I have been doing research into how the size of granular material affects the angle of repose. There have been several conflicting results on this topic. Several papers cite that the angle of repose decreases with the increase in particle size, while others say that it increases. However, in the papers where it decreases, the particles are perfect spheres, while where they are decreasing, the particles are granular material like sand. I was slightly confused by this, could anyone clarify why the angle of repose increases with an increase in size for sand but not for glass beads?