Dual-wavelength extinction rainbow refractometry for in-situ characterization of colloidal droplets

Abstract

Considerable interest and demand exists in multi-parameter characterization of colloidal droplets. However, an optical in-situ characterization of such droplets is a particular challenge due to its heterogeneous and multi-scale nature, motivating the investigation of a dual-wavelength extinction rainbow refractometry. An analytical model is proposed, in which dual-wavelength extinction effect of the inclusions on the amplitude of the primary rainbow peak allows measurement of inclusion concentration and particle size. The rainbow pattern itself yields the refractive index and diameter of the host droplet. Validation experiments using deionized monodisperse water droplets containing standard polystyrene nanoparticles with different concentrations (0%, 0.1%, 0.2%, 0.3%) and particle sizes (210 nm, 530 nm) were conducted. Measured refractive indices and droplet diameters agree well with the known values. Measurement errors of inclusion concentration and size were analyzed with and without a prior knowledge of the inclusion size. Measurement limits of this approach are also discussed.