Two-dimensional packing in prolate granular materials.

We investigate the two-dimensional packing of extremely prolate (aspect ratio alpha=L/D>10) granular materials, comparing experiments with Monte Carlo simulations. The average packing fraction of particles with aspect ratio alpha=12 is 0.68+/-0.03. We quantify the orientational correlation of particles and find a correlation length of two particle lengths. The functional form of the orientational correlation is the same in both experiments and simulations which three orders of magnitude in aspect ratio, all decaying over a distance of two particle lengths. It is possible to identify voids in the pile with sizes ranging over two orders of magnitude. The experimental void distribution function is a power law with exponent -beta=-2.43+/-0.08. Void distributions in simulated piles do not decay as a power law, but do show a broad tail. We extend the simulation to investigate the scaling at very large aspect ratios. A geometric argument predicts the pile number density to scale as alpha(-2). Simulations do indeed scale this way, but particle alignment complicates the picture, and the actual number densities are quite a bit larger than predicted.

Liquid polymorphism: water in nanoconfined and biological environments.

We demonstrate some recent progress in understanding the anomalous behavior of liquid water, by combining information provided by recent experiments and simulations on water in bulk, nanoconfined, and biological environments. We interpret evidence from recent experiments designed to test the hypothesis that liquid water may display 'polymorphism' in that it can exist in two different phases--and discuss recent work on water's transport anomalies as well as the unusual behavior of water in biological environments. Finally, we will discuss how the general concept of liquid polymorphism may prove useful in understanding anomalies in other liquids, such as silicon, silica, and carbon, as well as metallic glasses which have in common that they are characterized by two characteristic length scales in their interactions.

Phase transitions and dynamics of bulk and interfacial water.

New experiments on water at the surface of proteins at very low temperature display intriguing dynamic behaviors. The extreme conditions of these experiments make it difficult to explore the wide range of thermodynamic state points needed to offer a suitable interpretation. Detailed simulations suffer from the same problem, where equilibration times at low temperature become extremely long. We show how Monte Carlo simulations and mean field calculations using a tractable model of water help interpret the experimental results. Here we summarize the results for bulk water and investigate the thermodynamic and dynamic properties of supercooled water at an interface.

Heterogeneities in confined water and protein hydration water.

We report recent efforts to understand a broad range of experiments on confined water and protein hydration water, many initiated by a collaboration between workers at the University of Messina and MIT-the editors of this special issue. Preliminary calculations are not inconsistent with one tentative interpretation of these experiments as resulting from the system passing from the high-temperature high-pressure 'HDL' side of the Widom line (where the liquid might display non-Arrhenius behavior) to the low-temperature low-pressure 'LDL' side of the Widom line (where the liquid might display Arrhenius behavior). The Widom line-defined to be the line in the pressure-temperature plane where the correlation length has its maximum-arises if there is a critical point. Hence, interpreting the Messina-MIT experiments in terms of a Widom line is of potential relevance to testing, experimentally, the hypothesis that water displays a liquid-liquid critical point.