  We study motion of small particles in turbulence when the particle relaxation
time falls in the range of inertial time-scales of the flow. Due to inertia,
particles drift relative to the fluid. We show that the drift velocity is close
to the Lagrangian velocity increments of turbulence at the particle relaxation
time. We demonstrate that the collective drift of two close particles makes
them see local velocity increments fluctuate fast and we introduce the
corresponding Langevin description for separation dynamics. This allows to
describe the behavior of the Lyapunov exponent and give the analogue of
Richardson's law for separation above viscous scale.
