2009

Fragment particles use the same technique, with additional options when the
Thickness setting is greater than 0.
When Thickness is 0, all faces in the fragment are mapped the same as the
portion of the object surface from which they're derived.
When Thickness is greater than 0, the outer faces of the fragment copy the
surface of the distribution object, and are assigned the material ID specified
in the Outside ID spinner in the Particle Type rollout > Fragment Materials
group. The thickness edges use the Edge ID number, and the inner faces use
Backside ID. Thus, by assigning a multi/sub-object material to the object-based
emitter, you can specify a different material for the outer fragment surfaces,
the edges, and the inner surfaces. Note that the Outside ID spinner defaults
to a value of 0, which means "use whatever material is currently assigned."
Change this to a specific sub-material number to assign a sub-material to the
outside edges of the fragments.
Using Multi/Sub-Object Materials with Particle
Systems
Particles with a Multi/Sub-Object material
3012 | Chapter 14 Space Warps and Particle Systems