Wenze Ouyang

A molecular-dynamics simulation study on the dependence of Lennard-Jones gas-liquid phase diagram on the long-range part of the interactions

The particle-transfer molecular-dynamics technique is adopted to construct the Lennard-Jones fluid gas-liquid phase diagram. Detailed study of the dependence of the simulation results on the system size and the cutoff distance is performed to test the validity of the simulation technique. Both the traditional cutoff plus long-range correction (CPC) and Ewald summation methods are used in the simulations to calculate the interactions. In the intermediate range of temperatures, the results with the Ewald summation method are almost the same as those with the CPC method. However, in the range close to the critical point, the results with the CPC method deviate from those with the Ewald summation. Compared with the results obtained via the Ewald summation in a smaller system, simply increasing the system size in the CPC scheme may not give better results.

Influence of molecular topology on the static and dynamic properties of single polymer chain in solution.

The influence of molecular topology on the structural and dynamic properties of polymer chain in solution with ring structure, three-arm branched structure, and linear structure are studied by molecular dynamics simulation. At the same degree of polymerization (N), the ring-shaped chain possesses the smallest size and largest diffusion coefficient. With increasing N, the difference of the radii of gyration between the three types of polymer chains increases, whereas the difference of the diffusion coefficients among them decreases. However, the influence of the molecular topology on the static and the dynamic scaling exponents is small. The static scaling exponents decrease slightly, and the dynamic scaling exponents increase slightly, when the topology of the polymer chain is changed from linear to ring-shaped or three-arm branched architecture. The dynamics of these three types of polymer chain in solution is Zimm-like according to the dynamic scaling exponents and the dynamic structure factors.

Structural ordering and glass forming of soft spherical particles with harmonic repulsions.

We carry out dissipative particle dynamics simulations to investigate the dynamic process of phase transformation in the system with harmonic repulsion particles. Just below the melting point, the system undergoes liquid state, face-centered cubic crystallization, body-centered cubic crystallization, and reentrant melting phase transition upon compression, which is in good agreement with the phase diagram constructed previously via thermodynamic integration. However, when the temperature is decreased sufficiently, the system is trapped into an amorphous and frustrated glass state in the region of intermediate density, where the solid phase and crystal structure should be thermodynamically most stable.

Influence of the surface charge on the homogeneity of colloidal crystals

Five groups of suspensions composed of polystyrene particles, having similar size but different effective surface charge, were adopted to investigate the effects of surface charge and volume fraction on the homogeneity of colloidal crystals through checking the difference between D(exp) and D(uni) by reflection spectroscopy method (D(exp), D(uni) are the experimental and the expected value of the average nearest neighbor interparticle distance by assuming a uniform structure, respectively). We found volume fractions (ranging from 0.006 to 0.02) and structure types basically have no influence on the values of D(exp)/D(uni). Moreover, for crystals formed by lowly charged particles, D(exp)/D(uni) is approximately equal to 1, implying the crystals are homogeneous. With the increase of effective surface charge, D(exp) gradually deviates from D(uni) and the formed crystals become inhomogeneous. Our experimental observations are in accordance with the previous simulation results. Additionally, we also found D(exp)/D(uni) initially drops quickly with increasing effective surface charge and then it tends to an asymptotic value (~0.85), it is supposedly due to the saturation of effective charge. Our relevant computer simulations confirmed that the study scheme that using D(exp)/D(uni) as an indicator to assess the homogeneity of crystal structure is tenable and the simulation results are consistent with experiments.

Gas-liquid phase coexistence in quasi-two-dimensional Stockmayer fluids: A molecular dynamics study.

The Maxwell construction together with molecular dynamics simulation is used to study the gas-liquid phase coexistence of quasi-two-dimensional Stockmayer fluids. The phase coexistence curves and corresponding critical points under different dipole strength are obtained, and the critical properties are calculated. We investigate the dependence of the critical point and critical properties on the dipole strength. When the dipole strength is increased, the abrupt disappearance of the gas-liquid phase coexistence in quasi-two-dimensional Stockmayer fluids is not found. However, if the dipole strength is large enough, it does lead to the formation of very long reversible chains which makes the relaxation of the system very slow and the observation of phase coexistence rather difficult or even impossible.

From gas-liquid to liquid crystalline phase behavior via anisotropic attraction: A computer simulation study

The partial phase behavior of a continuum molecular model for self-assembling semiflexible equilibrium polymers is studied via Monte Carlo and molecular dynamics simulation. We investigate the transfer from ordinary gas-liquid coexistence to the appearance of liquid crystallinity driven by excluded volume interaction between rodlike aggregates. The transfer between the two types of phase behavior is governed by a tunable anisotropic attractive interaction between monomer particles. The relation to dipolar fluid models, which are also known to form reversible chains, is discussed.

Pattern of diffusion-limited aggregation on nonuniform substrate

Abstract Pattern of diffusion-limited aggregation (DLA) on nonuniform substrate was investigated by computer simulations. The nonuniform substrates are represented by Leath percolations with the probability p. p stands for the degree of nonuniformity and takes values in the range pc⩽p⩽1, where pc is the threshold of percolation. The DLA cluster grows up on the Leath percolation substrate. The patterns of the DLA clusters appear asymmetrical and nonuniform, and the branches are relative few for the case p is close to pc. In addition, the pattern depends on the shape of substrate. As p increases from pc to 1, cluster changes to pure DLA gradually. Correspondingly, the fractal dimension increases from 1.46 to 1.68. Also, the random walks on Leath percolations through the range pc⩽p⩽1 were examined. Our simulations show the Honda–Toyoki–Matsushita relation is still reasonable for fractional dimensional systems.

Crystal nucleation and metastable bcc phase in charged colloids: A molecular dynamics study

The dynamic process of homogenous nucleation in charged colloids is investigated by brute-force molecular dynamics simulation. To check if the liquid-solid transition will pass through metastable bcc, simulations are performed at the state points that definitely lie in the phase region of thermodynamically stable fcc. The simulation results confirm that, in all of these cases, the preordered precursors, acting as the seeds of nucleation, always have predominant bcc symmetry consistent with Ostwalds step rule and the Alexander-McTague mechanism. However, the polymorph selection is not straightforward because the crystal structures formed are not often determined by the symmetry of intermediate precursors but have different characters under different state points. The region of the state point where bcc crystal structures of large enough size are formed during crystallization is narrow, which gives a reasonable explanation as to why the metastable bcc phase in charged colloidal suspensions is rarely detected in macroscopic experiments.

Polymorph selection in the crystallization of hard-core Yukawa system

Colloid-colloid interactions in charge-stabilized dispersions can to some extent be represented by the hard-core Yukawa model. The crystallization process and polymorph selection of hard-core Yukawa model are studied by means of smart Monte Carlo simulations in the region of face-centered-cubic (fcc) phase. The contact value of hard-core Yukawa potential and the volume fraction of the colloids are fixed, while the Debye screening length can be varied. In the early stage of the crystallization, the precursors with relatively ordered liquid structure have been observed. Although the crystal structure of thermodynamically stable phase is fcc, the system crystallizes into a mixture of fcc and hexagonal close-packed (hcp) structures under small Debye screening length since the colloidal particles act as effective hard spheres. In the intermediate range of Debye screening length, the system crystallizes into a mixture of fcc, hcp, and body-centered-cubic (bcc). The existence of metastable hcp and bcc structures can be interpreted as a manifestation of the Ostwald’s step rule. Until the Debye screening length is large enough, the crystal structure obtained is almost a complete fcc suggesting the system eventually reaches to a thermodynamically stable state.