Robin,
        The materials you are looking for are called
"pola-tape."  A small motor with a rotating polarized filter
was attached to the bottom of an overhead projector lens:  the
parts of the overhead transparency where the tape had been
applied gave the optical effect of liquid motion.  Las I know,
there was a company in King of Prussia Pennsylvania marketing
these things but I do not have a name or addr.
        It used to be fairly common in the Audio-Visual field.
If I can get more information, I will let you know.