CiteULike: dchen Li

Colloidal building blocks with potential for magnetically configurable photonic crystals

Pedro Camargo — 2008-06-20T23:16:22-00:00

Soft Matter, 2007, 3, 1215 - 1222, DOI: 10.1039/b706565e

Ring-diffusion mediated homogeneous melting in the superheating regime

Xian Bai — 2008-05-03T15:47:24-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 77, No. 13. (2008)

Homogeneous melting in the superheating regime is investigated by using molecular dynamics simulation of a Lennard-Jones model system. We show that the commonly observed catastrophic melting at the superheating limit is caused by fast heating rate. By keeping the system isothermally at temperatures below the superheating limit, we observe intense self-diffusion motions as the precursor of melting. The highly correlated atomic motions are related to the self-diffusion loops or rings. Two types of loops are observed, closed loop and open loop, where the latter is directly related to the homogeneous nucleation of the liquid phase. Homogeneous melting occurs when the number density of diffusion loops reaches a critical value. Our results suggest that homogeneous melting in the superheating regime is a first-order thermodynamic phase transition triggered by the self-diffusion loops when the kinetic constraint imposed by heating rate is lessened.

Surface Diffusion Dynamics of a Single Polymer Chain in Dilute Solution

Hu Qian — 2008-03-18T21:08:38-00:00

Physical Review Letters, Vol. 99, No. 6. (2007)

Comprehensive three-dimensional dissipative particle dynamics simulations are carried out to elucidate the diffusion mechanism of a strongly adsorbed polymer chain on a solid surface in dilute solutions. We find Rouse and reptation dynamics for polymer chain diffusing on smooth and rough surfaces (with obstacles or sticking points), respectively. Combining with scaling analysis, we find that the interactions between the surface and the fluid screen the hydrodynamic interaction. The different scaling as found for a polymer chain diffusing on a fluid membrane [Phys. Rev. Lett. 82, 1911 (1999)] and on a solid surface [Nature (London) 406, 146 (2000)] may be explained by the solid surface inhomogeneity that induces reptation.

Temperature Oscillations in a Compartmentalized Bidisperse Granular Gas

Meiying Hou — 2008-03-18T00:51:22-00:00

Physical Review Letters, Vol. 100, No. 6. (2008)

A granular clock is observed in a vertically vibrated compartmentalized granular gas composed of two types of grains with the same size. The dynamics of the clock is studied in terms of an unstable evaporation or condensation model for the granular gas. In this model, the temperatures of the two types of grains are considered to be different, and they are functions of the composition of the gas. Oscillations in the system are driven by the asymmetric collisions properties between the two types of grains. Both our experiments and model show that the transition of the system from a homogeneous state to an oscillatory state is via a Hopf bifurcation.

Crystallization of hard-sphere colloids in microgravity

Zhu Jx — 2006-08-11T17:46:58-00:00

The structure of, and transitions between, liquids, crystals and glasses have commonly been studied with the hard-sphere model(1-5), in which the atoms are modelled as spheres that interact only through an infinite repulsion on contact. Suspensions of uniform colloidal polymer particles are good approximations to hard spheres(6-11), and so provide an experimental model system for investigating hard-sphere phases. They display a crystallization transition driven by entropy alone. Because the particles are much larger than atoms, and the crystals are weakly bound, gravity plays a significant role in the formation and structure of these colloidal crystals. Here we report the results of microgravity experiments performed on the Space Shuttle Columbia to elucidate the effects of gravity on colloidal crystallization. Whereas in normal gravity colloidal crystals grown just above the volume fraction at melting show a mixture of random stacking of hexagonally close-packed planes (r.h.c.p.) and face-centred cubic (f.c.c.) packing if allowed time to settle(7,8), those in microgravity exhibit the r.h.c.p. structure alone, suggesting that the f.c.c. component may be induced by gravity-induced stresses. We also see dendritic growth instabilities that are not evident in normal gravity, presumably because they are disrupted by shear-induced stresses as the crystals settle under gravity. Finally, glassy samples at high volume fraction which fail to crystallize after more than a year on Earth crystallize fully in less than two weeks in microgravity. Clearly gravity masks or alters some of the intrinsic aspects of colloidal crystallization.