  Lipid rafts are hypothesized to facilitate protein interaction, tension
regulation, and trafficking in biological membranes, but the mechanisms
responsible for their formation and maintenance are not clear. Insights into
many other condensed matter phenomena have come from colloidal systems, whose
micron-scale particles mimic basic properties of atoms and molecules but permit
dynamic visualization with single-particle resolution. Recently, experiments
showed that bidisperse mixtures of filamentous viruses can self-assemble into
colloidal monolayers with thermodynamically stable rafts exhibiting chiral
structure and repulsive interactions. We quantitatively explain these
observations by modeling the membrane particles as chiral liquid crystals.
Chiral twist promotes the formation of finite-sized rafts and mediates a
repulsion that distributes them evenly throughout the membrane. Although this
system is composed of filamentous viruses whose aggregation is entropically
driven by dextran depletants instead of phospholipids and cholesterol with
prominent electrostatic interactions, colloidal and biological membranes share
many of the same physical symmetries. Chiral twist can contribute to the
behavior of both systems and may account for certain stereospecific effects
observed in molecular membranes.
