  We dynamically simulate fractionation (partitioning of particle species)
during spinodal gas-liquid separation of a size-polydisperse colloid, using
polydispersity up to ~40% and a skewed parent size distribution. We introduce a
novel coarse-grained Voronoi method to minimise size bias in measuring local
volume fraction, along with a variety of spatial correlation functions which
detect fractionation without requiring a clear distinction between the phases.
These can be applied whether or not a system is phase separated, to determine
structural correlations in particle size, and generalise easily to other kinds
of polydispersity (charge, shape, etc.). We measure fractionation in both mean
size and polydispersity between the phases, its direction differing between
model interaction potentials which are identical in the monodisperse case.
These qualitative features are predicted by a perturbative theory requiring
only a monodisperse reference as input. The results show that intricate
fractionation takes place almost from the start of phase separation, so can
play a role even in nonequilibrium arrested states. The methods for
characterisation of inhomogeneous polydisperse systems could in principle be
applied to experiment as well as modelling.
