Fluids moving through soils and rocks mix and react within a network of tiny, interconnected pores. This mixing process controls how contaminants break down, and chemical reactions occur in natural environments. While it is common to describe mixing by looking at flow speeds, the way fluid twists, rotates, stretches, and folds inside pores is equally important because it determines how different fluids or chemicals come into contact.

This study examines what happens when air is introduced into a water-filled porous material. The presence of both air and water changes the flow pathways and alters how fluid parcels rotate and deform. Using advanced 3D computer simulations, we explore whether these air-induced flow changes promote spiral-like currents, how they enhance or suppress mixing at fluid interfaces, and whether patterns of 'flow twisting' (helicity) can predict mixing efficiency.

By focusing on the relationship between flow structure and mixing in realistic 3D unsaturated systems, this work helps reveal the mechanisms that control chemical reactions and transport in soils and aquifers, improving our ability to predict water quality and contaminant behavior in the natural subsurface.