1 Phase behavior of rod - like viruses and virus / sphere mixtures

The reasons physicists give for studying colloids are varied. Our initial motivation was that colloids can serve as model experimental systems to study simple fluids because with careful preparation, colloids approximate hard particles. Numerous studies have investigated the phase behavior, structure, and macroscopic viscoelastic properties of suspensions of spherical colloids (Poon and Pusey 1995). Far less studied have been colloids of anisotropic shape, in spite of their long recognized similarity to liquid crystals. Counterintuitively, hard rod fluids are theoretically simpler systems to understand than hard spheres (Forsyth et al. 1978). This surprising fact was first recognized by Onsager who realized that the isotropic nematic (I-N) transition in the rodlike colloid Tobacco Mosaic Virus (TMV) occurred at such low concentrations that only two-body interactions were necessary in order to quantitatively explain the I-N phase transition (Onsager 1949). In fact, in the limit of long thin rods Onsager’s theory becomes exact. This in contrast to the theory of phase transitions of hard spheres, for which no exact results exist (in three dimensions). For some years the Complex Fluids Group at Brandeis has studied the liquid crystalline behavior of suspensions of TMV (Fraden et al. 1985; Hurd et al. 1985; Wen and Meyer 1987; Oldenbourg et al. 1988; Fraden et al. 1989; Meyer 1990; Fraden et al. 1993; Wang et al. 1994; Fraden 1995; Adams and Fraden 1998) and filamentous phage fd (Tang and Fraden 1993; Tang and Fraden 1995; Fraden 1995; Tang and Fraden 1996; Dogic and Fraden 1997; Adams et al. 1998; Dogic et al. 2000; Dogic and Fraden 2001; Grelet and Fraden 2003; Dogic 2003; Purdy et al. 2003; Purdy and Fraden 2004b; Purdy and Fraden 2004a; Purdy et al. 2004). TMV is a beautiful colloidal rod (Kreibig and Wetter 1980; Wetter 1985). It is completely rigid and forms isotropic, nematic, smectic and colloidal crystalline phases. However, TMV is difficult to work with. One must cultivate tobacco plants, infect them with virus, harvest the crop, extract the virus which takes months plus all this must be done with care