  Unlike atoms, colloidal particles are not identical, but can only be
synthesised within a finite size tolerance. Colloids are therefore
polydisperse, i.e. mixtures of infinitely many components with sizes drawn from
a continuous distribution. We model the crystallisation of hard-sphere colloids
(with/without attractions) from an initially amorphous phase. Though the
polydisperse hard-sphere phase diagram has been widely studied, it is not
straightforwardly applicable to real colloidal crystals, since they are
inevitably out of equilibrium. The process by which colloidal crystals form
determines the size distribution of the particles that comprise them. Once
frozen into the crystal lattice, the particles are caged so that the
composition cannot subsequently relax to the equilibrium optimum. We predict
that the mean size of colloidal particles incorporated into a crystal is
smaller than anticipated by equilibrium calculations. This is because small
particles diffuse fastest and therefore arrive at the crystal in
disproportionate abundance.
