  We investigate the effect that the turbulent mixing strength parameter
$\alpha_{\rm{turb}}$ plays on near-infrared polarimetric and sub-millimetre
interferometric imaging observations of transitional discs (TDs) with a gap
carved by a planet. We generate synthetic observations of these objects with
ALMA and VLT/SPHERE-ZIMPOL by combining hydrodynamical, dust evolution,
radiative transfer and instrument models for values of
$\alpha_{\rm{turb}}=[10^{-4}, 10^{-3}, 10^{-2}]$. We find that, through a
combination of effects on the viscosity of the gas, the turbulent mixing and
dust evolution processes, $\alpha_{\rm{turb}}$ strongly affects the morphology
of the dust distribution that can be traced with these observations. We
constrain the value of $\alpha_{\rm{turb}}$ to be within an order of magnitude
of $10^{-3}$ in TD sources that show cavities in sub-mm continuum images while
featuring continuous distribution of dust or smaller cavities in
NIR-polarimetric images.
