Rheology of Granular Materials: Dynamics in a Stress Landscape

We present a framework for analyzing the rheology of dense driven granular materials, based on a recent proposal of a stress-based ensemble. In this ensemble fluctuations in a granular system near jamming are controlled by a temperature-like parameter, the angoricity, which is conjugate to the stress of the system. In this paper, we develop a model for slowly driven granular materials based on the stress ensemble and the idea of a landscape in stress space. The idea of an activated process driven by the angoricity has been shown by Behringer et al (2008) to describe the logarithmic strengthening of granular materials. Just as in the Soft Glassy Rheology (SGR) picture, our model represents the evolution of a small patch of granular material (a mesoscopic region) in a stress-based trap landscape. The angoricity plays the role of the fluctuation temperature in SGR. We determine (a) the constitutive equation, (b) the yield stress, and (c) the distribution of stress dissipated during granular shearing experiments, and compare these predictions to experiments of Hartley & Behringer (2003).Comment: 17 pages, 4 figure

Improved modelling of liquid GeSe2_2: the impact of the exchange-correlation functional

The structural properties of liquid GeSe$_2$ are studied by using first-principles molecular dynamics in conjuncton with the Becke, Lee, Yang and Parr (BLYP) generalized gradient approximation for the exchange and correlation energy. The results on partial pair correlation functions, coordination numbers, bond angle distributions and partial structure factors are compared with available experimental data and with previous first-principle molecular dynamics results obtained within the Perdew and Wang (PW) generalized gradient approximation for the exchange and correlation energy. We found that the BLYP approach substantially improves upon the PW one in the case of the short-range properties. In particular, the Ge$-$Ge pair correlation function takes a more structured profile that includes a marked first peak due to homopolar bonds, a first maximum exhibiting a clear shoulder and a deep minimum, all these features being absent in the previous PW results. Overall, the amount of tetrahedral order is significantly increased, in spite of a larger number of Ge$-$Ge homopolar connections. Due to the smaller number of miscoordinations, diffusion coefficients obtained by the present BLYP calculation are smaller by at least one order of magnitude than in the PW case.Comment: 6 figure

Pinpointing dynamic coupling in enzymes for efficient drug design

Enzymes are proteins that catalyze almost every chemical reaction in living systems, achieving rate enhancements of up to 21 orders of magnitude relative to the uncatalyzed reactions. However, despite a century of intense investigation, the biophysical basis of the enormous catalytic power of enzymes is not completely understood. Enzymes are not only central to living systems, but also to many industrial processes such as the production of food, textiles, detergents, pharmaceuticals and other chemicals where environmentally friendly, green methods are of ever increasing importance. Because of their central role for life, enzymes are key drug targets and enzyme inhibition is a central strategy in the design of new drugs. Acetylsalicylic acid, azidothymidine, acyclovir, allopurinol, chloramphenicol, exemestane, fosfomycin, isoniazid, methotrexate, profens, proguanil, statins, thiouracil and warfarin are but a small subset of approved drug substances that are used in the clinic to treat, among others, pain, fever, inflammation, malaria, cancer, HIV, bacterial and viral infections, rheumatoid arthritis, osteoarthritis and heart disease, through the inhibition of key enzymes

Transition from a simple yield stress fluid to a thixotropic material

From MRI rheometry we show that a pure emulsion can be turned from a simple yield stress fluid to a thixotropic material by adding a small fraction of colloidal particles. The two fluids have the same behavior in the liquid regime but the loaded emulsion exhibits a critical shear rate below which no steady flows can be observed. For a stress below the yield stress, the pure emulsion abruptly stops flowing, whereas the viscosity of the loaded emulsion continuously increases in time, which leads to an apparent flow stoppage. This phenomenon can be very well represented by a model assuming a progressive increase of the number of droplet links via colloidal particles.Comment: Published in Physical Review E. http://pre.aps.org/abstract/PRE/v76/i5/e05140

Chemistry in Evaporating Ices: Unexplored Territory

We suggest that three-body chemistry may occur in warm high density gas evaporating in transient co\textendash desorption events on interstellar ices. Using a highly idealised computational model we explore the chemical conversion from simple species of the ice to more complex species containing several heavy atoms, as a function of density and of adopted three body rate coefficients. We predict that there is a wide range of densities and rate coefficients in which a significant chemical conversion may occur. We discuss the implications of this idea for the astrochemistry of hot cores.Comment: Accepted in Ap

Self-learning Kinetic Monte-Carlo method: application to Cu(111)

We present a novel way of performing kinetic Monte Carlo simulations which does not require an {\it a priori} list of diffusion processes and their associated energetics and reaction rates. Rather, at any time during the simulation, energetics for all possible (single or multi-atom) processes, within a specific interaction range, are either computed accurately using a saddle point search procedure, or retrieved from a database in which previously encountered processes are stored. This self-learning procedure enhances the speed of the simulations along with a substantial gain in reliability because of the inclusion of many-particle processes. Accompanying results from the application of the method to the case of two-dimensional Cu adatom-cluster diffusion and coalescence on Cu(111) with detailed statistics of involved atomistic processes and contributing diffusion coefficients attest to the suitability of the method for the purpose.Comment: 18 pages, 9 figure

The pattern of childhood in the western Cape

An analysis of poisoning cases treated at the Red Cross War Memorial Children's Hospital dUring 1987 and of calls received on the poisons line is presented. Treatment of 1116 children was undertaken and 922 telephone calls were logged. Of the patients treated, 60% had ingested a drug and 30% had drunk paraffin. The high prevalence of paraffin poisoning in the western Cape is examined. Constant vigilance must be maintained if childhood poisoning is to be prevented.S Afr Med J 1990; 78: 22-2

Quarterly Status Report on LAMPRE Program for Period Ending August 20, 1960

The LAMPRE-I project is summarized in terms of capsule development, sodium system, LCX-III, gas system, and fuel transfer and storage. The 2-Mw Sodium Test Facility was initially placed in operation on Mar. 15, 1960. Since startup, operation has been continuous except for 12 forced shutdowns required for maintenance of auxiliary steam-system equipment and repair of refractory setting in the gas-fired sodium heater. Essentially no maintenance or adjustment hns been required on sodium components or the steam generator. The steam- generating unit is illustrated and design heat-transfer criteria and observed heattransfer data are given. The intermediate sodium heat exchanger, centrifugal sodium pumps, gas-fired sodium heater, sodium flow-control valves, and purification cold trap are discussed briefly. Research and development activities for the LAMPRE Program are reported in the core-development program, core-test facility, mobileblanket studies, fuel-alloy development, container- materials development, and fuel processing. (For preceding period see LAMS- 2438.) (W.D.M.

Bottlenecks to vibrational energy flow in OCS: Structures and mechanisms

Finding the causes for the nonstatistical vibrational energy relaxation in the planar carbonyl sulfide (OCS) molecule is a longstanding problem in chemical physics: Not only is the relaxation incomplete long past the predicted statistical relaxation time, but it also consists of a sequence of abrupt transitions between long-lived regions of localized energy modes. We report on the phase space bottlenecks responsible for this slow and uneven vibrational energy flow in this Hamiltonian system with three degrees of freedom. They belong to a particular class of two-dimensional invariant tori which are organized around elliptic periodic orbits. We relate the trapping and transition mechanisms with the linear stability of these structures.Comment: 13 pages, 13 figure

The changing trends of childhood poisoning at a tertiary children’s hospital in South Africa

Context. Information on childhood poisoning in the developingworld, including South Africa, is scarce, despite its contribution tomorbidity and mortality.Objective. We describe the profile of children with exposuresand poisonings presenting to Red Cross War Memorial Children’sHospital (RCWMCH) in Cape Town, South Africa, from 2003 to2008 and compare the trends of causative agents over the past twodecades.Methods. Cases were identified by review of the RCWMCH caserecords.Results. Of the total incidents (N=2 872), paraffin (kerosene)was the commonest agent (n=692, 24%) with 124 poisoningsincluding two deaths. Drugs were the most common toxin group(n=988, 34%), including 139 single-drug poisonings with 5deaths; 4 associated with traditional medicine use. Householdcleaning product incidents (n=302, 10%) resulted in 29 singleproductpoisonings with no deaths. Pesticide incidents (n=311,10%) included 6 deaths; 203 (65%) incidents were due toorganophosphates or carbamates. The suburban distribution ofthe main toxin groups varied. Comparing 1987 and 2008, thenumber of incidents decreased from 1 116 to 447; drug and paraffinincidents decreased respectively (from 673 to 150 and from 332 to87), household cleaning products and cosmetics increased (21 to69) and pesticide incidents increased (7 to 69).Conclusion. Despite a decrease in the overall number of incidentsover two decades at RCWMCH, paraffin and drugs remainthe principal agents responsible for paediatric exposures andpoisonings, with increasing incidents due to household cleaningproducts and pesticides. Identification of these toxin groups comingfrom specific suburbs allows for targeted prevention initiatives