I.M. Irurzun

Theoretical study of pattern formation during the catalytic oxidation of CO on Pt{100} at low pressures

Theoretical studies have thus far been unable to model pattern formation during the reaction in this system on physically feasible length and time scales. In this paper, we derive a computational reaction-diffusion model for this system in which most of the input parameters have been determined experimentally. We model the surface on a mesoscopic scale intermediate between the microscopic size of CO islands and the macroscopic length scale of pattern formation. In agreement with experimental investigations [M. Eiswirth et al., Z. Phys. Chem., Neue Folge 144, 59 (1985)], the results from our model divide the CO and O(2) partial pressure parameter space into three regions defined by the level of CO coverage or the presence of sustained oscillations. We see CO fronts moving into oxygen-covered regions, with the 1 x 1 to hex phase change occurring at the leading edge. There are also traveling waves consisting of successive oxygen and CO fronts that move into areas of relatively high CO coverage, and in this case, the phase change is more gradual and of lower amplitude. The propagation speed of these reaction waves is similar to those observed experimentally for CO and oxygen fronts [H. H. Rotermund et al., J. Chem. Phys. 91, 4942 (1989); H. H. Rotermund et al., Nature (London) 343, 355 (1990); J. Lauterbach and H. H. Rotermund, Surf. Sci. 311, 231 (1994)]. In the two-dimensional version of our model, the traveling waves take the form of target patterns emitted from surface inhomogeneities.

Increase in the embedding dimension in the heart rate variability associated with left ventricular abnormalities

In the present study, the authors report evidence that the existence of premature ventricular contractions increases the embedding dimension of the cardiac dynamics. They also analyze patients with congestive heart failure, a severe clinical condition associated with abnormal left ventricular function. Results also show an increase in the embedding dimension of the heart rate variability. They used electrocardiograms collected by themselves with quality standards that make them comparable with other databases.

Dynamics properties of polymer solutions (I). Dilute-semidilute transition

Measurements of viscosity and dielectric relaxation were carried out using several solutions of cellulose trinitrate polymer in isophorone in the dilute and the semidilute regime. In each case, the renormalization group theory was applied to the data using the De Genness blobs model for connecting dynamic and conformational quantities. There was adequate agreement between the experimental results and the theoretical predictions to show universal behavior. The values of β were independent of both molecular weight and polymer concentration and could be used to describe a variety of dynamic properties. These findings suggest that only one scaling variable is necessary for such purposes.

How to find the sunrise and sunset times from a Sun clock and calendar

The purpose of this article is to describe the construction of a Sun clock and calendar (SCandC) that will allow an observer to not only see the time but also the symmetry properties of the Sun–Earth relative movement. A set of circles drawn on the SCandC will allow the observer to see their associated dates as well as to perform a visual interpolation between any pair of consecutive circles to estimate an intermediate date. By introducing the sunrise and sunset horizon lines in the SCandC the observer will be able to find the sunrise and sunset times during most of the year. The observer will also be able to appreciate the difference in time duration between the spring–summer and autumn–winter periods, as a consequence of Keplers second law, as well as to observe that there is a small difference between the circle radii of equidistant dates from the solstices in the spring–summer versus autumn–winter periods as a consequence also of Keplers second law. The equations derived in the present article will be tested against data from a particular lighthouse.