Rhythmic cluster generation in strongly driven colloidal dispersions

We study the response of a nematic colloidal dispersion of rods to a driven probe particle which is dragged with high speed through the dispersion perpendicular to the nematic director. In front of the dragged particle, clusters of rods are generated which rhythmically grow and dissolve by rotational motion. We find evidence for a mesoscopic cluster-cluster correlation length, {\em independent} of the imposed drag speed. Our results are based on non-equilibrium Brownian dynamics computer simulations and in line with a dynamical scaling theory.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let

Concentration Dependen Sedimentation of Collidal Rods

In the first part of this paper, an approximate theory is developed for the leading order concentration dependence of the sedimentation coefficient for rod-like colloids/polymers/macromolecules. To first order in volume fraction $\phi$ of rods, the sedimentation coefficient is written as $1+\alpha \phi$. For large aspect ratio L/D (L is the rod length, D it's thickness) $\alpha$ is found to very like $\propto (\frac{L}{D})^2/\log (\frac{L}{D})$. This theoretical prediction is compared to experimental results. In the second part, experiments on {\it fd}-virus are described, both in the isotropic and nematic phase. First order in concentration results for this very long and thin (semi-flexible) rod are in agreement with the above theoretical prediction. Sedimentation profiles for the nematic phase show two sedimentation fronts. This result indicates that the nematic phase becomes unstable with the respect to isotropic phase during sedimentation.Comment: Submitted to J. Chem. Phys. See related webpage http://www.elsie.brandeis.ed

Notes from the Millennium Villages Project

Matt Bonds and Max Fraden document their experience unrolling a health insurance subsidization scheme in the Millennium Village in Mayenge, Rwanda. The Millennium Villages Project aims to achieve the eight Millennium Development Goals. These goals outline a global partnership to reduce extreme poverty with specific focuses on hunger, education, gender equality, health, and environmental sustainability. Matt and Max comment on critical lessons learned and observations on the field of sustainable development from their work in Rwanda