Finding the right rhythm to free droplets stuck in narrow pores

Subsurface reservoirs are underground rock formations that contain fluids like oil, natural gas, water, or carbon dioxide. During the transport of fluids through porous materials like these, capillary forces can cause a droplet to get stuck in a narrow pore while fluid flows around it.

Weitao Sun and Diyao Wang wanted to know if rhythmically changing the flow could help move a stuck droplet. Using 3D simulations, they studied how a trapped droplet deforms and interacts with the tiny fluid gaps around it.

The model showed that a droplet stuck in a pore under steady flow can pass when the flow is pulsated at the same average speed. However, the frequency of the pulsating flow is important. The stuck droplet mobilizes when the pulsations have an intermediate frequency but remains trapped with pulsations of a low or high frequency.

The results demonstrated that both the average strength of the flow and the timing of the pulsations must be considered when attempting to free a trapped droplet.

“The broader message is that, for a droplet trying to cross a narrow pore, when the flow changes can matter as much as how strongly it flows,” Sun said. “The findings offer a starting point for designing controlled experiments on pulsation-assisted fluid transport and for improving pore-scale models of multiphase flow.”

Next, the authors will explore how different pore geometries, droplet sizes, viscosity ratios, and surface conditions affect pulsation-assisted fluid transport. They also plan to examine connected pores.

“Connecting the model to deformable solids and wave-driven flow would also help assess its relevance to subsurface stimulation,” Sun said.

Source: “Frequency-dependent mobilization of a capillary-trapped droplet in porous constrictions under pulsating forcing,” by Weitao Sun and Diyao Wang, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0346749 .