Abstract
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 µm for 15 µm aerosol particles, and a separation of over 1500 µm between 5 µm and 10 µm particles when the laser is operated at 10 W. We show that a separation of about 421 µm can be achieved between two particles of the same size (10 µm) but having a refractive index difference of 0.1. Density based separations are also possible. Two 10 µm particles with a density difference of 600 kg/m3 can be separated by 193 µm. 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.
Highlights
The use of a laser to separate non-absorbing microscopic particles is based on the momentum exchange between the particles and the photons constituting the laser radiation; the resulting force is called optical or radiation pressure [1]
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
Separations performed using a mildly focused laser beam typically involve a counter propagating fluid flow, and this is termed optical chromatography (OC); when the fluid flow is perpendicular to the laser beam, the separation is termed as cross-type optical chromatography [5]
Summary
The use of a laser to separate non-absorbing microscopic particles is based on the momentum exchange between the particles and the photons constituting the laser radiation; the resulting force is called optical or radiation pressure [1]. In OC, particles in the laser beam with varying properties move differentially against an opposing fluid flow and come to rest where the optic and fluidic forces balance [5,6]. Such laser separations of particles in aqueous suspension have been shown in a number of our previous studies [3,7,8,9]. The airborne particles of interest are often collected and added to liquid by impingers, impactors [29] or growth tube like devices [30] In such suspensions, the particle concentration is often low for analysis and requires additional offline processing. An additional advantage of direct aerosol particle interrogation is the potential ability to sort particles of interest and make real time decisions regarding particle identity and the need to collect the sample or not
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