Biomedical subjects

Z A Daya

Publications and source records attributed to Z A Daya.

6 recordsLinked to original sources

PubMed2006

Experimental characterization of vibrated granular rings.

We report an experimental study of the statistical properties of vibrated granular rings. In this system, a linked rod and bead metallic chain in the form of a ring is collisionally excited by a vertically oscillating plate. The dynamics are driven primarily by inelastic bead-plate collisions and are simultaneously constrained by the rings' physical connectedness. By imaging many instances of the ring configurations, we measure the ensemble averages and distributions of several physical characteristics on the scale of individual beads and composite ring. We study local properties such as inter-bead separation and inter-bonds angles, and global properties such as the radius of gyration and center-of-mass motion. We characterize scaling with respect to the size of the chain.

Colloids↗

PubMed2005

Granular chains.

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Journal Article↗

PubMed2002

Prandtl-number dependence of interior temperature and velocity fluctuations in turbulent convection.

Temperature and vertical velocity fluctuations are measured in turbulent Rayleigh-Bénard convection at the center of an approximately unit aspect ratio container of cylindrical cross section. Our measurements show that the Rayleigh-number scaling exponent gamma of the interior temperature fluctuations (i.e., sigma(T)/deltaT approximately Ragamma) is a strong and nontrivial function of the Prandtl number in the range 2.9<Pr<12.4. Other measurements at constant Ra=2.0 x 10(9) show that the interior turbulent fluctuations decrease significantly with increasing Pr; the temperature and velocity fluctuations decrease by about approximately 45% and approximately 68% over our range in Pr.

Journal Article↗

PubMed2002

Entropic tightening of vibrated chains.

We investigate experimentally the distribution of configurations of a ring with an elementary topological constraint, a "figure-8" twist. Using a system far from thermal equilibrium, a vibrated granular chain, we show that configurations where one loop is small and the second is large are strongly preferred. Despite the highly non-equilibrium nature of the system, our results are consistent with recent predictions for equilibrium properties of topologically-constrained polymers. The dynamics of the tightening process weakly violates a (coarse-grained) detailed balance, indicating that the unexpected correspondence with an equilibrium entropic approach is not exact.

Journal Article↗

PubMed2001

Bifurcations in annular electroconvection with an imposed shear.

We report an experimental study of the primary bifurcation in electrically driven convection in a freely suspended film. A weakly conducting, submicron thick smectic liquid crystal film was supported by concentric circular electrodes. It electroconvected when a sufficiently large voltage V was applied between its inner and outer edges. The film could sustain rapid flows and yet remain strictly two dimensional. By rotation of the inner electrode, a circular Couette shear could be independently imposed. The control parameters were a dimensionless number R, analogous to the Rayleigh number, which is ~V2 and the Reynolds number Re of the azimuthal shear flow. The geometrical and material properties of the film were characterized by the radius ratio alpha, and a dimensionless number P, analogous to the Prandtl number. Using measurements of current-voltage characteristics of a large number of films, we examined the onset of electroconvection over a broad range of alpha, P, and Re. We compared this data quantitatively to the results of linear stability theory. This could be done with essentially no adjustable parameters. The current-voltage data above onset were then used to infer the amplitude of electroconvection in the weakly nonlinear regime by fitting them to a steady-state amplitude equation of the Landau form. We show how the primary bifurcation can be tuned between supercritical and subcritical by changing alpha and Re.

Journal Article↗

PubMed2001

Knots and random walks in vibrated granular chains.

We study experimentally statistical properties of the opening times of knots in vertically vibrated granular chains. Our measurements are in good qualitative and quantitative agreement with a theoretical model involving three random walks interacting via hard-core exclusion in one spatial dimension. In particular, the knot survival probability follows a universal scaling function which is independent of the chain length, with a corresponding diffusive characteristic time scale. Both the large-exit-time and the small-exit-time tails of the distribution are suppressed exponentially, and the corresponding decay coefficients are in excellent agreement with theoretical values.

Journal Article↗