  We investigate the distribution of relative velocities between small heavy
particles of different sizes in turbulence by analysing a statistical model for
bidisperse turbulent suspensions, containing particles with two different
Stokes numbers. This number, ${\rm St}$, is a measure of particle inertia which
in turn depends on particle size. When the Stokes numbers are similar, the
distribution exhibits power-law tails, just as in the case of equal ${\rm St}$.
The power-law exponent is a non-analytic function of the mean Stokes number
$\overline{\rm St}$, so that the exponent cannot be calculated in perturbation
theory around the advective limit. When the Stokes-number difference is larger,
the power law disappears, but the tails of the distribution still dominate the
relative-velocity moments, if $\overline{\rm St}$ is large enough.
