Evaporative Drying from Hydrophilic or Hydrophobic Homogeneous Porous Columns: Consequences of Wettability, Porous Structure and Hydraulic Connectivity

Evaporative drying from porous media is influenced by wettability and porous structures; altering these parameters impacts capillary effects and hydraulic connectivity, thereby achieving slower or faster evaporation. In this study, water was evaporated from a homogeneous porous column created with ~ 1165 glass (i.e., hydrophilic) or Teflon (i.e., hydrophobic) 2.38-mm-diameter spheres with an applied heat flux of 1000 W/m² supplied via a solar simulator; each experiment was replicated five times and lasted 7 days. This study investigates the combination of altered wettability on evaporation with an imposed heat flux to drive evaporation, while deploying X-ray imaging to measure evaporation fronts. Initial evaporation rates were faster (i.e., ~ 1.5 times) in glass than in Teflon. Traditionally, evaporation from porous media is categorized into three periods: constant rate, subsequent falling rate and slower rate period. Due to homogeneous porous structure and similar characteristic pore size (i.e., 0.453 mm), capillary effects were limited, resulting in an insignificant constant evaporation rate period. A sharp decrease in evaporation rate (i.e., falling rate period) was observed, followed by the slower rate period characterized by Fick’s law of diffusion. Teflon samples entered the slower rate period after 70 h compared to 90 h in glass, and combined with X-ray visualization, implying a lower rate of liquid island formation in the Teflon samples than the glass samples. The evaporative drying front, visualized by X-rays, propagated faster in glass with a final depth (after 7 days) of ~ 30 mm, compared to ~ 24 mm in Teflon. Permeability was modeled based on the geometry [e.g., 3.163E−9 m² (Revil, Glover, Pezard, and Zamora model), 3.287E−9 m² (Critical Path Analysis)] and experimentally measured for both glass (9.5 E−10 m²) and Teflon (8.9 E−10 m²) samples. Rayleigh numbers (Ra = 2380) and Nusselt (Nu = 4.1) numbers were calculated for quantifying natural evaporation of water from fully saturated porous media, and Bond (Bo = 193 E−3) and Capillary (Ca = 6.203 E−8) numbers were calculated and compared with previous studies.