Pore-scale interfacial delivery of PS/PEGDA microcapsules for surfactant transport and localized activation in porous media

Surfactant transport in porous media suffers from severe non-target loss. • A PS/PEGDA microcapsule system enables pore-scale interfacial delivery. • Oil contact induces microcapsule release and localized surfactant activation. • Carrier-mediated transport enhances flow redistribution and oil recovery. Transporting active agents to fluid-fluid interfaces within heterogeneous porous media remains challenging because dissolved species follow preferential flow pathways, undergo non-target loss during transport, and therefore act inefficiently at the confined interfacial regions where they are needed. Here, we develop oil-responsive microcapsules with a polystyrene/poly(ethylene glycol) diacrylate (PS/PEGDA) composite shell as a model platform for spatially interfacial delivery in multiphase porous media. Microfluidic fabrication produces monodisperse microcapsules with a tunable shell architecture, and the shell governs the trade-off between payload preservation during aqueous transport and oil-contact-induced release through enlarged shell pores due to PS dissolution. The microcapsules remain stable in saline, yet undergo interfacial localization upon oil exposure. Dynamic interfacial-tension measurements show that the petroleum-brine interfacial tension decreases from 28 mN/m to approximately 9.7 mN/m within 30 min after microcapsule addition, confirming interfacial activation upon arrival at the interfaces. Dynamic interfacial tension measurements and image velocimetry support the formation of local interfacial tension gradients and Marangoni-related interfacial convection, while apparent retention energy analysis indicates that interfacial trapping remains favorable after capsule contact. This effect enhances local flow perturbation after capsule arrival, assisting capsule migration toward the interface. In visualized micromodel experiments, surfactant-loaded microcapsules increase the oil recovery and produce a wide flow-field distribution through microparticle image velocimetry. These results demonstrate that carrier performance arises from the coupling of transport stability, interfacial localization, structural activation, and pore-scale accessibility rather than from interfacial tension reduction alone. This work establishes a general shell architecture-based framework for designing spatially selective delivery in multiphase porous transport. Download: Download high-res image (99KB) Download: Download full-size image