Microscopic mechanism of squeeze expulsion in granular size segregation

In the gravity-driven free-surface flow of grains, the mechanism of the segregation phenomenon in binary granular flow is mainly attributed to kinetic sieving and squeeze expulsion. Although much literature has delved into the penetration of small grains through random fluctuation sieving, independent research on the microscopic mechanism of squeeze expulsion remains insufficient. Our previous research found that squeeze expulsion is particularly prominent in quasi-two-dimensional binary disk grain flow. Based on this result, we used the discrete element method (DEM) and experiments to explore the mechanism of squeeze expulsion. The results show that the anisotropy of the contact force chain network and the velocity difference of the grains in different positions play a key role in the expulsion behavior of the grains. This expulsion behavior is influenced by the dynamics and instability of the force chains, manifesting itself as a probabilistic phenomenon. Through DEM simulations, we quantified the probabilities of large grains being expelled at different positions and under various slope angles. It was found that as the slope angle increases, the probability of large grains being expelled to the upper layer also increases, intensifying granular segregation. The probability of large grains being expelled is highest in the granular flow substrate layer. The revealed mechanism of squeeze expulsion in this study is crucial for understanding grain mixing and separation.