Schematic diagram showing the moment when the two granular aggregates just touch. The aggregates are equal in size with radius R and collide with a relative velocity υ. The point of contact is denoted by “C.” The solid green line connects the centers of the aggregates. The dashed gray lines are drawn from the centers of the aggregates along the direction of motion. The impact parameter of the collision, B, is the perpendicular distance between the gray lines, shown by black arrows. The collision angle is denoted by θ. The dashed black line represents the edge-on view of the common tangent plane of the two aggregates on the point of contact; it is denoted as the “collision plane.” The angle between the collision plane and the direction of motion is denoted as ψ.