On-the-fly cross flow laser guided separation of aerosol particles based on size, refractive index and density-theoretical analysis

Laser separation of particles is achieved using forces resulting from the momentum exchange between particles and photons constituting the laser radiation. Particles can experience different optical forces depending on their size and/or optical properties, such as refractive index. Thus, particles can move at different speeds in the presence of an optical force, leading to spatial separations. In this paper, we present a theoretical analysis on laser separation of non-absorbing aerosol particles moving at speeds (1-10 cm/sec) which are several orders of magnitude greater than typical particle speeds used in previous studies in liquid medium. The calculations are presented for particle deflection by a loosely focused Gaussian 1064 nm laser, which simultaneously holds and deflects particles entrained in flow perpendicular to their direction of travel. The gradient force holds the particles against the viscous drag for a short period of time. The scattering force simultaneously pushes the particles, perpendicular to the flow, during this period. Our calculations show particle deflections of over 2500 mu m for 15 mu m aerosol particles, and a separation of over 1500 mu m between 5 mu m and 10 mu m particles when the laser is operated at 10W. We show that a separation of about 421 mu m can be achieved between two particles of the same size (10 mu m) but having a refractive index difference of 0.1. Density based separations are also possible. Two 10 mu m particles with a density difference of 600 kg/m(3) can be separated by 193 mu m. Examples are shown for separation distances between polystyrene, poly(methylmethacrylate), silica and water particles. These large laser guided deflections represent a novel achievement for optical separation in the gas phase. (C) 2010 Optical Society of America