Penetration dynamics of non-Newtonian fluids into axially varying capillaries

The capillary-driven penetration of non-Newtonian fluids in capillaries with irregular walls is crucial in industrial applications, such as anode manufacturing for aluminum production, where a mixture of coal-tar pitch and fine petroleum coke particles (binder matrix) impregnates the open pores of coarse coke particles. Our study presents a semi-analytical model for capillary-driven flow of shear-thinning fluids in axially varying, wavy-walled microchannels, representative of coke open pore geometries. Incorporating weak inertia, viscous dissipation, and dynamic contact angle behavior (governed by a molecular kinetic theory), the model is systematically derived using lubrication theory and a power-law rheology, yielding a reduced-order equation for the advancing meniscus. The model is validated and calibrated via computational fluid dynamics simulations to extract the dynamic contact angle correction parameter. Our analysis quantifies three distinct penetration regimes and their transition dynamics: inertia-dominated, interfacial dissipation-dominated, and viscous dissipation-dominated. Analytical scaling laws and regime transition correlations are validated across varying power-law indices, Laplace numbers, contact angles, and geometrical features. The power-law index most strongly influences penetration, followed by static contact angle and geometric phase shift, while Laplace number affects early-time behavior. Dynamic contact angle analysis highlights the critical role of interfacial dissipation in irregular geometries. Applied to binder matrices with measured rheology, the model shows that increased fine coke content or channel irregularity significantly delays impregnation.