Swimming in Granular Media

We study a simple model of periodic contraction and extension of large intruders in a granular bed to understand the mechanism for swimming in an otherwise solid media. Using an event-driven simulation, we find optimal conditions that idealized swimmers must use to critically fluidize a sand bed so that it is rigid enough to support a load when needed, but fluid enough to permit motion with minimal resistance. Swimmers - or other intruders - that agitate the bed too rapidly produce large voids that prevent traction from being achieved, while swimmers that move too slowly cannot travel before the bed re-solidifies around them i.e., the swimmers locally probe the fundamental time-scale in a granular packing

Death kinetics of Escherichia coli in goat milk and Bacillus licheniformis in cloudberry jam treated by ohmic heating

In recent years, the world’s food industry has focused increasing attention on electrical techniques of food processing. Ohmic heating is one of these techniques that can be considered as a high temperature short time and a purely bulk heating method, having potential applications in processes such as blanching, evaporation and pasteurization in the food industry. However such technology would have to assure the microbiological safety obtained by the conventional cooking methods. Concerning this, the influence of heat treatment by ohmic and conventional technology on death kinetic parameters (D and z values) of Escherichia coli ATCC® 25922 was studied in goat milk. In ohmic treatment lower D values were obtained (D60ºC = 4.2 min, D63ºC = 1.9 min, D65ºC = 0.86 min) as compared to conventional treatment (D63ºC = 3.9 min, D65ºC = 3.5, D67ºC = 2.8 min, D75ºC = 1.5 min). The increase of temperature required for a ten fold decrease in D value was also lower in the ohmic inactivation (z = 8.4 ºC) comparing with the conventional inactivation (z = 23.1 ºC). The death kinetics for Bacillus licheniformis ATCC® 14580 spores in cloudberry jam were also studied under both types of heat inactivation (ohmic and conventional) and similar conclusions were drawn for the D values; lower D values were also obtained for ohmic treatment (D70ºC = 57.1 min, D75ºC = 25.2 min, D80ºC = 7.2 min) as compared to conventional treatment (D70ºC = 85.3 min, D75ºC = 51.0, D80ºC = 18.1 min, D85ºC = 6.0 min, D90ºC = 1.6 min). However, between the z values obtained for those treatments (z ohmic = 11.1 ºC and z conventional = 11.4 ºC) the differences were not significant. In general the results of present work indicate that the ohmic heating provides quicker death kinetics. This opens the perspective for shorter, less aggressive treatments

N=2 Gauge Theories: Congruence Subgroups, Coset Graphs and Modular Surfaces

We establish a correspondence between generalized quiver gauge theories in four dimensions and congruence subgroups of the modular group, hinging upon the trivalent graphs which arise in both. The gauge theories and the graphs are enumerated and their numbers are compared. The correspondence is particularly striking for genus zero torsion-free congruence subgroups as exemplified by those which arise in Moonshine. We analyze in detail the case of index 24, where modular elliptic K3 surfaces emerge: here, the elliptic j-invariants can be recast as dessins d'enfant which dictate the Seiberg-Witten curves.Comment: 42+1 pages, 5 figures; various helpful comments incorporate

MnAs dots grown on GaN(0001)-(1x1) surface

MnAs has been grown by means of MBE on the GaN(0001)-(1x1) surface. Two options of initiating the crystal growth were applied: (a) a regular MBE procedure (manganese and arsenic were delivered simultaneously) and (b) subsequent deposition of manganese and arsenic layers. It was shown that spontaneous formation of MnAs dots with the surface density of 1$\cdot 10^{11}$ cm$^{-2}$ and $2.5\cdot 10^{11}$ cm$^{-2}$, respectively (as observed by AFM), occurred for the layer thickness higher than 5 ML. Electronic structure of the MnAs/GaN systems was studied by resonant photoemission spectroscopy. That led to determination of the Mn 3d - related contribution to the total density of states (DOS) distribution of MnAs. It has been proven that the electronic structures of the MnAs dots grown by the two procedures differ markedly. One corresponds to metallic, ferromagnetic NiAs-type MnAs, the other is similar to that reported for half-metallic zinc-blende MnAs. Both system behave superparamagnetically (as revealed by magnetization measurements), but with both the blocking temperatures and the intra-dot Curie temperatures substantially different. The intra-dot Curie temperature is about 260 K for the former system while markedly higher than room temperature for the latter one. Relations between growth process, electronic structure and other properties of the studied systems are discussed. Possible mechanisms of half-metallic MnAs formation on GaN are considered.Comment: 20+ pages, 8 figure