  The rheology of suspensions of Brownian, or colloidal, particles (diameter $d
\lesssim 1$ $\mu$m) differs markedly from that of larger grains ($d \gtrsim 50$
$\mu$m). Each of these two regimes has been separately studied, but the flow of
suspensions with intermediate particle sizes (1 $\mu\textrm{m} \lesssim d
\lesssim 50$ $\mu$m), which occur ubiquitously in applications, remains poorly
understood. By measuring the rheology of suspensions of hard spheres with a
wide range of sizes, we show experimentally that shear thickening drives the
transition from colloidal to granular flow across the intermediate size regime.
This insight makes possible a unified description of the (non-inertial)
rheology of hard spheres over the full size spectrum. Moreover, we are able to
test a new theory of friction-induced shear thickening, showing that our data
can be well fitted using expressions derived from it.
