Surface freezing and a two-step pathway of the isotropic-smectic phase transition in colloidal rods

We study the kinetics of the isotropic-smectic phase transition in a colloidal rod/polymer mixture by visualizing individual smectic layers. First, we show that the bulk isotropic-smectic phase transition is preceded by a surface freezing transition in which a quasi two-dimensional smectic phase wets the isotropic-nematic interface. Next, we identify a two step kinetic pathway for the formation of a bulk smectic phase. In the first step a metastable isotropic-nematic interface is formed. This interface is wetted by the surface induced smectic phase. In the subsequent step, smectic layers nucleate at this surface phase and grow into the isotropic bulk phase.Comment: 5 pages, 4 figure, accepted by PR

Self-diffusion of rod-like viruses in the nematic phase

We measure the self-diffusion of colloidal rod-like virus {\it fd} in an isotropic and nematic phase. A low volume fraction of viruses are labelled with a fluorescent dye and dissolved in a background of unlabelled rods. The trajectories of individual rods are visualized using fluorescence microscopy from which the diffusion constant is extracted. The diffusion parallel ($D_{\parallel}$) and perpendicular ($D_{\perp}$) to the nematic director is measured. The ratio ($D_{\parallel}/D_{\perp}$) increases monotonically with increasing virus concentration. Crossing the isotropic-nematic phase boundary results in increase of $D_{\parallel}$ and decrease of $D_{\perp}$ when compared to the diffusion in the isotropic phase ($D_{iso}$).Comment: 7 pages, 4 figures, to appear in Europhysics Letter

Trains, tails and loops of partially adsorbed semi-flexible filaments

Polymer adsorption is a fundamental problem in statistical mechanics that has direct relevance to diverse disciplines ranging from biological lubrication to stability of colloidal suspensions. We combine experiments with computer simulations to investigate depletion induced adsorption of semi-flexible polymers onto a hard-wall. Three dimensional filament configurations of partially adsorbed F-actin polymers are visualized with total internal reflection fluorescence microscopy. This information is used to determine the location of the adsorption/desorption transition and extract the statistics of trains, tails and loops of partially adsorbed filament configurations. In contrast to long flexible filaments which primarily desorb by the formation of loops, the desorption of stiff, finite-sized filaments is largely driven by fluctuating filament tails. Simulations quantitatively reproduce our experimental data and allow us to extract universal laws that explain scaling of the adsorption-desorption transition with relevant microscopic parameters. Our results demonstrate how the adhesion strength, filament stiffness, length, as well as the configurational space accessible to the desorbed filament can be used to design the characteristics of filament adsorption and thus engineer properties of composite biopolymeric materials

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

Structure and Intermolecular Interactions between L-Type Straight Flagellar Filaments.

Bacterial mobility is powered by rotation of helical flagellar filaments driven by rotary motors. Flagellin isolated from the Salmonella Typhimurium SJW1660 strain, which differs by a point mutation from the wild-type strain, assembles into straight filaments in which flagellin monomers are arranged in a left-handed helix. Using small-angle x-ray scattering and osmotic stress methods, we investigated the structure of SJW1660 flagellar filaments as well as the intermolecular forces that govern their assembly into dense hexagonal bundles. The scattering data were fitted to models, which took into account the atomic structure of the flagellin subunits. The analysis revealed the exact helical arrangement and the super-helical twist of the flagellin subunits within the filaments. Under osmotic stress, the filaments formed two-dimensional hexagonal bundles. Monte Carlo simulations and continuum theories were used to analyze the scattering data from hexagonal arrays, revealing how the bundle bulk modulus and the deflection length of filaments in the bundles depend on the applied osmotic stress. Scattering data from aligned flagellar bundles confirmed the theoretically predicated structure-factor scattering peak line shape. Quantitative analysis of the measured equation of state of the bundles revealed the contributions of electrostatic, hydration, and elastic interactions to the intermolecular forces associated with bundling of straight semi-flexible flagellar filaments