In porous materials (soil, rocks, filters) fluids move through tiny, winding channels. When two immiscible fluids, such as water and air, are present simultaneously, their interface constantly shifts: air bubbles push water out of some pores and open paths through others. It resembles a crowded subway where the crowd constantly reshuffles, forcing individuals to change their trajectories.

Researchers studied how such unsteady two-phase flow affects the mixing of dissolved substances. They found that the movement of the phase interface makes the fluid stretch and fold like dough that is repeatedly rolled and folded. This process is called chaotic mixing: neighboring fluid particles rapidly separate, and substances spread much more efficiently than in a calm single-phase flow.

Using numerical models, the scientists established that the mixing rate depends on the flow rate. If the flow is too slow, the fluid barely deforms; if too fast, the phase interface moves so often that it cannot effectively stretch the fluid. Mixing peaks at an optimal speed, where shear deformation is balanced by the frequency of flow direction changes.

This finding helps understand how mixing and chemical reactions occur in multiphase systems in nature and technology — from contaminant migration in groundwater to processes in chemical reactors. By controlling the flow rate, one can enhance or suppress mixing, opening new possibilities for water purification, oil recovery, and many other applications.