Research on pressure sensors for biomedical instruments

The development of a piezo-resistive pressure transducer is discussed suitable for recording pressures typically encountered in biomedical applications. The pressure transducer consists of a thin silicon diaphragm containing four strain-sensitive resistors, and is fabricated using silicon monolithic integrated-circuit technology. The pressure transducers can be as small as 0.7 mm outer diameter, and are, as a result, suitable for mounting at the tip of a catheter. Pressure-induced stress in the diaphragm is sensed by the resistors, which are interconnected to form a Wheatstone bridge

Some Finite Size Effects in Simulations of Glass Dynamics

We present the results of a molecular dynamics computer simulation in which we investigate the dynamics of silica. By considering different system sizes, we show that in simulations of the dynamics of this strong glass former surprisingly large finite size effects are present. In particular we demonstrate that the relaxation times of the incoherent intermediate scattering function and the time dependence of the mean squared displacement are affected by such finite size effects. By compressing the system to high densities, we transform it to a fragile glass former and find that for that system these types of finite size effects are much weaker.Comment: 12 pages of RevTex, 4 postscript figures available from W. Ko

Observations of Fallout from the Fukushima Reactor Accident in San Francisco Bay Area Rainwater

We have observed fallout from the recent Fukushima Dai-ichi reactor accident in samples of rainwater collected in the San Francisco Bay area. Gamma ray spectra measured from these samples show clear evidence of fission products - 131,132I, 132Te, and 134,137Cs. The activity levels we have measured for these isotopes are very low and pose no health risk to the public.Comment: 5 pages, 2 figure

The relationship between fragility, configurational entropy and the potential energy landscape of glass forming liquids

Glass is a microscopically disordered, solid form of matter that results when a fluid is cooled or compressed in such a fashion that it does not crystallise. Almost all types of materials are capable of glass formation -- polymers, metal alloys, and molten salts, to name a few. Given such diversity, organising principles which systematise data concerning glass formation are invaluable. One such principle is the classification of glass formers according to their fragility\cite{fragility}. Fragility measures the rapidity with which a liquid's properties such as viscosity change as the glassy state is approached. Although the relationship between features of the energy landscape of a glass former, its configurational entropy and fragility have been analysed previously (e. g.,\cite{speedyfr}), an understanding of the origins of fragility in these features is far from being well established. Results for a model liquid, whose fragility depends on its bulk density, are presented in this letter. Analysis of the relationship between fragility and quantitative measures of the energy landscape (the complicated dependence of energy on configuration) reveal that the fragility depends on changes in the vibrational properties of individual energy basins, in addition to the total number of such basins present, and their spread in energy. A thermodynamic expression for fragility is derived, which is in quantitative agreement with {\it kinetic} fragilities obtained from the liquid's diffusivity.Comment: 8 pages, 3 figure

Coarse-grained microscopic model of glass formers

We introduce a coarse-grained model for atomic glass formers. Its elements are physically motivated local microscopic dynamical rules parameterized by observables. Results of the model are established and used to interpret the measured behaviors of supercooled fluids approaching glass transitions. The model predicts the presence of a crossover from hierarchical super-Arrhenius dynamics at short length scales to diffusive Arrhenius dynamics at large length scales. This prediction distinguishes our model from other theories of glass formers and can be tested by experiment.Comment: 5 pages, 5 figure

Microscopic theory of network glasses

A molecular theory of the glass transition of network forming liquids is developed using a combination of self-consistent phonon and liquid state approaches. Both the dynamical transition and the entropy crisis characteristic of random first order transitions are mapped out as a function of the degree of bonding and the density. Using a scaling relation for a soft-core model to crudely translate the densities into temperatures, the theory predicts that the ratio of the dynamical transition temperature to the laboratory transition temperature rises as the degree of bonding increases, while the Kauzmann temperature falls relative to the laboratory transition. These results indicate why highly coordinated liquids should be "strong" while van der Waals liquids without coordination are "fragile".Comment: slightly revised version that has been accepted for publication in Phys. Rev. Let

Dynamic Transitions in a Two Dimensional Associating Lattice Gas Model

Using Monte Carlo simulations we investigate some new aspects of the phase diagram and the behavior of the diffusion coefficient in an associating lattice gas (ALG) model on different regions of the phase diagram. The ALG model combines a two dimensional lattice gas where particles interact through a soft core potential and orientational degrees of freedom. The competition between soft core potential and directional attractive forces results in a high density liquid phase, a low density liquid phase, and a gas phase. Besides anomalies in the behavior of the density with the temperature at constant pressure and of the diffusion coefficient with density at constant temperature are also found. The two liquid phases are separated by a coexistence line that ends in a bicritical point. The low density liquid phase is separated from the gas phase by a coexistence line that ends in tricritical point. The bicritical and tricritical points are linked by a critical $\lambda$-line. The high density liquid phase and the fluid phases are separated by a second $\tau$ critical line. We then investigate how the diffusion coefficient behaves on different regions of the chemical potential-temperature phase diagram. We find that diffusivity undergoes two types of dynamic transitions: a fragile-to-strong trans ition when the critical $\lambda$-line is crossed by decreasing the temperature at a constant chemical potential; and a strong-to-strong transition when the $\tau$-critical line is crossed by decreasing the temperature at a constant chemical potential.Comment: 22 page

What does the potential energy landscape tell us about the dynamics of supercooled liquids and glasses?

For a model glass-former we demonstrate via computer simulations how macroscopic dynamic quantities can be inferred from a PEL analysis. The essential step is to consider whole superstructures of many PEL minima, called metabasins, rather than single minima. We show that two types of metabasins exist: some allowing for quasi-free motion on the PEL (liquid-like), the others acting as traps (solid-like). The activated, multi-step escapes from the latter metabasins are found to dictate the slowing down of dynamics upon cooling over a much broader temperature range than is currently assumed

Liquid crystal phase and waterlike anomalies in a core-softened shoulder-dumbbells system

Using molecular dynamics we investigate the thermodynamics, dynamics and structure of 250 diatomic molecules interacting by a core-softened potential. This system exhibits thermodynamics, dynamics and structural anomalies: a maximum in density-temperature plane at constante pressure and maximum and minimum points in the diffusivity and translational order parameter against density at constant temperature. Starting with very dense systems and decreasing density the mobility at low temperatures first increases, reach a maximum, then decreases, reach a minimum and finally increases. In the pressure-temperature phase diagram the line of maximum translational order parameter is located outside the line of diffusivity extrema that is enclosing the temperature of maximum density line. We compare our results with the monomeric system showing that the anisotropy due to the dumbbell leads to a much larger solid phase and to the appearance of a liquid crystal phase. the double ranged thermodynamic and dynamic anomalies.Comment: 14 pages, 5 figure

Damage spreading in the mode-coupling equations for glasses

We examine the problem of damage spreading in the off-equilibrium mode coupling equations. The study is done for the spherical $p$-spin model introduced by Crisanti, Horner and Sommers. For $p>2$ we show the existence of a temperature transition $T_0$ well above any relevant thermodynamic transition temperature. Above $T_0$ the asymptotic damage decays to zero while below $T_0$ it decays to a finite value independent of the initial damage. This transition is stable in the presence of asymmetry in the interactions. We discuss the physical origin of this peculiar phase transition which occurs as a consequence of the non-linear coupling between the damage and the two-time correlation functions.Comment: 5 pages, 2 figures, Revtex fil