Directional Drying in Bilayer Porous Films: Funnel vs Ink-Bottle Geometries.

We investigate the drying dynamics of mesoporous silicon bilayers with controlled pore geometries during ethanol evaporation. Using Reflective Interferometric Fourier Transform Spectroscopy (RIFTS), we track the evolution of liquid saturation in each layer. We find that placing smaller pores atop larger ones─mimicking an ink-bottle geometry─can unexpectedly accelerate evaporation under specific thickness ratios. This behavior suggests a complex interplay of mass transport mechanisms beyond simple geometrical constraints. Additionally, we introduce a novel image-based method for spatially resolved saturation mapping over millimeter-scale areas, complementing the localized RIFTS measurements. Our results demonstrate that evaporation dynamics in porous films can be modulated through structural design, providing new experimental tools to study and tailor fluid transport in hierarchical porous materials.