Evaluation of bis-GMA/MMA resin adhesion to silica-coated and silanized titanium

The effects of pH value and alcohol solvent type of a silane solution on the bonding of an experimental resin to the silica-coated titanium (Ti) surface were studied. First, Ti surfaces underwent tribochemical Rocatec ™ treatment followed by silanization of the surface with 3-methacryloxypropyltrimethoxysilane (MPS). Then, resin stubs based on a mixture of bisphenol-A-glycidyl dimethacrylate and methyl methacrylate were bonded and light-cured onto each silica-coated Ti surface (n = 6 per group). Two different solvents for MPS, namely iso-propanol (i-PrOH)/H2O and ethanol (EtOH)/H2O were used, at pH values of 4.5, 5.0, and 5.5, and shear bond strengths were tested both under dry storage conditions and after water sorption induced by accelerated aging (i.e. thermo-cycling). The shear bond strengths were also re-determined after the silane solutions had been stored at 4°C for 15 weeks before the silanization step. For dry samples, the shear bond strengths ranged from 7.5 to 10.6 MPa (ANOVA, p < 0.05) when the Ti surface had been silanized with MPS in i-PrOH/H2O, and from 6.5 to 12.4 MPa (ANOVA, p < 0.05) when the Ti surface had been silanized with MPS in EtOH/H2O at pH 4.5. Fifteen weeks of storage of the silane solution increased the shear bond strength of dry samples by ca. 1-4 MPa per test group. In contrast, thermo-cycling reduced the shear bond strength in both solvent systems. The weight of the test sample stubs increased by ca. 3.5 wt% after 187 days of being subjected to the water sorption test. © 2009 VSP.postprin

Susceptibility and Percolation in 2D Random Field Ising Magnets

The ground state structure of the two-dimensional random field Ising magnet is studied using exact numerical calculations. First we show that the ferromagnetism, which exists for small system sizes, vanishes with a large excitation at a random field strength dependent length scale. This {\it break-up length scale} $L_b$ scales exponentially with the squared random field, $\exp(A/\Delta^2)$. By adding an external field $H$ we then study the susceptibility in the ground state. If $L>L_b$, domains melt continuously and the magnetization has a smooth behavior, independent of system size, and the susceptibility decays as $L^{-2}$. We define a random field strength dependent critical external field value $\pm H_c(\Delta)$, for the up and down spins to form a percolation type of spanning cluster. The percolation transition is in the standard short-range correlated percolation universality class. The mass of the spanning cluster increases with decreasing $\Delta$ and the critical external field approaches zero for vanishing random field strength, implying the critical field scaling (for Gaussian disorder) $H_c \sim (\Delta -\Delta_c)^\delta$, where $\Delta_c = 1.65 \pm 0.05$ and $\delta=2.05\pm 0.10$. Below $\Delta_c$ the systems should percolate even when H=0. This implies that even for H=0 above $L_b$ the domains can be fractal at low random fields, such that the largest domain spans the system at low random field strength values and its mass has the fractal dimension of standard percolation $D_f = 91/48$. The structure of the spanning clusters is studied by defining {\it red clusters}, in analogy to the ``red sites'' of ordinary site-percolation. The size of red clusters defines an extra length scale, independent of $L$.Comment: 17 pages, 28 figures, accepted for publication in Phys. Rev.

Three-dimensional molecular dynamics simulations of void coalescence during dynamic fracture of ductile metals

Void coalescence and interaction in dynamic fracture of ductile metals have been investigated using three-dimensional strain-controlled multi-million atom molecular dynamics simulations of copper. The correlated growth of two voids during the coalescence process leading to fracture is investigated, both in terms of its onset and the ensuing dynamical interactions. Void interactions are quantified through the rate of reduction of the distance between the voids, through the correlated directional growth of the voids, and through correlated shape evolution of the voids. The critical inter-void ligament distance marking the onset of coalescence is shown to be approximately one void radius based on the quantification measurements used, independent of the initial separation distance between the voids and the strain-rate of the expansion of the system. The interaction of the voids is not reflected in the volumetric asymptotic growth rate of the voids, as demonstrated here. Finally, the practice of using a single void and periodic boundary conditions to study coalescence is examined critically and shown to produce results markedly different than the coalescence of a pair of isolated voids.Comment: Accepted for publication in Physical Review

The superconducting strand for the CMS solenoid conductor

The Compact Muon Solenoid (CMS) is one of the general-purpose detectors to be provided for the LHC project at CERN. The design field of the CMS superconducting magnet is 4 T, the magnetic length is 12.5 m and the free bore is 6 m. Approximately 2000 km of superconducting strand is under procurement for the conductor of the CMS superconducting solenoid. Each strand length is required to be an integral multiple of 2.75 km. The strand is composed of copper- stabilized multifilamentary Nb-Ti with Nb barrier. Individual strands are identified by distinctive patterns of Nb-Ti filaments selected during stacking of the monofilaments. The statistics of piece length, measurements of I/sub c/, n-value, copper RRR, (Cu+Nb)/Nb-Ti ratio, as well as the results of independent cross checks of these quantities, are presented. A study was performed on the CMS strands to investigate the critical current degradation due to various heat treatments. The degradation versus annealing temperature and duration are reported. (4 refs)

Nonlinear Dynamics of Aeolian Sand Ripples

We study the initial instability of flat sand surface and further nonlinear dynamics of wind ripples. The proposed continuous model of ripple formation allowed us to simulate the development of a typical asymmetric ripple shape and the evolution of sand ripple pattern. We suggest that this evolution occurs via ripple merger preceded by several soliton-like interaction of ripples.Comment: 6 pages, 3 figures, corrected 2 typo

Open string theory and planar algebras

In this note we show that abstract planar algebras are algebras over the topological operad of moduli spaces of stable maps with Lagrangian boundary conditions, which in the case of the projective line are described in terms of real rational functions. These moduli spaces appear naturally in the formulation of open string theory on the projective line. We also show two geometric ways to obtain planar algebras from real algebraic geometry, one based on string topology and one on Gromov-Witten theory. In particular, through the well known relation between planar algebras and subfactors, these results establish a connection between open string theory, real algebraic geometry, and subfactors of von Neumann algebras.Comment: 13 pages, LaTeX, 7 eps figure

Phytoplankton pigments and dissolved organic matter distribution in the Gulf of Riga

http://www.elsevier.com/locate/issn/09247963The results of field studies of phytoplankton and dissolved organic matter DOM.in the Gulf of Riga in August-September 1993, June 1994 and April 1995 are presented. Actively excited fluorescence in UV- and visible spectral regions was used to investigate spatial distributions of DOM, Chl a, carotenoids and phycoerythrin in surface water. The fluorescent data were collected by means of laser remote sensing spectrometer LIDAR., spectrofluorometers in underway flow-through mode and in the measurements of the water samples. Pronounced variable spatial structures of phytoplankton were observed in spring 1995 and late summer 1993, while in early summer 1994 the pigment distribution was rather homogeneous. The spatial modulation of high-resolution profiles of phytoplankton with the period 3.5-5 km was detected across the gulf in all seasons probably caused by variability of hydrophysical conditions. A negative correlation of DOM concentration in surface waters and salinity was revealed in August-September 1993 and June 1994

Statistical Physics of Fracture Surfaces Morphology

Experiments on fracture surface morphologies offer increasing amounts of data that can be analyzed using methods of statistical physics. One finds scaling exponents associated with correlation and structure functions, indicating a rich phenomenology of anomalous scaling. We argue that traditional models of fracture fail to reproduce this rich phenomenology and new ideas and concepts are called for. We present some recent models that introduce the effects of deviations from homogeneous linear elasticity theory on the morphology of fracture surfaces, succeeding to reproduce the multiscaling phenomenology at least in 1+1 dimensions. For surfaces in 2+1 dimensions we introduce novel methods of analysis based on projecting the data on the irreducible representations of the SO(2) symmetry group. It appears that this approach organizes effectively the rich scaling properties. We end up with the proposition of new experiments in which the rotational symmetry is not broken, such that the scaling properties should be particularly simple.Comment: A review paper submitted to J. Stat. Phy