204a

Kirstin R. Purdy^{1}, Szabolcs Varga^{2}, Amparo Galindo^{3}, Seth Franden^{4}, and **George Jackson**^{3}. (1) Martin Fisher School of Physics, Brandeis University, Waltham, MA 02454, (2) Department of Physics, University of Veszprém, PO Box 158, Veszprém, H-8201, Hungary, (3) Chemical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, United Kingdom, (4) Department of Physics, Brandeis University, Waltham, MA 02454

We report experimental measurements of the phase behavior of mixtures of thin (charged semi-flexible fd virus) and thick (fd-PEG, fd virus covalently coated with poly-(ethylene glycol)) rods with diameter ratio varying from 3.7 to 1.1 [K. R. Purdy, S. Varga, A. Galindo, G. Jackson, and S. Fraden, Phys. Rev. Lett., 94, 057801 (2005); Phys. Rev. E, 72, 051704 (2005)]. The phase diagrams of the rod mixtures reveal isotropic-nematic, isotropic-nematic-nematic and nematic-nematic coexisting phases with increasing concentration. In stark contrast to predictions from earlier theoretical work, we find a nematic-nematic coexistence region bound by a lower critical point. Moreover, we show that a rescaled Onsager-type theory for binary hard rod mixtures qualitatively describes the observed phase behavior. Although all mesogenic molecules possess a rigid non-spherical core the fd virus also possesses a chiral geometry. This imparts a chirality on the bulk phase which is strongly temperature dependent. The effect of this molecular chirality on the bulk phase behaviour is examined by simulating a system of hard spherocylinders of aspect ratio L/D=5 with a central chiral interaction. For sufficiently large values of the chiral interaction parameter, a chiral nematic (and even a blue phase) is found. The problem with the use of the standard periodic boundary conditions for such systems is that the chiral pitch of the phase corresponds to the dimension of the simulation box. As a consequence the chiral pitch shows no dependence on the temperature, which is in keeping with experimental findings. One can circumvent this problem by confining the system between parallel hard walls, thus breaking the symmetry of the twist between the two ends of the box [S. Varga and G. Jackson, Chem. Phys. Lett., 377, 6 (2003)]. It is gratifying to see that the pitch increases with increasing temperature as is seen experimentally in dispersions of fd virus [Z. Dogic and S. Fraden, Langmuir, 16, 7820 (2000)]; for values of the chiral interaction which are similar to the kinetic energy, a pitch of the order of hundreds of nanometers is observed.

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