Transition from static to kinetic friction: insights from a 2D model

We describe a 2D spring-block model for the transition from static to kinetic friction at an elastic-slider-rigid-substrate interface obeying a minimalistic friction law (Amontons-Coulomb). By using realistic boundary conditions, a number of previously unexplained experimental results on precursory microslip fronts are successfully reproduced. From the analysis of the interfacial stresses, we derive a prediction for the evolution of the precursor length as a function of the applied loads, as well as an approximate relationship between microscopic and macroscopic friction coefficients. We show that the stress buildup due to both elastic loading and microslip-related relaxations depends only weakly on the underlying shear crack propagation dynamics. Conversely, crack speed depends strongly on both the instantaneous stresses and the friction coefficients, through a nontrivial scaling parameter.     

Activation of the pre-supplementary motor area but not inferior prefrontal cortex in association with short stop signal reaction time – an intra-subject analysis

Background  

Our previous work described the neural processes of motor response inhibition during a stop signal task (SST). Employing the race model, we computed the stop signal reaction time (SSRT) to index individuals' ability in inhibitory control. The pre-supplementary motor area (preSMA), which shows greater activity in individuals with short as compared to those with long SSRT, plays a role in mediating response inhibition. In contrast, the right inferior prefrontal cortex (rIFC) showed greater activity during stop success as compared to stop error. Here we further pursued this functional differentiation of preSMA and rIFC on the basis of an intra-subject approach.     

Long-range transport of trace elements to Southern Norway studied by INAA of air filters

The present work is a multi-element study of air concentrations at Birkenes, Southern Norway, based on daily filter samples during 1985. Sixteen elements are determined regularly by INAA; i.e. Na, Al, Cl, Sc, V, Cr, Mn, Fe, Co, Zn, As, Se, Br, Ag, Sb, La. In addition sulfate is determined by ion chromatography and Cu, Ni, Cd and Pb by electrothermal AAS. The main aim of the study is to see whether the air concentrations of long-range transported pollutants have changed significantly since 1979 when a similar study was last performed. Analysis of inter-element correlations in the two data series serves to define four main aerosol compounds: One general pollution, one marine, one soil, and one fossil-fuel component. Classification of air samples with respect to emission areas according to calculated air trajectories indicates from the limited number of 1985 samples analyzed so far that the contribution of air pollutants from Eastern Europe is at least as high as in 1979 while that from Western Europe has decreased. This decrease is evident for Pb in particular.     

Groundwater management: Waiting for a drought*

In this article we present a stylized model for optimal management of an unconfined groundwater resource when the threat of drought exists. The drought is modeled as a stochastic event that hits at an uncertain date and two benchmark management policies are investigated: (a) A policy of optimal dynamic management ignoring the threat of drought; and (b) an economically optimal policy that accounts for the threat of a drought. We show that the optimal predrought steady‐state equilibrium stock size of groundwater under policy b is larger than that under policy (a) Furthermore, we show that an increase in the probability of a drought gives rise to two counteracting effects: One in the direction of a larger predrought steady‐state equilibrium stock size (a recovery effect) and one in the direction of a lower predrought steady‐state equilibrium stock (an extinction effect). We find that the recovery effect dominates the extinction effect. Recommendations for Resource Managers: We analyze two groundwater extraction policies that can be used when a threat of drought exists: (a) Dynamic optimal management ignoring the threat of drought; and (b) dynamic optimal management taking the threat of drought into account. We show that the predrought steady‐state equilibrium stock size of water should be larger under the policy (b) than under policy (a). This conclusion has three implications for resource managers:

• Current groundwater management should take future extraction possibilities into account.

• A resource manager ought to take the threat of drought into account in groundwater management.

• A buffer stock of water should be built‐up before the drought to be drawn upon during the event.

     

Assessing toxicity and mobilisation of impregnation salts at a contaminated site

Severe soil contamination is often encountered at wood-impregnation plants due to spills, dripping and deposition of sludge associated with dissolved salts of copper, chromium and arsenic (CCA). Soil samples from a CCA-plant in southern Norway were analysed via a factorial extraction design to investigate mobilisation of contaminated soils. Various concentrations of organic acids, sea-salts, and pH showed that contaminants were not stable, and could be mobilised to the aqueous phase. To further investigate mobilisation of impregnation salts, soil solution collectors were installed at various depths at the site. Concentrations varied considerably. Hydrological changes revealed elevated levels of dissolved salts, which agree with the factorial experiment. Soil chemical processes (not total solid-phase concentrations) dominated the mobilisation and subsequent leaching. Soil solutions were tested for changes in toxicity by chemical analysis and degree of inhibition of luminescence in Vibrio fisheri (Microtox). Changes in toxicity corresponded to changes in soil solution.     

Microscale immunoaffinity SPE and MEKC in fast determination of testosterone in male urine

Conventional methods for the determination of testosterone in body fluids typically suffer from poor recovery, lack of specificity, complex sample pretreatment, or the need for derivatization. Here, a simple, specific, and fast analysis method for testosterone was developed, with a methodology based on testosterone-specific immunoaffinity SPE (IA-SPE) and subsequent analysis by partial filling MEKC (PF-MEKC). An immunosorbent consisting of a recombinant antitestosterone Fab fragment covalently attached to activated Sepharose was prepared. IA-SPE and PF-MEKC were set up in hyphenated and off-line constructions, and the applicability of the two constructions in analysis of testosterone in male urine was investigated. The results obtained with the hyphenated construction proved to be only indicative of the presence of testosterone. The off-line IA-SPE and PF-MEKC construction, however, was successfully used in the determination of free testosterone in male urine samples after enzymatic hydrolysis of the glucuronide conjugates. Except for the hydrolysis reaction, no sample pretreatment was required. After hydrolysis, the overall analysis time per sample was only 14 min. The off-line IA-SPE and PF-MEKC method proved to be robust, sensitive (LOQ 35 mug/L), and specific, enabling separation of testosterone from four related steroids. Thus, it provides attractive features when compared to traditional methods for determination of testosterone in male urine.     

On the absence of a density effect for inner shell ionization

It is shown that the observed absence of a density effect in inner shell ionization in solids is due to the fact that the required momentum transfer has a wavelength of the order of, or smaller than, the lattice constant, and thus cannot be transmitted to the lattice. This leads to a cut-off in the dielectric function corresponding to electronic binding energies above 1—1.5 keV, depending on the lattice constant. For more strongly bound electrons there shoul be no density effect.     

On the semiclassical impulse approximation for electron capture in asymmetric ion-atom collisions

A derivation of the impulse approximation for the capture of a targetK-shell electron by a light projectile in ion-atom collisions is given in the framework of the semiclassical approximation. The impact-parameter dependence of the capture probability is calculated numerically without further approximations, and shows good agreement with recent experimental results for protons colliding with Ne and Ar. The validity of several peaking approximations and the relation to ionisation theories is briefly discussed.     

Space-time in ordered electromagnetic radiation outside a massive plane

The Einstein-Maxwell equations are solved with an energy-momentum tensor corresponding to the field of a standing electromagnetic wave outside a massive plane, in the limit where the test particle is much larger than the wavelength of the electromagnetic field. Physical properties of the solution are discussed.