Features of colloidal silica deposits dip coated onto porous alumina membranes from aqueous suspensions

The present work is directed towards understanding the development of coating morphologies and the accompanying hydrodynamics of suspension flows during dip coating on porous substrates. Colloidal silica particles are deposited from aqueous suspension onto nanoporous alumina membranes. The choice of colloidal silica as the model system is motivated by its popularity in fabrication of antireflection coatings, and as nanostructured material templates for electrochemical and catalytic applications. Nanoporous alumina as substrate is notable for its chemical and thermal stability, and its ability to form self-organized pore structures. The objective here is to characterize the dependence of coating morphologies on membrane pore size, wettability, dip coating speed, and size of colloidal particles in suspension. A coating regime map is formulated based on different morphologies obtained for coatings deposited under different operating parameter spaces. The map indicates existence of two perceptibly distinct regimes which emanate from particle-induced interfacial deformation of the meniscus: Scarcely populated particulate deposition regime and densely packed coating regime having significant amount of particle deposition. The experimental results are analysed and explained using numerical results from earlier mathematical models in literature which cater to modified Landau-Levich formulation for porous substrates, as well as the hydrodynamics of colloidal assembly on moving substrates.