Evaluation of configurational entropy of a model liquid from computer simulations

Computer simulations have been employed in recent years to evaluate the configurational entropy changes in model glass-forming liquids. We consider two methods, both of which involve the calculation of the `intra-basin' entropy as a means for obtaining the configurational entropy. The first method involves the evaluation of the intra-basin entropy from the vibrational frequencies of inherent structures, by making a harmonic approximation of the local potential energy topography. The second method employs simulations that confine the liquid within a localized region of configuration space by the imposition of constraints; apart from the choice of the constraints, no further assumptions are made. We compare the configurational entropies estimated for a model liquid (binary mixture of particles interacting {\it via} the Lennard-Jones potential) for a range of temperatures, at fixed density.Comment: 10 pages, 5 figures, Proceedings of "Unifying Concepts in Glass Physics" Trieste 1999 (to appear in J. Phys. Cond. Mat.

Arachnoid web: a mystery unravelled

A 47-year-old lady came with complaints of dull aching low backache and numbness over both the lower limbs since six months, aggravated since one week. She had tenderness over dorsal (D8-D10) and lumbar (L4-L5) spinal and paraspinal regions. Radiographs were inconclusive. Plain and contrast Magnetic resonance imaging (MRI) of the dorso-lumbar (D-L) spine with whole spine screening were also inconclusive initially. But close observation and serial viewing of all the sections revealed features of an arachnoid web at D7-D8 vertebral level. Patient underwent D7, D8 laminectomy with durotomy and arachnoid web excision. She had considerable improvement in her symptoms after the surgery. Hence, prompt recognition and early surgery ensure good outcomes in this uncommon yet relevant clinical entity

Alignments of the Dominant Galaxies in Poor Clusters

We have examined the orientations of brightest cluster galaxies (BCGs) in poor MKW and AWM clusters and find that, like their counterparts in richer Abell clusters, poor cluster BCGs exhibit a strong propensity to be aligned with the principal axes of their host clusters as well as the surrounding distribution of nearby (< 20/h Mpc) Abell clusters. The processes responsible for dominant galaxy alignments are therefore independent of cluster richness. We argue that these alignments most likely arise from anisotropic infall of material into clusters along large-scale filaments.Comment: 8 pages, 5 figure

Liquid Limits: The Glass Transition and Liquid-Gas Spinodal Boundaries of Metastable Liquids

The liquid-gas spinodal and the glass transition define ultimate boundaries beyond which substances cannot exist as (stable or metastable) liquids. The relation between these limits is analyzed {\it via} computer simulations of a model liquid. The results obtained indicate that the liquid - gas spinodal and the glass transition lines intersect at a finite temperature, implying a glass - gas mechanical instability locus at low temperatures. The glass transition lines obtained by thermodynamic and dynamic criteria agree very well with each other.Comment: 5 pages, 4 figures, to appear in Phys. Rev. Let

Potential Energy Landscape Equation of State

Depth, number, and shape of the basins of the potential energy landscape are the key ingredients of the inherent structure thermodynamic formalism introduced by Stillinger and Weber [F. H. Stillinger and T. A. Weber, Phys. Rev. A 25, 978 (1982)]. Within this formalism, an equation of state based only on the volume dependence of these landscape properties is derived. Vibrational and configurational contributions to pressure are sorted out in a transparent way. Predictions are successfully compared with data from extensive molecular dynamics simulations of a simple model for the fragile liquid orthoterphenyl.Comment: RevTeX4, 4 pages, 5 figure

Mechanical properties of nanotube sheets: Alterations in joint morphology and achievable moduli in manufacturable materials

Nanotube sheets, or “bucky papers,” have been proposed for use in actuating, structural and electrochemical systems, based in part on their potential mechanical properties. Here, we present results of detailed simulations of networks of nanotubes/ropes, with special emphasis on the effect of joint morphology. We perform detailed simulations of three-dimensional joint deformation, and use the results to inform simulations of two-dimensional (2D) networks with intertube connections represented by torsion springs. Upper bounds are established on moduli of nanotube sheets, using the 2D Euler beam-network simulations. Comparisons of experimental and simulated response for HiPco-nanotube and laser-ablated nanotube sheets, indicate that ∼2–30-fold increases in moduli may be achievable in these materials. Increasing the numbers of interrope connections appears to be the best target for improving nanotube sheet stiffnesses in materials containing straight segments. © 2004 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70283/2/JAPIAU-95-8-4335-1.pd