Insight into the behavior and dynamics of extended thin films in shear-thinning liquids

Extended liquid thin films are essential and ubiquitous in the field of microfluidics. Mass and energy transfer in microfluidic systems, such as micro-scale heat pipes, falling film reactors, etc., depend on the forces acting near the three-phase contact line. Within the extended thin film region, the solid–liquid intermolecular force becomes significant along with the surface force. Several experiments have been conducted to understand and optimize the forces involved in mass and energy transport for Newtonian liquids. However, in real-world situations, these extended thin films are usually made of non-Newtonian liquids. The impact of high viscous forces and the complex rheology of non-Newtonian liquids on the extended thin film remains largely unexplored. This work pioneers a detailed experimental investigation into the extended thin film behavior of a shear-thinning polymeric liquid solution, offering new insights into this understudied phenomenon. The polymeric solution is supplemented with a surfactant to adjust the surface tension. The interplay between surfactant and the intrinsic nature of polymer solutions is studied by measuring their rheological properties. The extended thin film thickness is measured using image-analysis interferometry for polymer solutions with varying concentrations. The Hamaker constant is calculated from the slope and curvature profiles. A theoretical model is developed using the augmented Young–Laplace equation. The model can predict the extended film thickness profile near the three-phase contact line region. The model's predictions are favorably compared with experimental results. This work advances the understanding of extended thin film dynamics in non-Newtonian fluids, with broad implications for industrial and scientific applications.