Part of the fun of experimenting with granular materials, says Stephen W. Morris, is the showmanship. In one stunt that he has demonstrated in settings ranging from high school classrooms to television studios, the University of Toronto
loads clear plastic tubes with white table salt and black sand and starts them rotating. What transpires in the tubes usually knocks the socks off of any
bystander. Instead of mixing into a drab gray sameness, the sand particles slowly separate into crisp black bands cutting across a long, narrow field of salt. As the spinning continues, some bands disappear and new ones arise. "It's a parlor trick," Morris says. Not to deny its entertainment value, this
of how strangely granular materials can behave is also an authentic experiment in a field both rich in fundamental physics and major practical consequences. Yet granular mixing today remains more of an art than a
, says chemical engineer Fernando J.