  There is a growing consensus that shear thickening of concentrated
dispersions is driven by the formation of stress-induced frictional contacts.
The Wyart-Cates (WC) model of this phenomenon, in which the microphysics of the
contacts enters solely via the fraction $f$ of contacts that are frictional,
can successfully fit flow curves for suspensions of weakly polydisperse
spheres. However, its validity for "real-life", polydisperse suspensions has
yet to be seriously tested. By performing systematic simulations on bidisperse
mixtures of spheres, we show that the WC model applies only in the monodisperse
limit and fails when substantial bidispersity is introduced. We trace the
failure of the model to its inability to distinguish large-large, large-small
and small-small frictional contacts. By fitting our data using a polydisperse
analogue of $f$ that depends separately on the fraction of each of these
contact types, we show that the WC picture of shear thickening is incomplete.
Systematic experiments on model shear-thickening suspensions corroborate our
findings, but highlight important challenges in rigorously testing the WC model
with real systems. Our results prompt new questions about the microphysics of
thickening for both monodisperse and polydisperse systems.
