Condensed Matter Theory

Abstract:

The fact that ice oats because of the hydrogen bonding imposing a perfect tetrahedrally coordinated network, linking them into six-membered rings with much empty space between the molecules (Franzese and Stanley, p. 105), is perhaps the best known of what are widely seen as a long list of curiosities. Water’s maximum density at 4°C and its unusually high thermal capacity are also familiar anomalies. Many others, however, are less celebrated but are surely as noteworthy. Both the melting and boiling points of water are unexpectedly high when it is placed in the sequence of group VI hydrides. So Lyndell-Bell and Debenedetti remind us by this extrapolation, although not by this imagery, that ice placed in a gin and tonic would melt at -100°C and a cup of tea should be prepared at -80°C. Not only that, but the effect of supercooling is also remarkable, so that at ambient pressure it can reach -41°C, whereas at 2 kbar it may be as low as -92°C (Franzese and Stanley, p. 102). These authors also remind us that if the supercooling is very rapid the water fails to crystallize and becomes a glass. This is of more than passing interest because in its high density form it is “the most abundant ice in the universe, where it is found as a frost on interstellar grains” (Franzese and Stanley, p. 103). This is not the only regime in which water becomes amorphous. In the hydration layer associated with a peptide, the water again has glasslike properties “with a very rough potential-energy landscape and slow hopping between local potential minima” (Ball, p. 56).

Abstract:

Division in Staphylococci occurs equatorially and on specific sequentially orthogonal planes in three dimensions, resulting, after incomplete cell separation, in the 'bunch of grapes' cluster organization that defines the genus. The shape of Staphylococci is principally maintained by peptidoglycan. In this study, we use Atomic Force Microscopy (AFM) and fluorescence microscopy with vancomycin labelling to examine purified peptidoglycan architecture and its dynamics in Staphylococcus aureus and correlate these with the cell cycle. At the presumptive septum, cells were found to form a large belt of peptidoglycan in the division plane before the centripetal formation of the septal disc; this often had a 'piecrust' texture. After division, the structures remain as orthogonal ribs, encoding the location of past division planes in the cell wall. We propose that this epigenetic information is used to enable S. aureus to divide in sequentially orthogonal planes, explaining how a spherical organism can maintain division plane localization with fidelity over many generations. © 2010 Macmillan Publishers Limited. All rights reserved.

Abstract:

We study the propulsion of a micron-size paramagnetic colloidal doublet dispersed in water and driven above a surface by an external precessing magnetic field. The applied field forces the doublet to precess around an axis parallel to the plane of motion and the rotation of the colloidal assembly is rectified into translation due to a periodic asymmetry in dissipation close to the bounding plate. These recent experimental findings [P. Tierno, R. Golestanian, I. Pagonabarraga, and F. Sagués, Phys. Rev. Lett. 101, 218304 (2008)] are complemented here with a theoretical analysis of the system and extended to more complex magnetic modulations such as elliptical driving fields. Experimental results show a good agreement with numerical simulations with the aim to find the best conditions toward the optimization of propulsion speed and swimming efficiency.