section 1.9 section 1.9
Many colloidal particles are non-spherical.
For instance, clay consists of
platelets, red blood cells are toroidal and other
colloids have a rodlike shape.
Examples of
colloidal rods are TMV and fd viruses, inorganic 
-particles
and boehmite needles. Rodlike particles have a richer phase diagram
than spherical colloids, because the rods can form a variety of
liquid-crystalline phases [24]. The term ``liquid crystal'' refers to a
structure that
is in between a crystalline solid and an isotropic liquid. Only
sufficiently non-spherical objects can form liquid
crystals.
Already in the 1940's, Onsager showed that an isotropic dispersion of hard, infinitely long rods, must undergo a first order orientational ordering transition at sufficiently high density [25]. The resulting nematic liquid-crystalline phase is characterized by long-range orientational ordering of the particles. There is, however, no translational ordering: i.e. there are no long-range positional correlations of the centers of mass of the particles.
When compressed further, the system of hard rods can transform into another liquid crystalline phase, called smectic. This phase has not only orientational ordering but it also forms layers, in which the particles are still free to move. There is only long-range positional order in one direction. Eventually, at sufficiently high pressures, crystallization will take place, with long-range positional order in all three directions. We show the sequence of liquid crystalline ordering in figure 1.4 for the case of a simple model of rodlike particles, namely hard spherocylinders.
Non-adsorbing polymers will also induce depletion interactions between non-spherical colloids. In fact, the addition of polymer can have a large effect on the stability of the liquid crystalline phases in dispersions of rodlike colloids. The phase behavior of mixtures of spherocylinder and polymers is one of the issues that will be addressed in this thesis.
section 1.9 