Biomedical subjects

Yu-Qiang Ma

Publications and source records attributed to Yu-Qiang Ma.

6 recordsLinked to original sources

PubMed2006

Confined crystallization of cylindrical diblock copolymers studied by dynamic Monte Carlo simulations.

We report dynamic Monte Carlo simulations of polymer crystallization confined in the cylindrical microdomains of diblock copolymers. The microdomains were prepared via spontaneous microphase separation from homogeneous melt, and the major component was then frozen in a vitreous amorphous state to make a hard confinement to the crystallization of the minor component. We found that during the isothermal crystallization at high temperatures, crystal orientations are dominantly perpendicular to the cylinder axis at the early stage of crystal nucleation and remain to the final state; while if the block junctions are broken before crystallization, crystal orientations are dominantly parallel at the early stage of crystal nucleation, and eventually other orientations take the place of parallel preferences. Analysis of bond orientations in the heterogeneous melts demonstrates the microscopic origin of oriented crystal nucleation.

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PubMed2004

Effects of quenched randomness induced by car accidents on traffic flow in a cellular automata model.

In this paper we numerically study the impact of quenched disorder induced by car accidents on traffic flow in the Nagel-Schreckenberg (NS) model. Car accidents occur when the necessary conditions proposed by [J. Phys. A 30, 3329 (1997)]] are satisfied. Two realistic situations of cars involved in car accidents have been considered. Model A is presented to consider that the accident cars become temporarily stuck. Our studies exhibit the "inverse- lambda form" or the metastable state for traffic flow in the fundamental diagram and wide-moving waves of jams in the space-time pattern. Model B is proposed to take into account that the "wrecked" cars stay there forever and the cars behind will pass through the sites occupied by the "wrecked" cars with a transmission rate. Four-stage transitions from a maximum flow through a sharp decrease phase and a density-independent phase to a high-density jamming phase for traffic flow have been observed. The density profiles and the effects of transmission rate and probability of the occurrence of car accidents in model B are also discussed.

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PubMed2003

Fast growth in phase-separating A-B-copolymer ternary mixtures with a chemical reaction.

We study the dynamics of phase separation of a binary A-B- polymer mixture with copolymer C, which is produced by the reaction of two counterpart reactive polymers A and B at the interface via the chemical reaction A+B right harpoon over left harpoon C. For low interfacial energy between the A and B phases, where the copolymer prefers to locate at interfaces, we show that the chemical reaction accelerates the phase separation of the system dramatically, because the backward reaction always drives the creation of immiscible A and B pairs at interfaces, which speed up the phase separation of the system, while the forward reaction process becomes more and more difficult as the interfaces are gradually saturated by copolymers. We also indicate that for a fixed chemical reaction rate constant, as the initial concentration of the copolymers increases, the domain growth at the late stage is speeded up as a result of the backward chemical reaction. However, when the interfacial energy is high, both forward and backward reactions coexist due to the occurrence of unsaturated interfaces, but the relative strength of reaction rates has no appreciable effect on domain growth during spinodal decomposition, because the interfacial energy dominates phase separation.

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PubMed2002

Hysteresis phenomena in CO catalytic oxidation system in the presence of inhomogeneities of the catalyst surface.

The hysteresis phenomena in the CO catalytic oxidation system are studied by Monte Carlo simulation in the presence of the inhomogeneities of the catalyst surface. We show that the O-passivated state is destroyed due to the inhomogeneities of the surface, in contrast to the classical Ziff-Gulari-Barshad model. The defects on the surface have a significant effect on the hysteresis transition points. Most importantly, the supercritical nucleation and growth of the O adatom island during the transition from a low reactivity to a high reactivity states are closely related to the inhomogeneities of the catalyst surface. It is shown that the width of the hysteresis loop shrinks as the scan rate beta(CO) of y(CO) (the fraction of CO in gas phase) decreases, but there exists a finite width of the hysteresis loop even if beta(CO) becomes infinitely small. On the other hand, the width of the hysteresis loop decreases with decreasing the diffusion rate, and even the hysteresis loop may disappear for a slow diffusion. These simulation results are in good consistency with the previous relevant experimental results.

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PubMed2002

Nonequilibrium kinetic phase transition of a monomer-dimer reaction model with sequential dimer adsorption in two dimensions.

We study a monomer-dimer reaction model in two dimensions in which the adsorption process of a dimer on the surface sites is separated into two steps (i.e., B2+*-->B2 *, and B2 * +*-->2B*, where * is an empty site), as first introduced by Evans and co-workers. It is clear that the dissociation of a dimer is dependent on the complicated configuration of the adsorbate. We show that the continuous transition from the reactive state to the O-passivated state and the discontinuous transition to the CO-passivated state both shift toward higher values of the fraction p of the monomer in gas phase but the reaction window decreases compared to the Ziff-Gulari-Barshad model. For the model studied here, the critical exponents of the continuous phase transition still exhibit a directed percolation character as expected.

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PubMed2002

Phase separation in two-dimensional binary fluid mixtures: spontaneous pinning effect.

The dynamics of phase separation, under asymmetric quench, is studied for binary mixtures by molecular dynamics simulations. We consider two kinds of systems, i.e., the small molecular liquid mixture and the flexible chain blend. The domain growth is found to be dependent crucially upon the relative composition of the mixture. For a near symmetric quench with volume fraction not far away from the critical value, we find that the domain growth is obviously slowed down (the spontaneous pinning effect) when the phase-separating structure undergoes the percolation-to-cluster transition. However, as the volume fraction of the minority phase is decreased, a pinning-depinning transition of the domain growth is observed for certain asymmetric quenches, due to the correlated motion of droplets. For sufficiently asymmetric quenches where the volume fraction of the minority phase is low, the domain growth is suppressed at late stages for the flexible polymer blend, in contrast to the fast growth for the small molecular system. The results are in good agreement with previous studies, and confirm the possibility of the intermittent or final pinning of phase separation for isolated droplets structure.

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