Cholesteric Phase in Virus Suspensions

We report measurements of the cholesteric pitch and twist elastic constant $(K_{22})$ in monodisperse suspensions of the rod-like virus filamentous bacteriophage {\it fd}. Measurements were taken for concentrations spanning the entire cholesteric region at several ionic strengths and temperatures. In the limit of high ionic strength the cholesteric pitch $(P_0)$ scales with concentration (c) as $P_0 \propto c^{-1.66}$. As the ionic strength decreases the scaling exponent systematically changes to lower values.