Week of 10/10/08

In attempt to make these posts more regular I will be trying a different approach of simply posting links to various papers of interest, without much graphics or commentary.

• V. A. Martinez, G. Bryant and W. van Megen, “Slow Dynamics and Aging of a Colloidal Hard Sphere Glass,” Physical Review Letters 101, 135702-4 (2008)
• J. M. Brader, M. E. Cates and M. Fuchs, “First-Principles Constitutive Equation for Suspension Rheology,” Physical Review Letters 101, 138301-4 (2008)  also reviewed by
  N. Wagner, “How colloidal dispersions relax under stress,” Physics 1, 22 (2008)
• D. K. Satapathy, O. Bunk, K. Jefimovs et al., “Colloidal Monolayer Trapped near a Charged Wall: A Synchrotron X-Ray Diffraction Study,” Physical Review Letters 101, 136103-4 (2008)

Secrets of universal random close packing, plus wedging the phase diagrams

Two quick highlights – one from soft matter and another from hard condensed matter.

First highlight is a PRL with an intriguing title ” Why is Random Close Packing Reproducible?” by Randall Kamien and Andrea Liu.

In very simple terms, the random close packing – jammed configuration where each sphere is confined by its neighbours – is invariably at .64 of volume filling. The ordered hexagonally close packed can achieve higher ratios, of about 0.74, and one would think that the special number of 0.64 can be derived and/or proven mathematically – not so.

Kamien and Liu show that 0.64 is the number for which the number of available configuration states disappears (goes to zero).

The second highlight is another PRL by Oleg Krupin, Eli Rotenberg and Steve Kevan “Controlling the Magnetic Ground State in Cr1-xVx Films”.

Here they used a very clever “wedge” design by making a wedge of CrV material, where the thickness varies along one direction and composition (V doping) varies along the perpendicular direction. This allows mapping out the magnetic phase diagram (C-SDW, I-SDW AFM phase vs. Paramagnetic phase) as a function of temperature, thickness of the film and V composition without having to change the samples.

Glass on cooling OR heating

This week’s item is a paper that just appeared on arxiv.org by Simon Mochrie’s Yale group and Argonne collaborators on a unique situation that occurs in a liquid that becomes a glass upon cooling OR heating: X. Lu et al., “How a liquid becomes a glass both on cooling and on heating” arxiv.org/0708.3663v1

Typically we think of glasses that vitrify when the temperature is (suddenly) lowered. The unique situation explored in this paper is a situation where at high temperature system forms a glass dominated by repulsive interactions, and at low temperatures due to attractive interactions.

By tracking the glass transition using coherent x-ray scattering techniques (XPCS in this case) the authors look in great detail at the logarithmically decaying slow fluctuations in the same system under very different circumstances – and therefore able to study not a single but two glass transitions with either repulsive or attractive interactions.

Monatomic Glass

This week’s item is a recent Nature paper “Vitrification of a monatomic metallic liquid” by Bhatt et. al from Angell’s group at Arizona State (Nature 448, 787-790 (16 August 2007)).

It is accompanied by News and Views article “Metal turned to Glass”, which does a better job of placing these developments in historical and scientific context than I ever could.

But very basic summary is that under pressure pure Germanium appears to be capable of vitrification – or glassy state. Previously most metallic glasses were alloys, with two or typically more than 3 components. Directional bonding due to multi-component nature of the alloy always plays a role in these cases, so the search for a simple monatomic system capable of forming a glass has been undergoing since 1960ies.

The samples appear to be too small for x-ray diffraction analysis, it would be interesting to see if large enough samples can be produced – in Ge or other monatomic metals.

Two PRLs on Colloidal Glasses

Confocal laser 3D microscopy studies of colloidal dynamics (typical size of particles on the order of a micron) have been producing a lot of results in the literature lately. Here are two PRLs, pulled almost by random out of selection of several other papers on similar topics just this month alone. Both have Eric Weeks of Emory as an author, who also happens to have a nice writeup about confocal microscopy technique.