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Continuous-feed optical sorting of aerosol particles



John J. Curry, Zachary H. Levine


We consider the problem of sorting, by size, spherical particles of order 100 nm radius. The scheme we analyze consists of a heterogeneous stream of spherical particles flowing at an oblique angle across a Gaussian mode optical standing wave. Sorting is achieved by the combined spatial dependence and size dependence of the optical force. Particles of all sizes enter the flow at a point, but exit at different locations depending on their size. Here they may be detected optically or separated for further processing. The scheme has the advantages of producing a continuous stream of sorted particles, preserving the particles without alteration, and utilizing a simple, low numerical aperture optical system. We derive the optical force exerted by a plane standing wave on a spherical particle from first principles. The result reduces to the well-known Rayleigh formula in the limit of small radius. For larger radii, the force undergoes irregular oscillations, vanishing for certain values of particle radius. This leads to a number of disjoint ranges of radii which may be sorted relatively easily. In particular, with an optical wavelength of $1064$~nm and particle index of refraction of $1.57$, particles with radius in the neighborhood of 275~nm may be sorted with a resolution below 1~nm. We performed detailed Monte Carlo simulations of particle trajectories through the optical field under the influence of a convective air flow. We also developed a method for deriving effective velocities and diffusion constants from the Fokker-Planck equation that can generate equivalent results much more quickly. This makes it possible to explore various configurations at low computational cost.
Optics Express


Curry, J. and Levine, Z. (2016), Continuous-feed optical sorting of aerosol particles, Optics Express, [online], (Accessed April 17, 2024)
Created June 15, 2016, Updated June 2, 2021