possible formation of ordered structures of a binary polymer blend by introducing mobile particles in a periodically oscillating driving field. The particles which have a preferential attraction to one of the immiscible phases, will significantly perturb the phase separation of the system and breakup the isotropy of the system, so that some interesting structures such as lamellar and cylinder phases are observed by appropriate selection of the simulation parameters. We examine in detail the dependence of formed morphology and domain size on the oscillating fields, the relative composition of mixtures, the diffusion coefficient, and quench depth, and then discuss how to realize stable and highly ordered structures.

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PubMed2002

Self-assembling morphology induced by nanoscale rods in a phase-separating mixture.

A symmetric binary mixture containing mobile nanoscale rods is quenched from a one-phase state into an unstable phase-separating region. Incorporating the motion of rods, the system undergoes spinodal decomposition to form a dropletlike structure. The rod-rod interaction and the preferential adsorption of one of the two immiscible phases onto the mobile rods play an important role in driving the system to self-assemble into this special structure. Within each cluster of the wetting phase, the rods align parallel to each other as in a nematic liquid crystal, while the rod orientations between these clusters are randomly distributed. However, an interconnected structure is recovered in the presence of hydrodynamic interaction. In the present phase-separating mixture with rods, the growth dynamics is examined in detail, and our simulations reveal a crossover from a rod-mobility-dependent to an independent regime. The system always exhibits slowing-down growth behavior where the well-known diffusive Lifshitz-Slyozov mechanism and hydrodynamic effects are both suppressed.

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