  Dip-coating consists in withdrawing a substrate from a bath to coat it with a
thin liquid layer. This process is well-understood for homogeneous fluids, but
heterogeneities such as particles dispersed in the liquid lead to more complex
situations. Indeed, particles introduce a new length scale, their size, in
addition to the thickness of the coating film. Recent studies have shown that
at first order, the thickness of the coating film for monodisperse particles
can be captured by an effective capillary number based on the viscosity of the
suspension, providing that the film is thicker than the particle diameter.
However, suspensions involved in most practical applications are polydisperse,
characterized by a wide range of particle sizes, introducing additional length
scales. In this study, we investigate the dip coating of suspensions having a
bimodal size distribution of particles. We show that the effective viscosity
approach is still valid in the regime where the coating film is thicker than
the diameter of the largest particles, although bidisperse suspensions are less
viscous than monodisperse suspensions of the same solid fraction. We also
characterize the intermediate regime that consists of a heterogeneous coating
layer and where the composition of the film is different from the composition
of the bath. A model to predict the probability of entraining the particles in
the liquid film depending on their sizes is proposed and captures our
measurements. In this regime, corresponding to a specific range of withdrawal
velocities, capillarity filters the large particles out of the film.
