Generating circulation in nonlinear optical fluids with weak laser radiation

We report on the observations of circulations induced by a 10-mW red He-Ne laser (633-nm wavelength) in nonlinear absorbing solutions of 2,9,16,23-Tetrakis(phenylthio)-29H,31H-phthalocyanine. Two solutions were prepared with the same concentrations (approximately 1mM/L) of phthalocyanine in chlorobenzene. Polymer poly(methyl methacrylate) was added to the second solution to increase its viscosity. When the laser beam passed through a solution, the diffraction ring pattern was observed on a screen. As the power density of the laser radiation within a solution increased, more rings could be seen (up to 15 were observed). In the less viscous solution, the rings appeared squeezed from the top half, indicating the presence of convection. In the more viscous solution, however, the rings were perfectly circular. These results were confirmed visually when the fluid circulation (at about .2 rev/s) was seen with the aid of micron size alumina particles in the less viscous solution. No circulation was seen in the more viscous solution when the beam was relatively far below the meniscus. When the beam was positioned at less than .12mm below the meniscus, circulation did occur. Then, the 100mm thick layer of the less viscous solution was positioned horizontally so that the beam was focused from the bottom. An ordered set of Benard-Rayleigh convection cells was observed (for the first time, to our knowledge, without conventional heating). This opto-mechanical effect can be implemented in light driven micro-pumps and similar fluidic devices.