Dynamical models of dust evolution in discs with mean molecular weight μ=2.3 (left column) and μ=28 (right column), with the same density profile (with the gas midplane density at 65 au equal to ρ_g,0=10⁻¹⁷ g cm⁻³, and the Gaussian profile width σ_g=15 au), at 10 Myr after particle release in the assumed belt indicated by grey bands. Top panels show the normalized geometric optical depth per log unit of size as a function of radius and particle size from the results of our dynamical models (yellow-red colour plots), the stationary radii where the drift velocity due to gas and radiative effects is zero (solid black lines; see Eq. (10)), and the radii where β(s)=η(r). Bottom panels show normalized surface brightness as a function of radius in the disc for scattered light at 1.6 μm assuming a face-on disc. For details, see Section 4.3.