filament concentration

1. The cell cytoskeleton as active soft matterM. Cristina Marchetti, Syracuse University, DMR-0305407 The cell cytoskeleton is a network of polar biopolymers cross-linked by a variety of smaller proteins. Among these are motor proteins - “nanomachines” that convert chemical energy into mechanical work and continuously remodel the network, controlling its structure and mechanical properties. The cytoskeleton is an example of a new type of active soft matter that is maintained out of equilibrium by external or internal energy sources. A central goal of this project is to use methods from condensed matter physics to study the structure and rheological properties of active materials. The actin-myosin cytoskeleton in a fish keratocyte controls cell crawling. (Svitkina et al. JCB 1997)

2. shear viscosity N Active solution Passive solution P filament concentration Isotropic Nematic Iso concentration motor activity Contractile forces from active crosslinkers yield a diverging viscosity as the concentration is increased across the Iso-N transition. (cond-mat/0607285) “Phase diagram” of a solution of polar filaments & motors. Filaments (represented by arrows) have random orientation in the Isotropic (Iso) phase. The Polarized (P) and Nematic (N) phases exhibit liquid-crystalline order. Homogeneus states are destabilized by filament bundling above the dashed blue line. (cond-mat/0607287) One graduate student and one postdoctoral researcher have been involved in this interdisciplinary project which gives students a broad education at the interface of physics and biology.