Radial drift of solids in a gas disk ($v_{pr}=-2\eta v_K\mathrm{St}/(1+{\mathrm{St}}^2)$)

For test particles with linear drag, negligible radial gas flow and negligible dust feedback, Sub-Keplerian rotation removes solid-particle angular momentum. The displayed inward velocity uses Stokes number $\mathrm{St}={\Omega }_K{t}_s$ and the pressure-gradient parameter of a protoplanetary disk. Drift is fastest at $\mathrm{St}=1$; the physical particle size at that point depends on the local gas and drag law.

A region that concentrates solids by opposing their loss through radial drift. Near a radial gas-pressure maximum, the pressure-support drift changes sign and carries particles toward the maximum from both sides. Gas advection and turbulent diffusion can limit retention.