  In order to investigate the coupling between the degrees of alignment of
elongated particles in binary nematic dispersions, surfactant stabilized
single-wall carbon nanotubes (CNTs) have been added to nematic suspensions of
colloidal rodlike viruses in aqueous solution.We have independently measured
the orientational order parameter of both components of the guest-host system
by means of polarized Raman spectroscopy and by optical birefringence,
respectively. Our system allows us therefore to probe the regime where the
guest particles (CNTs) are shorter and thinner than the fd virus host
particles. We show that the degree of order of the CNTs is systematically
smaller than that of the fd virus particles for the whole nematic range. These
measurements are in good agreement with predictions of an Onsager-type
second-viral theory, which explicitly includes the flexibility of the virus
particles, and the polydispersity of the CNTs.
