A two-dimensional schematic picture of spherical particles on the lattice.
Left: a particle is allowed
to move continuously across the lattice, and the position of the
-surface defines fluid lattice sites (light blue) and solid lattice
-sites (dark red). The discrete surface is defined by links
+surface defines fluid lattice sites (gray) and solid lattice
+sites (black). The discrete surface is defined by links
where propagation would intersect the surface (arrows). Note the
discrete shape of the two particles is different. Right: post-collision
distributions are reversed at the surface by the process of bounce-back
