Many lyotropic liquid crystals are composed of mesogens that display a considerable spread in size or shape affecting their material properties and thermodynamics via various demixing and multi-phase coexistence scenarios. Starting from a generalized Onsager theory, we formulate a generic framework that enables locating spinodal polydispersities as well as identifying the nature of incipient size fractionation for arbitrary model potentials and size distributions. We apply our theory to nematic phases of both hard rods and disks whose main particle dimension is described by a unimodal log-normal distribution. We find that both rod-based and discotic nematics become unstable at a critical polydispersity of about 20%. We also investigate the effect of doping nematic assemblies with a small fraction of large species and highlight their effect on the stability of the uniform nematic fluid. Our main finding is that while rod-based are only weakly affected by the presence of large species, doping discotic nematics with very large platelets leads to a remarkable suppression of the spinodal instabilities. This could open up routes towards controlling the mechanical properties of nematic materials by manipulating the local stability of nematic fluid and its tendency to undergo fractionation-driven microphase separation. Published by AIP Publishing.
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