Pore-scale influence of surfactants on evaporation in a porous medium.

Despite their widespread use in controlling interfacial behavior in porous media, the pore-scale influence of surfactants on evaporation is still not well understood. This study examines how varying concentrations of sodium dodecyl sulfate (SDS) modify evaporation dynamics within a polydimethylsiloxane (PDMS) microfluidic network. Evaporation experiments were conducted using pure water and SDS solutions at 0.10 wt.% (below the critical micelle concentration, CMC), 0.23 wt.% (at CMC), 0.3 wt.%, and 0.5 wt.% (above CMC). High-resolution imaging and image analysis enabled direct quantification of liquid saturation, meniscus evolution, and contact angle behavior. At the CMC, SDS reduced surface tension from 72 mN/m to 40 mN/m, lowered capillary entry pressures, and promoted air invasion that completed evaporation 47% faster than water. Below the CMC, partial surface tension reduction improved early drainage but sustained thin films that delayed complete evaporation. The results show that tuning surfactant concentration near the CMC effectively controls pore-scale drainage and evaporation kinetics. These findings provide a foundation for developing predictive models and optimizing the design of porous materials for applications involving controlled evaporation.