title: Laser photophoretic migration with periodic expansion-contraction motion of photo-absorbing microemulsion droplets in water.
abstract: When the water-in-oil (w/o) microemulsion droplets including the Co(III)-pyridylazo complex as the photo-absorber were irradiated with a continuous-wave Ar(+) ion laser (514.5 nm), we have observed unexpected phenomenon that photo-absorbing microemulsion droplets in water repeated the expansion and the sudden contraction during the laser photophoretic migration. The period of the expansion-contraction cycle was inversely proportional to both the concentration of the complex and the irradiated laser power and was independent of the initial size of the droplet. The mechanism of the periodic motion of the droplet was investigated by local temperature measurement and Raman microscope spectroscopy. It was suggested that the first step was the phase separation of the bicontinuous microemulsion droplet into the normal w/o microemulsion outer phase and the aqueous inner phase in the droplet, which was caused by the laser-induced temperature gradient inside the droplet. Subsequently, an expansion of the inner aqueous phase was induced by the percolation of the external water by thermo-osmosis, which was caused by the laser-induced temperature gradient between the inside and the outside of the microemulsion liquid membrane of the droplet. When the liquid membrane became thinner to a critical thickness, the inner aqueous phase was released and the droplet shrank into the original size. The proposed mechanism can give an account of the unique cyclical motion.
authors.affiliation: Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan.
authors.forename: Makiko
authors.initials: M
authors.lastname: Tanaka
authors.forename: Hideaki
authors.initials: H
authors.lastname: Monjushiro
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Watarai