De-mixing of polydisperse fluids: experimental test of a universal relation

Colloids, emulsions, polymer blends, and other important complex fluids, are polydisperse, i.e. there are variations among their constituent particles. Polydispersity is usually regarded as an ubiquitous, uncontrollable nuisance causing experimental inconsistencies. We have varied the polydispersity of a complex fluid, whilst keeping all other parameters constant, and report the first measurements of some universal physics. At coexistence (e.g. between liquid and vapour), fractionation occurs—each phase receives a different mix of the various ingredients, e.g. with the liquid disproportionately abundant with larger particles. Theory predicts, at low polydispersity, that this demixing becomes universal, irrespective of the material, with chemical differences between the phases proportional to polydispersity to the power two. We have studied colloid– polymer suspensions at two-phase coexistence and, using light scattering, measured the exponent as 2.16(plus_minus)0.44.