Jewel Agbara

The flow of liquid films on discrete objects is encountered in coating processes for a wide range of products such as biomedical devices, automobiles, and food. These products have complex geometries making them more challenging to coat compared to a flat object. Due to the non-flat surface of these objects, the liquid film drains under the effect of gravity making the coating non-uniform. Two major instabilities affect the coating uniformity in the axial direction, namely the Rayleigh-Plateau (RP) and Rayleigh-Taylor (RT) instability. The RP instability dominates with a sufficiently high rotation rate or thin enough film and leads to the formation of rings or bands of liquid enveloping the cylinder. On the other hand, the RT instability dominates with a sufficiently low rotation rate or thick enough film and results in the breakup of the coating into droplets of liquid which form at the bottom of the cylinder. Results outlined from past work carried out using Newtonian fluids show that there is a critical rotation rate above which a steady, smooth, and asymmetric coating is supported by the cylinder rotation. This rate is dependent on the liquid viscosity, the liquid density, the mean coating thickness, and the cylinder radius. In order to discover the regime where a smooth coating can be obtained for a non-Newtonian fluid, fluid visualization experiments were first carried out with a Newtonian fluid (dyed glycerol-water mixture) and results from these experiments were validated with the results from past work to ensure a proper functioning experimental setup.