  We report a study of the cholesteric phase in monodisperse suspensions of the
rod-like virus fd sterically stabilized with the polymer polyethylene glycol
(PEG). After coating the virus with neutral polymers, the phase diagram and
nematic order parameter of the fd-PEG system then become independent of ionic
strength. Surprisingly, the fd-PEG suspensions not only continue to exhibit a
cholesteric phase, which means that the grafted polymer does not screen all
chiral interactions between rods, but paradoxically the cholesteric pitch of
this sterically stabilized fd-PEG system varies with ionic strength.
Furthermore, we observe that the cholesteric pitch decreases with increasing
viral contour length, in contrast to theories which predict the opposite trend.
Different models of the origin of chirality in colloidal liquid crystals are
discussed.
