Sreeram K Kalpathy

Coating thin liquid films with complex rheological behaviour on permeable substrates is often an important requirement in several applications such as contact lenses, textiles, and paper-based electronics. Here, we extend the classical Landau-Levich problem of dip coating of Newtonian liquids on rigid substrates to liquids of power-law rheology on permeable substrates. Our results suggest distinct deviation from the classical Landau-Levich relation through exhibition of different regimes of varying dependence of coating film thickness on withdrawal speed. A process map is presented depicting these coating thickness regimes for a wide range of operating parameters such as the substrate permeability factor, power-law exponent of the liquid, and a rescaled capillary number.

The present work examines the effect of substrate surface porosity on the coating thickness and meniscus profile during dip coating under saturated porous media conditions. The classical Landau-Levich formulation is modified by encoding the influence of porosity in an effective Navier slip boundary condition at the porous substrate surface. It is shown that simplified Navier slip-based model works well for creeping flow through the porous medium. The film height profile equation is derived as a function of a rescaled capillary number (Ca‾) and a substrate permeability factor, with inertial effects neglected. Numerical solutions show that the classical 2/3rd power dependence of film thickness on capillary number is recovered only at sufficiently high Ca‾ values. As Ca‾ is decreased, a marked deviation is seen. The shrinking of the entrainment meniscus and the change in meniscus curvature are analyzed in detail. The theoretical results are also validated with a suitable experimental system.

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.