Biosurfactant-Mediated Alteration of Pore-Scale Evaporation: Surface Tension Effects

Abstract

Abstract. The Ogallala Aquifer, the largest freshwater source in North America, has lost more than 400 km3 of storage since the 1930s. Reducing soil evaporation is one potential approach to extend water availability. This study investigates the effects of the biosurfactant, Surfactin, on pore-scale evaporation. Surfactin lowers surface tension and alters wettability, thereby affecting how water behaves in porous media. We investigated evaporation in a three-bead, single-pore model with hydrophilic glass and hydrophobic Teflon 2.38-mm-diameter beads. Droplets of de-ionized water and Surfactin solution (50 ppm) were placed in the pores. High-speed imaging from side and top views measured evaporation time, contact angle, and projected length of contact (PLOC). For glass pores, Surfactin produced a modest reduction in mean evaporation time (from 54 to 52 min), whereas the reduction was more pronounced for Teflon pores (from 62 to 57 min). Surfactin also advanced liquid island rupture by approximately 3 min in glass and 6 min in Teflon pores. It promoted earlier depinning of contact lines, as indicated by sharper PLOC contraction and fluctuating contact angles consistent with stick–slip dynamics. To interpret these effects, the Priestley–Taylor framework was modified to include an effective latent heat term linked to surface tension and a material-specific coefficient representing wettability. These results demonstrate that biosurfactants alter interfacial dynamics and evaporation pathways at the pore scale, offering pore-scale mechanistic insight relevant to evaporation-stage transitions in porous media.