Disclosed is an ink composition which comprises an aqueous phase, an oil phase, a photochromic material, and a surfactant, said ink exhibiting a liquid crystalline gel phase at a first temperature and a liquid microemulsion phase at a second temperature higher than the first temperature....http://www.google.de/patents/US5551973?utm_source=gb-gplus-sharePatent US5551973 - Photochromic microemulsion ink compositions
