Biomedical subjects

D C Hong

Publications and source records attributed to D C Hong.

4 recordsLinked to original sources

PubMed2001

Density-functional theory of hard-sphere condensation under gravity.

The onset of condensation of hard spheres in a gravitational field is studied using density-functional theory (DFT). We find that the local density approximation yields results identical to those obtained previously using the kinetic theory [Physica A 271, 192, (1999)], and a weighted density-functional theory gives qualitatively similar results, namely, that the temperature at which condensation begins at the bottom scales linearly with the weight, diameter, and number of layers of particles. We find also that the different DFT approaches give quantitatively different results for the density profiles at low temperatures. In particular, the weighted density-functional approach reveals the layering of hard spheres in the solid regime.

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PubMed2001

Condensation of hard rods under gravity: exact results in one dimension.

We present exact results for the density profile of a one dimensional array of N hard rods of diameter D and mass m under gravity g. For a strictly one dimensional system, the liquid-solid transition occurs at zero temperature, because the close-packed density straight phi(c) is 1. However, if we relax this condition slightly such that straight phi(c)=1-delta, we find a series of critical temperatures Tc(i)=mgD(N+1-i)/mu0 with mu0=1/delta-1, at which the ith particle undergoes the liquid-solid transition. The functional form of the onset temperature, Tc(1)=mgDN/mu0, is consistent with the previous result [Physica A 271, 192 (1999)] obtained by the Enskog equation. We also show that the increase in the center of mass is linear in T before the transition, but it becomes quadratic in T after the transition because of the formation of solid near the bottom.

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PubMed2001

Reverse Brazil nut problem: competition between percolation and condensation.

In the Brazil nut problem (BNP), hard spheres with larger diameters rise to the top. There are various explanations (percolation, reorganization, convection), but a broad understanding or control of this effect is by no means achieved. A theory is presented for the crossover from BNP to the reverse Brazil nut problem based on a competition between the percolation effect and the condensation of hard spheres. The crossover condition is determined, and theoretical predictions are compared to molecular dynamics simulations in two and three dimensions.

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PubMed2000

Surface instability in windblown sand.

We investigate the formation of ripples on the surface of windblown sand based on the one-dimensional model of Nishimori and Ouchi [Phys. Rev. Lett. 71, 197 (1993)], which contains the processes of saltation and grain relaxation. We carry out a nonlinear analysis to determine the propagation speed of the restabilized ripple patterns, and the amplitudes and phases of their first, second, and third harmonics. The agreement between the theory and our numerical simulations is excellent near the onset of the instability. We also determine the Eckhaus boundary, outside which the steady ripple patterns are unstable.

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