Experimental validation of viscosity-invariant slip length at fluid–porous interfaces using water and oleic acid

This study experimentally validates that the effective slip length at a free-flow and porous medium interface remains invariant with the fluid's viscosity. Micro-Particle Image Velocimetry measurements were performed in a Hele–Shaw-type channel with a permeable lateral wall using water (1 cSt) and oleic acid (43.9 cSt). As for the latter, a robust methodology for transferring fluorescent polystyrene particles from a water solution to oleic acid is also presented. Despite the significant viscosity contrast, the effective slip length (Ls) remains constant within the spatial resolution of the measurements, ranging from 24 to 27 μm and matching reference values within a 5% error margin. Furthermore, water and oleic acid yield virtually identical normalized interfacial velocity gradients, supporting the assumption that tangential momentum transfer is predominantly governed by the geometric structure of the interface. These findings validate both the viscosity independence of Ls and the practical applicability of the proposed particle-transfer method, offering a solid framework for designing microfluidic systems with porous boundaries.