Light scattering by microdroplets of water and water suspensions

We investigated elastic light scattering on isolated evaporating droplets of radius between 1 and 20 µm. The droplets were either pure water or a water based suspension, they carried electric charge and were contained in an electrodynamic trap. The evolution of the trapped droplet was investigated by means of scatterometry. A numerical model of such evolution, incorpo- rating the kinetic effects near the droplet surface was constructed. For water droplets with spherical inclusions the radius as well as effective refractive index was determined. An essential deviation, in the form of a resonance, from predictions by standard effective medium theories was encountered. Simple analysis of the phenomenon was conducted and a qualitative explanation is proposed. Similar analysis was applied to fullerene water suspension droplets in order to investigate the real part of refraction index. The observation of light scattered on various objects is a most common method of investigation of the reality. In this paper we study the scattering of light on water and water suspensions particle of the fundamental, ideal shape of a sphere, with the radius comparable to the wavelength of the used light - a few micrometers. Under normal atmospheric conditions - below 100% relative humidity S - the droplets of pure water are not stable. They grow for S>1 or shrink for S<1. Careful observa- tion of light scattering together with the appropriate use of theory allows to determine the radius and the refraction index n (or dielectric function ε: ε=n2) of the droplet. The issue of refractive index is especially interesting for droplets of suspen- sions, which are so omnipresent. In the first part of this paper we present the study of evolution of pure water microdroplet with well known refraction index. This investigation made it possible to look into kinetic regime of droplet evolution - the region of droplet sizes of the order of the free path of air molecules. In this region it is necessary to supplement diffusion coefficient with so called evaporation coefficient αC describing the ratio of the number of molecules crossing the liquid-vapor interface to the number of molecules impinging on it: αC =nevap/ncol. Similarly, the thermal conductivity of moist air must be supplemented with the thermal accommodation coefficient αT, determines the probability that a molecule on impinging the interface attains the thermal equilibrium with the medium on the opposite side. The literature yields a very imprecise value for αC and αT ranging from 0.01 to 1 (compare e.g.: (1, 2, 3, 4)). The aim of our first experiments was to find the value of αC and αT. 2. MODEL