Dust growth model for the silicate dust component for the supergiant μ Cep. (a) Radial variation of the fraction f of the element Si condensed into silicate dust. (b) Outflow velocity of the gas, v_gas, caused by radiation pressure on the silicate grains (solid blue line) and drift velocity of the silicate grains with respect to the gas, v_drift, calculated for spherical grains in the tight coupling approximation (solid red line) and the simplified velocity law defined by Eq. (19) (dashed black line). For comparison, also the variation of sonic velocity, c_sonic, and escape velocity from the star, v_excape, with radial distance are shown.