It is about fluid flow among grains.
8jce6Ruben Juanes, Yue Meng, Bauyrzhan K. Primkulov, "Multiphase flow and granular mechanics", Physical Review Fluids [Phys Rev Fluids], vol 5, no 11, p 110516, Nov 2020, url https://doi.org/10.1103/PhysRevFluids.5.110516.
It is from a colleague in the last week of previous month.