Fig. 12. Definition of three kinds of angles, where e₁ and e₂ are the orientation vectors of cell 1 and 2, respectively. _in is the angle between e_1,in and e_2,in when two cells surfaces are at a distance of L before the collision. _out is the angle between e_1,out and e_2,out when two cell surfaces are at a distance of L after the collision. In order to describes the change of orientation of cell 2 relative to cell 1, a frame is fixed to cell 1 so that the frame rotates when cell 1 rotates, and d is defined as change in the angle of cell 2 relative to this rotating frame.