Feasibility Studies on Photophoretic Effects in Field-Flow Fractionation of Particles

Abstract

Various mechanisms of particles photophoresis, both of direct and indirect type, are considered theoretically. The analytical expressions are obtained for photophoretic and photo-thermophoretic mobilities of particles, and their dependence on particle size, optical, and physicochemical properties is analyzed. The motion of latex spheres, glass beads, and carbon black particles of 3–22 μm diameter, in water, under the action of focused Ar+-ion laser beam, was studied experimentally at LD=514.5 nm and power 0.1–0.8 W, using two arrangements. In the first one, the particles' motion was observed through the microscope. The positive photophoresis (away from the light source) was registered for 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 axis, in the direction of suspension flow inside the capillary. The elution curves for polydisperse carbon black particles were registered in the gravity-sedimentation FFF mode with the laser power switched on and off. Typical curves possessed a strong initial maximum, attributed to the fraction of smaller particles, and a substantially lower secondary maximum related to large particles. The action of light changed the 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 particles in the range 1–100 mm/sec, depending on the light intensity, which are sufficient to accomplish their separation relative to size, optical, and surface charge properties.