Tuning the viscosity of a "smart" fluid using light.
Research of Aimee Ketner and Rakesh Kumar.

Is it possible to switch a fluid from a thick jelly state to a thin water-like solution by shining light? And moreover, is it possible to do make such a fluid using simple, cheap chemicals? We show that this can be done in our our JACS (2007) paper, where the "recipe" for the fluid depicted in this movie is given. The movie shows our photoresponsive fluid that is initially very viscous, but becomes very thin and pourable upon UV irradiation.

http://www.complexfluids.umd.edu
http://complexfluids.umd.edu/papers/39_2007g.pdf (less info)