Create wall and apply reflection rule when particle cross the wall.
If the style bounce_back is used, the position of the particle crossing the wall will be reversed and the direction of its velocity and force components are reversed.
If the style bounce_forward is used, the position of the particle crossing the wall will be specular reflected and the direction of its velocity and force components are reversed.
If the style specular is used, the position of the particle crossing the wall will be specular reflected and only the normal direction of its velocity and force components will be reversed.