Deciphering surface-area-dependent foam stability: a combined experimental and molecular dynamics study of SiO 2 /SDS systems

Nanoparticle enhanced foam flooding shows great promise for enhanced oil recovery, yet the underlying nanoscale mechanisms that govern foam stability remain poorly understood. This study systematically investigates the effect of nanoparticle surface area - and thus effective particle size – on sodium dodecyl sulfate (SDS) foam stability using four hydrophilic SiO2 nanoparticles with specific surface areas ranging from 100 to 380 m2/g. The results demonstrate that the concentration and specific surface areas exhibit negligible influence on the foamability, they profoundly affect foam stability. At a fixed concentration of 1 wt%, increasing the specific surface area from 100 m2/g to 380 m2/g extends the foam drainage half-life by 90 s (from 310 s to 400 s), representing a∼29% improvement. To elucidate the underlying physics, a two-phase molecular dynamics model at the gas-liquid interface was constructed. The simulation revealed a size dependent stabilization mechanism: foam stability increases as the nanoparticle size decreases, reaching an optimum at 1.0 nm. At this optimal size, the nanoparticles not only adsorb at the gas-liquid interface but also geometrically confined and crowded within the narrow liquid film. This dual action mechanism kinetically suppresses water diffusion while effectively reinforcing the structural integrity of nanoparticles at the gas-liquid interface and in the liquid film. Consequently, both liquid drainage and bubble coarsening are synergistically retarded, leading to an enhancement in macroscopic foam stability. These findings provide a new mechanistic understanding of nanoparticle stabilized foams and offer a quantitative design guideline for selecting optimal nanoparticle dimensions in enhanced oil recovery applications.