FEASIBILITY STUDIES ON PHOTOPHORETIC EFFECTS IN FIELD-FLOW FRACTIONATION OF PARTICLES

Various mechanisms of particles photophoresis, both of direct and indi rect type, are considered theoretically. The analytical expressions ar e obtained for photophoretic and photothermophoretic mobilities of par ticles, and their dependence on particle size, optical, and physicoche mical properties is analyzed. The motion of latex spheres, glass beads , and carbon black particles of 3-22 mu m diameter, in water, under th e action of focused Ar+-ion laser beam, was studied experimentally at lambda=514.5 nm and power 0.1-0.8 W, using two arrangements. In the fi rst one, the particles' motion was observed through the microscope. Th e positive photophoresis (away from the light source) was registered f or all kinds of particles. Photophoretic velocities of particles were evaluated in connection with their size, optical properties, and laser power density. In another arrangement, the laser power was focused at the entrance glass window of a round metallic capillary, along its ax is, in the direction of suspension flow inside the capillary. The elut ion curves for polydisperse carbon black particles were registered in the gravity-sedimentation FFF mode with the laser power sn-itched on a nd off. Typical curves possessed a strong initial maximum, attributed to the fraction of smaller particles, and a substantially lower second ary maximum related to large particles. The action of light changed th e shape of the first maximum and shifted to a smaller time the second one. Both experimental and theoretical results show the possibility to generate, under FFF conditions, the photophoretic velocities of parti cles in the range 1-100 mm/sec, depending on the light intensity, whic h are sufficient to accomplish their separation relative to size, opti cal, and surface charge properties.