from JSphCpu::InteractionForcesDEM function it seems that fn (normal contact force) is applied in the normal between the particles (as mentioned in the source code). it is valid if particles from two interacting solid falls exactly on top of each other but what happens to the case that particle from solid A falls in between two particles from solid B. For this case the normal force is not going to be aligned with the normal vector of the face that these particles belong to. would you please clarify? thank you in advance.