Optical size determination for single cubic particles suspended in a laser beam

Abstract

An experimental technique by which the angular distribution of light scattered from individual cubic NaCl particles suspended in a laser beam can be measured and correlated with their size is described. This method takes advantage of the fact that the growth of an inorganic salt aerosol by water vapor condensation can be predicted from theory. In practice, the salt particle is exposed to a predetermined constant humidity and grown to a solution droplet in equilibrium with its environment. Since the optical and thermodynamic properties of the NaCl-H 2O system are well known, the size of the solution droplet is readily calculated from Mie theory. The actual volume of the dry salt particle, which is unambiguously determined from the size and composition of the solution droplet, may then be compared with the optical size deduced from the light scattering data assuming a sphere of equal volume. Good agreement is obtained in the size range of 0.5 to 0.7 μm in diameter.