Analogies with optical bistability in ferroelectrics

Mathematical analogies are developed between the formalism used to describe regenerative pulsations accompanying optical bistability in ferroelectrics and that used to parametrize the transition from steady flow to laminar vortex generation in viscous fluids. An “effective Reynolds number” and an “effective specific viscosity” are both defined for the optical bistability model. The characteristic length in the problem is defined in a way similar to that in Abrikosov's theory of Type II superconductors.     

Ventilatory management in the postanesthesia care unit

Postoperative ventilatory depression is common in patients who have received intravenous and inhalational anesthetic agents. Prompt assessment and treatment of ventilatory depression are essential to minimize morbidity and mortality.     

A mutation spectra database for bacterial and mammalian genes: 1998

This database consists of over 24 000 mutations in 18 viral, bacterial, yeast or mammalian genes. The data are grouped as sets of DNA base sequence changes or spectra caused by a particular mutagen under defined conditions. The spectra are available on the World Wide Web at http://info.med.yale.edu/mutbase/ in two formats; in text format that can be browsed on-line or downloaded for use with a text editor and in dBASEIII format for use, after downloading, by relational database programs or by spreadsheets. Researchers are encouraged to submit DNA sequence changes to a suitable mutation database such as ours. A data entry program, MUTSIN, can be retrieved from this site. MUTSIN diagrams each mutation on the computer screen and alerts the user to any discrepancies.     

Surface mass transport of alumina

The kinetics of thermal grooving at the intersection of rhombohedral twin boundaries with the \((10\bar 10)\) plane in aluminium oxide were measured from 1773 to 2273 K. Analysis of the data using the model of Mullins showed that surface diffusion was the dominant mechanism for mass transport. The results were compared with other similar published work on alumina, and the following equation for surface diffusion was determined: $$D_s (cm^2 sec^{ - 1} ) = 4.05 x 10^5 exp - (452kJ mol^{ - 1} /RT).$$      

Autocatalytic reactions in the isothermal,continuous stirred tank reactor: Isolas and other forms of multistability

Autocatalytic reactions are often complicated, and analyses of their behaviour in open systems can seem too particular to permit useful generalisation. We study here the simplest of circumstances (uniform temperatures and concentrations in the isothermal CSTR) and the simplest of reaction schemes: (i) quadratic autocatalysis (A + B→2B); and (ii) cubic autocatalysis (A + 2B→3B). The catalyst B may be stable or have a finite lifetime (B→ inert products). Allowing for this finite lifetime adds another dimension to the interest.The phenomena encountered include multistability, hysteresis, critical extinctions, critical ignitions, and anomalous relaxation times (though infinite values do not arise). Patterns of stationary states as function of residence time can show isolas and mushrooms. All these aspects yield to simple algebraic analysis. The presence of the catalyst B in the inflow can make qualitative differences of a kind parallelled by an additional, non-catalytic reaction of the same stoichiometry (e.g. A→B). Invoking the reversibility of the reactions neither increases nor diminishes their variety, and thermodynamic considerations have little to do with the many different patterns of reactivity displayed.The local stability of the various stationary states has also been characterized. Quadratic autocatalysis shows limited variety (stable node, stable focus); cubic autocatalysis generates all the kinds of stationary state possible in a two-variable system. Again all the algebra is straightforward if not always simple. Sustained oscillatory behavior (limit cycles) also occur.All these remarks relate to isothermal systems, but there are the most striking parallels between isothermal autocatalysis and the exothermic, first-order reaction in the CSTR. Behaviour with an autocatalyst of complete stability corresponds to perfect heat insulation (adiabatic operation) in the non-isothermal, exothermic system.     

Mechanisms of antioxidant action: The effect of spin traps during the processing and photooxidation of PVC

It is demonstrated by the use of spin traps that during the early stages of thermal processing of PVC considerable concentrations (> 3 × 10^?6 mol g^?1) of free radicals are produced which are primarily responsible for the initial products (hydroperoxides, unsaturation, and hydrogen chloride) previously shown to be formed in the polymer. From a semiquantitative analysis of these products, it is estimated that more than 50% of the radicals are formed from hydroperoxides and the rest by mechanoscission of the polymer chain. The spin traps are effective processing stabilizers in combination with a tin maleate HC1 scavenger. One of them (2-methyl-2-nitroso propane, MNP) has also been shown to be a photoantioxidant.