Coating processes employed in several applications involve substrates that are not truly smooth, rigid and impermeable. They could possess inherent surface porosity and permeability that can influence the dynamics of coating flows. Such surface heterogeneities compounded with the rheology of the coating liquid can have a significant impact on the coating morphology and quality. The above mentioned effects are relevant to many applications such as protective wall coatings, paper and textile-based electronics, porous contact lens, and clinical sensors, which require thin films of polymeric liquids and colloidal suspensions to be coated onto permeable, rough, or structured substrates. In the present work, a series of fundamental problems addressing these complexities are examined.
First, liquid film entrainment during dip coating on a saturated porous substrate is modelled as an extension of the classical Landau-Levich problem. The results suggest that the classical 2/3rd power law relation between the limiting film thickness and capillary number may deviate into a distorted power law relation or a capillary number-independent thickness regime for porous substrates. This model is subsequently extended to study dip coating of non-Newtonian liquids with power law rheology. We use insights from these studies to understand morphology of coatings from a colloidal silica suspension onto porous alumina membranes. Finally, the stability of thin polymer films spin coated onto permeable surfaces is studied experimentally as well as theoretically from a fluid mechanical perspective.

Keywords: Coating flows, Thin liquid films, Landau-Levich problem, Darcy’s law, Colloidal silica, Dewetting.