Biomedical subjects

Justin Stambaugh

Publications and source records attributed to Justin Stambaugh.

4 recordsLinked to original sources

PubMed2005

Polymerization transitions in two-dimensional systems of dipolar spheres.

We investigate the self-organization of dipolar spheres into polymer chains as a fundamental model of the self-assembly of particles having anisotropic interparticle interactions. Our study involves a combination of modeling with vertically vibrated magnetic beads simulating a quasi-two-dimensional fluid at equilibrium and corresponding Monte Carlo simulations of hard spheres with embedded extended dipoles. We find a transition from a gas-like phase to a polymerized phase upon cooling, in accord with the analytic theory of equilibrium polymerization.

Colloids↗

PubMed2004

Segregation in a monolayer of magnetic spheres.

Segregation and pattern formation is investigated for binary mixtures of granular magnetic spheres in a vertically vibrated monolayer. The spheres, all of equal mass and size, have a maximum surface magnetic field B induced by encased cylindrical magnetic cores of length l. For binary mixtures of particles with equal l but different B, we find that the particles spontaneously segregate when driven. For fixed vibration frequency, the segregation rate increases roughly linearly with driving acceleration over the amplitudes investigated. For systems of fixed particle number density, the rate of segregation also decreases as the volume fraction of "strong" (high B) particles increases. We find that segregation also occurs in binary mixtures of particles with equal B, but different l. Finally, using a simple model of spheres with dipolar and higher magnetic moments, we show that the observed segregation phenomena occur in conjunction with a decrease in magnetic energy.

Journal Article↗

PubMed2004

Transient and oscillatory granular shear flow.

The forces and particle motion during transient and oscillatory shear of granular material are investigated experimentally. In a shear cell of Taylor-Couette-type we find that how a granular shear flow starts depends strongly on the prior shear direction. If the shear direction is reversed, the material goes through a transient period during which the material compacts, the shear force is small, and the shear band is wide. Three-dimensional confocal imaging of particle rearrangements during shear reversal shows that bulk and surface flows are comparable. Repeated reversals, or oscillations of the shear direction, lead to additional compaction, which can be described by a stretched exponential, similar to compaction induced by tapping.

Anisotropy↗

PubMed2003

Pattern formation in a monolayer of magnetic spheres.

Pattern formation is investigated for a vertically vibrated monolayer of magnetic spheres. The spheres of diameter D encase cylindrical magnetic cores of length l. For large D/l, we find that the particles form a hexagonal-close-packed pattern in which the particles' dipole vectors assume a macroscopic circulating vortical pattern. For smaller D/l, the particles form concentric rings. The static configurational magnetic energy (which depends on D/l) appears to be a determining factor in pattern selection even though the experimental system is driven and dissipative.

Journal Article↗