(230f) Two-Layer Spin Coating Flow of Newtonian Liquids: A Computational Study 

Spin coating is the most commonly used method in industries to form coating films of desired thickness and functionality. In case of two-layer spin coating process, stratified layers of two immiscible liquids are deposited onto the substrate simultaneously. When the substrate is rotated, these layers spread radially to form thin two-layer coating film of finite thickness. Questions concerning the effect of surface/interfacial tension on uniformity of these films and contact line evolution need thorough investigation. Here, we have developed an axisymmetric model governing the flow of two-layer spin coating process. Liquids used for the study are assumed to be Newtonian and fully wetting. The contact line singularity is resolved using a precursor layer model and the governing equations are simplified using lubrication approximation. The resulting fourth order non-linear PDEs are solved using a numerical technique based on Galerkin finite-element method (G/FEM). It is observed that a uniform two-layer film surrounded by thin single layer film is formed when the viscosity ratio is small. On the contrary, when viscosity ratio is large, a thin two-layer film surrounded by bulky capillary ridges is formed. Similarly, the results also show that sharpness of capillary ridge increases with decrease in the surface tension ratio. Further, it is found that increase in the precursor layer thickness increases the spreading rate, thereby making the film more uniform. Finally, the uniformity of the final two-layer film does not get affected by the initial volume of fluid present in the upper layer. The results of our numerical study may serve as a basis for the design and optimization of multi-layer spin coating process for a given application. ^[1]

Reference:

[1] S. Sahoo, A. Arora, and P. Doshi, â??Two-layer spin coating flow of Newtonian liquids: A computational study,â? Comput. Fluids 131, 180 (2016).