  We propose a simple model, supported by contact-dynamics simulations as well
as rheology and friction measurements, that links the transition from
continuous to discontinuous shear-thickening in dense granular pastes to
distinct lubrication regimes in the particle contacts. We identify a local
Sommerfeld number that determines the transition from Newtonian to
shear-thickening flows, and then show that the suspension's volume fraction and
the boundary lubrication friction coefficient control the nature of the
shear-thickening transition, both in simulations and experiments.
