Single droplet size, velocity, and optical characteristics by the polarization properties of scattered light

A method is described for obtaining size, velocity, and optical properties of transparent spherical droplets employing the polarization characteristics of scattered light. A preliminary analysis of the Lorenz-Mie solution, in comparison with geometrical optics, points out the importance of surface waves in the side scattering region between theta = 85 degrees and theta = 120 degrees . Here the horizontal component of scattered light due to surface waves prevails over that due to external reflection for droplets smaller than 100 microm and the polarization ratiogamma = C(HH)/C(VV) can be employed for determining the particle size. A dual-beam system is made of two equal intensity circularly counterotating polarized laser beams which generate a polarized fringe pattern in the interference volume. The polarization ratio of scattered light, at a fixed scattering angle theta, and velocity is obtained by analysis of the bursts produced by individual droplets. The method was tested by determining the size and velocity distribution functions of droplet arrays produced by a Berglund-Liu atomizer operated either in monodisperse or in bidisperse regimes. The angular pattern of the polarization ratio was determined on calibrated streams of transparent droplets with different refractive indices, and the influence of this parameter on the role of surface waves in different angular scattering regions is discussed.