Size separation of granular particles.
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Biomedical subjects
S R Nagel
Publications and source records attributed to S R Nagel.
Convection in a fully immersed granular slurry.
Using vertical vibrations, we have induced convection in a granular system completely immersed in fluid inside a cylindrical container. Convection is established in a single convection roll, which carries the material upward in the center and downward along the side walls of the container. We measure the rise times of tracer particles embedded in the medium at various depths along the central axis of the system. By fitting this data to an appropriate functional form, we obtain information on the depth dependence of convection velocity. Significant differences are found in the frequency and acceleration amplitude dependence of convection between wet and otherwise identical dry systems. In addition, we find that a power-law form provides a better fit to our data than the logarithmic form used in dry systems.
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Using selective withdrawal to coat microparticles.
We report a method that uses the process of selective withdrawal of one fluid through a second immiscible fluid to coat small particles with polymer films. Fluid is withdrawn through a tube with its orifice slightly above a water-oil interface. Upon increasing the flow rate, there is a transition from a state where only oil is withdrawn to a state where the water, containing the particles to be coated and appropriate prepolymer reagents, is entrained in a thin spout along with the oil. The entrained particles eventually cause the spout interface to break, producing a thin coat of controllable thickness around each particle, which can be subsequently polymerized using chemical reagents, light, or heat. This method allows flexibility in the chemical composition and thickness of the conformal coatings.
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Force distributions in three-dimensional granular assemblies: effects of packing order and interparticle friction.
We present a systematic investigation of the distribution of normal forces at the boundaries of static packings of spheres. A method for the efficient construction of large hexagonal-close-packed crystals is introduced and used to study the effect of spatial ordering on the distribution of forces. Under uniaxial compression we find that the form for the probability distribution of normal forces between particles does not depend strongly on crystallinity or interparticle friction. In all cases the distribution decays exponentially at large forces and shows a plateau or possibly a small peak near the average force but does not tend to zero at small forces.
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Noise stabilization of self-organized memories.
We investigate a nonlinear dynamical system which "remembers" preselected values of a system parameter. The deterministic version of the system can encode many parameter values during a transient period, but in the limit of long times, almost all of them are forgotten. Here we show that a certain type of stochastic noise can stabilize multiple memories, enabling many parameter values to be encoded permanently. We present analytic results that provide insight both into the memory formation and into the noise-induced memory stabilization. The relevance of our results to experiments on the charge-density wave material NbSe3 is discussed.
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