Effect of interface friction on the mechanics of indentation of a finite layer resting on a rigid substrate

Copper strips of 2.5 mm thickness resting on stainless steel anvils were normally indented by wedges under nominal plane strain conditions. Inflections in the hardness-penetration characteristics were identified. Inflections separate stages where each stage has typical mechanics of deformation. These are arrived at by studying the distortion of 0.125 mm spaced grids inscribed on the deformation plane of the strip. The sensitivity of hardness and deformation mechanics to wedge angle and the interfacial friction between strip and anvil were investigated within the framework of existing slip line field models of indentation of semi-infinite and finite blocks.

Dynamics of sheared inelastic dumbbells

We study the dynamical properties of the homogeneous shear flow of inelastic dumbbells in two dimensions as a first step towards examining the effect of shape on the properties of flowing granular materials. The dumbbells are modelled as smooth fused disks characterized by the ratio of the distance between centres (L) and the disk diameter (D), with an aspect ratio (L/D) varying between 0 and 1 in our simulations. Area fractions studied are in the range 0.1-0.7, while coefficients of normal restitution (e(n)) from 0.99 to 0.7 are considered. The simulations use a modified form of the event-driven methodology for circular disks. The average orientation is characterized by an order parameter S, which varies between 0 (for a perfectly disordered fluid) and 1 (for a fluid with the axes of all dumbbells in the same direction). We investigate power-law fits of S as a function of (L D) and (1 - e(n)(2)) There is a gradual increase in ordering as the area fraction is increased, as the aspect ratio is increased or as the coefficient of restitution is decreased. The order parameter has a maximum value of about 0.5 for the highest area fraction and lowest coefficient of restitution considered here. The mean energy of the velocity fluctuations in the flow direction is higher than that in the gradient direction and the rotational energy, though the difference decreases as the area fraction increases, due to the efficient collisional transfer of energy between the three directions. The distributions of the translational and rotational velocities are Gaussian to a very good approximation. The pressure is found to be remarkably independent of the coefficient of restitution. The pressure and dissipation rate show relatively little variation when scaled by the collision frequency for all the area fractions studied here, indicating that the collision frequency determines the momentum transport and energy dissipation, even at the lowest area fractions studied here. The mean angular velocity of the particles is equal to half the vorticity at low area fractions, but the magnitude systematically decreases to less than half the vorticity as the area fraction is increased, even though the stress tensor is symmetric.

Optimization of two-dimensional NMR by matched accumulation

It is well known that in the time-domain acquisition of NMR data, signal-to-noise (S/N) improves as the square root of the number of transients accumulated. However, the amplitude of the measured signal varies during the time of detection, having a functional form dependent on the coherence detected. Matching the time spent signal averaging to the expected amplitude of the signal observed should also improve the detected signal-to-noise. Following this reasoning, Barna et al. (J Magn. Reson.75, 384, 1987) demonstrated the utility of exponential sampling in one- and two-dimensional NMR, using maximum-entropy methods to analyze the data. It is proposed here that for two-dimensional experiments the exponential sampling be replaced by exponential averaging. The data thus collected can be analyzed by standard fast-Fourier-transform routines. We demonstrate the utility of exponential averaging in 2D NOESY spectra of the protein ubiquitin, in which an enhanced SIN is observed. It is also shown that the method acquires delayed double-quantum-filtered COSY without phase distortion.

Temperature and concentration dependence of adsorption properties of methane in NaY: a molecular dynamics study

Molecular dynamics calculations on methane sorbed in NaY (Si/Al = 3.0) employing realistic methane-methane and methane-zeolite intermolecular potential functions at different temperatures (50, 150, 220, and 300 K) and concentrations (2, 4, 6, and 8 molecules/cage) are reported. The thermodynamic results are in agreement with the available experimental data. Guest-guest and guest-host radial distribution functions (rdfs), energy distribution functions, distribution of cage occupancy, center-of-cage-center-of-mass (coc-com) rdfs, velocity autocorrelation functions for com and angular motion and the Fourier transformed power spectra, and diffusion coefficients are presented as a function of temperature and concentration. At 50 K, methane is localized near the adsorption site. Site-site migration and essentially free rotational motion are observed at 150 K. Molecules preferentially occupy the region near the inner surface of the alpha-cage. The vibrational frequencies for the com of methane shift toward higher values with decreasing temperature and increasing adsorbate concentration. The observed frequencies for com motion are 36, 53, and 85 cm-1 and for rotational motion at 50 K, 95 and 150 cm-1 in agreement with neutron scattering data. The diffusion coefficients show a type I behavior as a function of loading in agreement with NMR measurements. Cage-to-cage diffusion is found to be always mediated by the surface.

Nucleosome core histone complex isolated gently and rapidly in 2 M NaCl is octameric

A relatively stable specific complex of the chromatin core histones H2A, H2B, H3, and H4 has been obtained in 2 M NaCl/25 mM sodium phosphate buffer, pH 7.0. The histone core complex has an apparent specific volume of 0.73 ml/g. Its sedimentation coefficient was dependent on rotor speed (angular velocity, omega) and attained different stable values at low and high rotor speeds. The drop in sedimentation coefficient occurred sharply between omega 2 values of about 9 x 10(6) and 1.1 x 10(7) (radians/sec)2. The s020,w corresponding to zero angular velocity (1 atmosphere pressure) was 6.6 S +/- (SEM) 0.1 S. At high rotor speeds the value decreased to 3.8 S +/- 0.1 S. The core complex has a diffusion coefficient, D20,w, of 5.4 x 10(-7) cm2/sec and a molecular weight of 108,000 +/- (SD) 2500.

Wigner distribution for angle coordinates in quantum mechanics

The method of Wigner distribution functions, and the Weyl correspondence between quantum and classical variables, are extended from the usual kind of canonically conjugate position and momentum operators to the case of an angle and angular momentum operator pair. The sense in which one has a description of quantum mechanics using classical phase‐space language is much clarified by this extension.

Physical properties of a boreal clay soil under differently managed perennial vegetation

The physical properties of surface soil horizons, essentially pore size, shape, continuity and affinity for water, regulate water entry into the soil. These properties are prone to changes caused by natural forces and human activity. The hydraulic properties of the surface soil greatly impact the generation of surface runoff and accompanied erosion, the major concern of agricultural water protection. The general target of this thesis was to improve our understanding of the structural and hydraulic properties of boreal clay soils. Physical properties of a clayey surface soil (0 - 10 cm, clay content 51%), with a micaceous/illitic mineralogy subjected to three different management practices of perennial vegetation, were studied. The study sites were vegetated buffer zones located side by side in SW Finland: 1) natural vegetation with no management, 2) harvested once a year, and 3) grazed by cattle. The soil structure, hydraulic properties, shrinkage properties and soil water repellency were determined at all sites. Two distinct flow domains were evident. The surface soil was characterized by subangular blocky, angular blocky and platy aggregates. Hence, large, partially accommodated, irregular elongated pores dominated the macropore domain at all sites. The intra-aggregate pore system was mostly comprised of pores smaller than 30 μm, which are responsible for water storage. Macropores at the grazed site, compacted by hoof pressure, were horizontally oriented and pore connectivity was poorest, which decreased water and air flux compared with other sites. Drying of the soil greatly altered its structure. The decrease in soil volume between wet and dry soil was 7 - 10%, most of which occurred in the moisture range of field conditions. Structural changes, including irreversible collapse of interaggregate pores, began at matric potentials around -6 kPa indicating, instability of soil structure against increasing hydraulic stress. Water saturation and several freezethaw cycles between autumn and spring likely weakened the soil structure. Soil water repellency was observed at all sites at the time of sampling and when soil was dryer than about 40 vol.%. (matric potential < -6 kPa). Therefore, water repellency contributes to water flow over a wide moisture range. Water repellency was also observed in soils with low organic carbon content (< 2%), which suggests that this phenomenon is common in agricultural soils of Finland due to their relatively high organic carbon content. Aggregate-related pedofeatures of dense infillings described as clay intrusions were found at all sites. The formation of these intrusions was attributed to clay dispersion and/or translocation during spring thaw and drying of the suspension in situ. These processes generate very new aggregates whose physical properties are most probably different from those of the bulk soil aggregates. Formation of the clay infillings suggested that prolonged wetness in autumn and spring impairs soil structure due to clay dispersion, while on the other hand it contributes to the pedogenesis of the soil. The results emphasize the dynamic nature of the physical properties of clay soils, essentially driven by their moisture state. In a dry soil, fast preferential flow is favoured by abundant macropores including shrinkage cracks and is further enhanced by water repellency. Increase in soil moisture reduces water repellency, and swelling of accommodated pores lowers the saturated hydraulic conductivity. Moisture- and temperature-related processes significantly alter soil structure over a time span of 1 yr. Thus, the pore characteristics as well as the hydraulic properties of soil are time-dependent.

The chemistry of vetivalene-type vetivalene-type naturally-occurring sesquiterpenoids

Vetivalene, 1,4-dimetiiyl-6-isopropylnaphl1tlmlenc(1 ) represents a new sesquiterpene skelcton which 15 presumed to originate from eudesmane by a shift of the angular nicthyl group. Novel sesquiterpenm related to vetivalene have been isolated from plant sources ill ~s c e o ig ears. A suriey dealing xith the chemistry (structure, synthesis and configuration) of members of this interesting new class OF yesquiterpenes, coniprising occidol, risbitinol and the various ernmotinn is presented

The chemistry of vetivalene-type naturally occurring sesqufterpenoids

Vetivalene, 1,4-dimetiiyl-6-isopropylnaphl1tlmlenc(1 ) represents a new sesquiterpene skelcton which 15 presumed to originate from eudesmane by a shift of the angular nicthyl group. Novel sesquiterpenm related to vetivalene have been isolated from plant sources ill ~s c e o ig ears. A suriey dealing xith the chemistry (structure, synthesis and configuration) of members of this interesting new class OF yesquiterpenes, coniprising occidol, risbitinol and the various ernmotinn is presented.

Stereochemical studies on cyclic peptides. Part XI. Conformation of cyclic pentapeptides having intramolecular 3 leads to 1 hydrogen bonds.

Conformational analysis of cyclic pentapeptides having two intra-ring 3 leads to 1 hydrogen bonds has been carried out. It is found that the structure can easily be formed with trans planar peptide units without causing significant angular strain at the alpha-carbon atoms. Four different types of conformations designated Types I--IV are possible for the backbone structure. Details of these four types of conformations and also the accommodating possibility of these types for allglycyl and all-alanyl residues are presented. Three of the four types have relatively low energies for glycyl residues whereas the other one has a slightly higher energy. When alanyl residues are introduced at the five alpha-carbon atoms, the types that are energetically favourable depend upon the sequence of isomers. Energy calculations have also been carried out for the combinations of glycyl, L- and D-alanyl residues. The theoretical results are compared with available experimental observations both from solution and solid state studies.

On measurement of top polarization as a probe of t(t)over-bar production mechanisms at the LHC

In this note we demonstrate the use of top polarization in the study of t (t) over bar resonances at the LHC, in the possible case where the dynamics implies a non-zero top polarization. As a probe of top polarization we construct an asymmetry in the decay-lepton azimuthal angle distribution (corresponding to the sign of cos phi(l)) in the laboratory. The asymmetry is non-vanishing even for a symmetric collider like the LHC, where a positive z axis is not uniquely defined. The angular distribution of the leptons has the advantage of being a faithful top-spin analyzer, unaffected by possible anomalous tbW couplings, to linear order. We study, for purposes of demonstration, the case of a Z' as might exist in the little Higgs models. We identify kinematic cuts which ensure that our asymmetry reflects the polarization in sign and magnitude. We investigate possibilities at the LHC with two energy options: root s = 14TeV and root s = 7TeV, as well as at the Tevatron. At the LHC the model predicts net top quark polarization of the order of a few per cent for M-Z' similar or equal to 1200GeV, being as high as 10% for a smaller mass of the Z' of 700GeV and for the largest allowed coupling in the model, the values being higher for the 7TeV option. These polarizations translate to a deviation from the standard-model value of azimuthal asymmetry of up to about 4% (7%) for 14 (7) TeV LHC, whereas for the Tevatron, values as high as 12% are attained. For the 14TeV LHC with an integrated luminosity of 10 fb(-1), these numbers translate into a 3 sigma sensitivity over a large part of the range 500 less than or similar to M-Z' less than or similar to 1500GeV.

An EXAFS investigation of the Zn environment in K2ZnCl4 during solid-to-solid phase transformations

Extended X-ray absorption fine structure (EXAFS) spectroscopy is applied to an investigation of the structural environment around Zn in polycrystalline K2ZnCi4 over the temperature range associated with its solid-to-solid phase transformations at 127 degrees C and 282 degrees C. The results show a reversible increase in thermal disorder and in the tetrahedral distortion of the ZnCl42- anion upon transformation into the incommensurate phase.

Time-Dependent Dynamics of a Planar Galaxy Model

Time-dependent models of collisionless stellar systems with harmonic potentials allowing for an essentially exact analytic description have recently been described. These include oscillating spheres and spheroids. This paper extends the analysis to time-dependent elliptic discs. Although restricted to two space dimensions, the systems are richer in that their parameters form a 10-dimensional phase space (in contrast to six for the earlier models). Apart from total energy and angular momentum, two additional conserved quantities emerge naturally. These can be chosen as the areas of extremal sections of the ellipsoidal region of phase space occupied by the system (their product gives the conserved volume). The present paper describes the construction of these models. An application to a tidal encounter is given which allows one to go beyond the impulse approximation and demonstrates the effects of rotation of the perturbed system on energy and angular-momentum transfer. The angular-momentum transfer is shown to scale inversely as the cube of the encounter velocity for an initial configuration of the perturbed galaxy with zero quadrupole moment.

Effect of Turbulence on Emerging Magnetic Flux Tubes in the Convection Zone

Working under the hypothesis that magnetic flux in the sun is generated at the bottom of the convection zone, Choudhuri and Gilman (1987; Astrophys. J. 316, 788) found that a magnetic flux tube symmetric around the rotation axis, when released at the bottom of the convection zone, gets deflected by the Coriolis force and tends to move parallel to the rotation axis as it rises in the convection zone. As a result, all the flux emerges at rather high latitudes and the flux observed at the typical sunspot latitudes remains unexplained. Choudhuri (1989; Solar Physics, in press) finds that non-axisymmetric perturbations too cannot subdue the Coriolis force. In this paper, we no longer treat the convection zone to be passive as in the previous papers, but we consider the role of turbulence in the convection zone in inhibiting the Coriolis force. The interaction of the flux tubes with the turbulence is treated in a phenomenological way as follows: (1) Large scale turbulence on the scale of giant cells can physically drag the tubes outwards, thus pulling the flux towards lower latitudes by dominating over the Coriolis force. (2) Small scale turbulence of the size of the tubes can exchange angular momentum with the tube, thus suppressing the growth of the Coriolis force and making the tubes emerge at lower latitudes. Numerical simulations show that the giant cells can drag the tubes and make them emerge at lower latitufes only if the velocities within the giant cells are unrealistically large of if the radii of the flux tubes are as small as 10 km. However, small scale turbulence can successfully suppress the growth of the Coriolis force if the tubes have radii smaller than about 300 km which may not be unreasonable. Such flux tubes can then emerge at low latitudes where sunspots are seen.

Molecular Mechanics Studies of Homopolymeric and Mixed Sequence Py.Pu*Pu Triple Helices,

DNA triple helices containing two purine strands and one pyrimidine strand (C.G*G and T.A*A) have been studied, using model building followed by energy minimisation, for different orientations of the third strand resulting from variation in the hydrogen bonding between the Watson-Crick duplex and the third strand and the glycosidic torsion angle in the third strand. Our results show that in the C.G*G case the structure with a parallel orientation of the third strand, resulting from Hoogsteen hydrogen bonds between the third strand and the Watson-Crick duplex, is energetically the most favourable while in the T.A*A case the antiparallel orientation of the third strand, resulting from reverse Hoogsteen hydrogen bonds, is energetically the most favourable. These studies when extended to the mixed sequence triplexes, in which the second strand is a mixture of G and A, correspondingly the third strand is a mixture of G and APT, show that though the parallel orientation is still energetically more favourable, the antiparallel orientation becomes energetically comparable with an increasing number of thymines in the third strand. Structurally, for the mixed triplexes containing G and T in the third strand, it is seen that the basepair non-isomorphism between the C.G*G and the T.A*T triplets can be overcome with some changes in the base pair parameters without much distortion of either the backbone or the hydrogen bonds.