We present a theoretical and experimental study of the role of hydrodynamic interactions on the motion and dispersion of metal rodlike particles in the presence of an externally applied electric field. In these systems, the electric field polarizes the particles and induces an electroosmotic flow relative to the surface of each particle. The simulations include the effect of the gravitational body force, buoyancy, far-field hydrodynamic interactions, and near-field lubrication forces. The particles in the simulations and experiments were observed to experience repeated pairing interactions in which they come together axially with their ends approaching each other, slide past one another until their centers approach, and then push apart. These interactions were confirmed in measurements of particle orientations and velocities, pair distribution functions, and net dispersion of the suspension. For large electric fields, the pair distribution functions show accumulation and depletion regions consistent with many pairing events. For particle concentrations of 10;{8}particles/mL and higher, dispersion within the suspension dramatically increases with increased field strength.

Previous research demonstrated that methanogenic cultures enriched from Baltimore Harbor (Baltimore, MD) sediments were able to degrade naphthalene and phenanthrene. In this report, the degradation activity was maintained through a sequential transfer without adding additional sediments and the established polycyclic aromatic hydrocarbon (PAH)-degrading methanogenic communities were characterized via comparative sequence analysis of clone libraries of 16S rRNA genes amplified using bacteria-specific and Archaea-specific primers. The phylogenetic analysis indicated that the addition of PAHs clearly shifted the structure of the methanogenic community and resulted in an increase in populations of species previously found in other hydrocarbon-degrading communities. Of particular interest is the fact that the dominant microbial population of the naphthalene cultures was different from that of the phenanthrene cultures, suggesting that different species are involved in the degradation. Finally, this information may lead to the identification and isolation of methanogenic populations that can degrade PAHs.

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We examine the collective dynamics of polarizable, Brownian, sedimenting rods of high aspect ratio. Previous work of Koch and Shaqfeh (J. Fluids Mech., vol. 209, 1989 pp. 521–542) has shown that in the absence of Brownian motion, sedimenting suspensions of rods are unstable to concentration fluctuations and form dense streamers via interparticle hydrodynamic interactions. Recently, Saintillan, Shaqfeh & Darve (Phys. Fluids, vol. 18 (121701), 2006b p. 1) demonstrated that electric fields can act to stabilize these non-Brownian suspensions of polarizable rods through induced-charge electrokinetic rotation, which forces particle alignment. In this paper, we employ a mean-field linear stability analysis as well as Brownian dynamics simulations to study the effect of thermal motion on the onset of instability. We find that in the absence of electric fields, Brownian motion consistently suppresses instability formation through randomization of particle orientation. However, when electric fields are applied, thermal motion can act to induce instability by counteracting the stabilizing effect of induced-charge orientation.

We describe the Brownian demixing of sedimenting suspensions, a recently discovered phenomenon in which increases in the thermal energy can destabilize a system of orientable particles subjected to a torque to fluctuations in concentration. Through use of Brownian dynamics simulation and a mean-field analysis, we demonstrate that demixing occurs in a model system composed of slender rigid rods sedimenting between no-slip walls. Additionally, we describe the effects of wall separation distance on suspension stability.