Biomedical subjects

R J Wijngaarden

Publications and source records attributed to R J Wijngaarden.

4 recordsLinked to original sources

PubMed2004

Extremal dynamics and the approach to the critical state: experiments on a three dimensional pile of rice.

The evolution of the growth of a ricepile is studied in three dimensions. With time, the pile approaches a critical state with a certain slope. Assuming extremal dynamics in the evolution of the pile, the way the critical state is approached is dictated by the scaling properties of the critical state itself. Experimentally, we determine the envelope of the maximal slope, which is a measure for the distance from the critical state, as well as the growth of the average avalanche size with time. These quantities obey power-law scaling, where the experimental exponents are in good agreement with those obtained from an earlier determination of the critical state properties and extremal dynamics. Furthermore, we discuss the influence of the transient state on the avalanche size distribution, which may have applications in the prevention of large avalanches in natural systems.

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PubMed2003

Avalanche dynamics, surface roughening, and self-organized criticality: Experiments on a three-dimensional pile of rice.

We present a two-dimensional system that exhibits features of self-organized criticality. The avalanches that occur on the surface of a pile of rice are found to exhibit finite size scaling in their probability distribution. The critical exponents are tau=1.21(2) for the avalanche size distribution and D=1.99(2) for the cutoff size. Furthermore, the geometry of the avalanches is studied, leading to a fractal dimension of the active sites of d(B)=1.58(2). Using a set of scaling relations, we can calculate the roughness exponent alpha=D-d(B)=0.41(3) and the dynamic exponent z=D(2-tau)=1.56(8). This result is compared with that obtained from a power-spectrum analysis of the surface roughness, which yields alpha=0.42(3) and z=1.5(1) in excellent agreement with those obtained from the scaling relations.

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PubMed2003

Refraction and reflection of diffusion fronts.

Diffusion waves form the basis of several measurement technologies in materials science as well as in biological systems. They are, however, so heavily damped that their observation is a real challenge to the experimentalist. We show that accurate information about the refraction-like and reflection-like behavior of diffusion waves can be obtained by studying diffusion fronts. For this we use hydrogen in a metal as a model system and visualize its 2D migration with an optical indicator. The similarities between classical optics and diffusion, in particular, the applicability of Snell's law to diffusive systems are discussed. Our measurements are in good agreement with numerical simulations.

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PubMed2001

Light-induced metal-insulator transition in a switchable mirror.

Rare earth hydride films can be converted reversibly from metallic mirrors to insulating windows simply by changing the surrounding hydrogen gas pressure at room temperature. At low temperatures, in situ doping is not possible in this way as hydrogen cannot diffuse. However, our finding of persistent photoconductivity under ultraviolet illumination offers an attractive possibility to tune yttrium hydride through the T = 0 metal-insulator transition. Conductivity and Hall measurements are used to determine critical exponents. The unusually large value for the product of the static and dynamical critical exponents appears to signify the important role played by electron-electron interactions.

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