Induced Flow and Motions of Fine Solid Particles in Water With Irradiation of YAG Laser

Induced flow and motions of fine solid particles with irradiation of YAG laser beam in water droplet, often called laser trapping, are discussed in this paper. While the absorption of the near infrared laser beam into glass plate and water can be generally small as they have high transmissivity, the absorption should be effective for the high intensity of laser beam by being focused under a microscope. A micro-sized heat spot could be produced by the absorption of the laser beam, and heat transfer occurred from the heated glass plate to the water droplet with the particles. The local flow was induced by the effect of the heat transfer. The force of the flow exerting the particles was so great that the radiation pressure on the particle would not be effective any more. The local flow caused the motions of the particles far from the laser beam, e.g., rotating, clustering and unexpected motions induced by a produced bubble on the bottom of the glass plate. These interesting phenomena were observed under microscope and recorded by a VHS connecting to a CCD camera. The particles were rotated with the irradiation of the laser beam, the revolution was too high compared to the VHS capture rate. Also one of these interesting phenomena was that the scattered particles in the water droplet were clustered around the focal spot of the laser beam. The theoretical backgrounds on the induced flow and the motion of the fine particles are also discussed.