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Proceedings Paper

Phase transition in photo-absorbing single microemulsion droplets during the laser photophoretic migration
Author(s): Hideaki Monjushiro; Makiko Tanaka; Hitoshi Watarai
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Paper Abstract

Laser-photophretic behavior of the micrometer-sized single droplets of photo-absorbing microemulsion was observed in water by irradiating a cw Ar+ ion laser. We have made an unexpected observation that the droplets repeated the expansion and the sudden contraction motion during the laser photophoretic migration. The period of the expansion-contraction cycle was decreased as the irradiated laser power was increased and was independent of the initial size of the droplet. The origin of the cyclical motion is the temperature rise of the droplet due to the photo-thermal conversion. The three-step mechanism for the periodic motion of the droplet was proposed. The first step is 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. In the second step, 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 inside and outside of the microemulsion liquid membrane of the droplet. In the third step, the liquid membrane became thinner to a critical thickness which induced its break and the droplet shrank into the original shape. The proposed mechanism for the expansion process was confirmed by the numerical simulation of the volume change of the droplet during the laser irradiation, taking account of the heat evolution in the droplet, the heat transfer from the droplet, and the percolation of the external water by thermo-osmosis and by the high-speed CCD imaging of the contraction motion of the droplet.

Paper Details

Date Published: 26 August 2005
PDF: 11 pages
Proc. SPIE 5930, Optical Trapping and Optical Micromanipulation II, 593018 (26 August 2005); doi: 10.1117/12.617973
Show Author Affiliations
Hideaki Monjushiro, Osaka Univ. (Japan)
Makiko Tanaka, Osaka Univ. (Japan)
Hitoshi Watarai, Osaka Univ. (Japan)

Published in SPIE Proceedings Vol. 5930:
Optical Trapping and Optical Micromanipulation II
Kishan Dholakia; Gabriel C. Spalding, Editor(s)

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