Impact of orientation on evaporation limited spreading of ethanol on thin porous strips

Wicking is a widely studied phenomenon with applications spanning natural processes and industrial systems, such as the evaporation of liquids in heat pipes for temperature control. This research investigates a simplified scenario where ethanol spreads horizontally over thin filter papers, followed by evaporation. Experiments were conducted using three types of filter paper with varying permeability, and data were captured using optical and thermal imaging techniques. The findings for horizontally oriented filter papers revealed that the steady-state spreading length $\left(L_c\right)$ was significantly shorter than predicted by Jurin's law, highlighting evaporation as the primary limiting factor in the spreading process. Interestingly, $L_c$ values for horizontal configurations were ∼30 % greater than their vertical counterparts, underscoring the significant influence of gravitational forces in vertical cases. Thermal imaging further revealed a non-uniform temperature distribution, with an inversion observed near the midpoint of the liquid's spread. Using this nonlinear temperature profile, we applied the previously developed Non-Constant Evaporation Model (NCEM) (Murthy and Kumar 2025 [1]), originally proposed for vertical cases, to the horizontal configuration by neglecting gravitational effects $\left(g=0\right)$. The NCEM, which assumes a power-law dependence of the evaporation rate on the local wicked length, served as an improvement over the Constant Evaporation Model (CEM) (Fries et al., 2008 [2]), which presumes uniform evaporation across the wicked length. The modified NCEM (Murthy and Kumar 2025 [1]) yielded $h-t$ curves that closely aligned with the experimental data. Additionally, we utilized the previously developed dimensionless numbers, which effectively consolidated various $h-t$ curves into a unified master curve. These findings provide deeper insights into the dynamics of wicking and evaporation, with significant implications for thermal management technologies such as wicks and heat pipes.