Size and shape characterization of individual flowing droplets by laser light scattering

A new technique for determining both the size changes and the shape distortions of individual droplets flowing in a linear stream is presented. This technique is based on the whispering gallery modes or more accurately morphology dependent resonances (MDRs) associated with microparicles having a rotational axis symmetry. For size and shape determination, the MDRs in the fluorescence spectra from ethanol droplets tagged with Rhodamine 6G dye were measured. Highly monodisperse tagged ethanol droplets are generated by a modified Berglund-Liu vibrating orifice generator. The droplets have a radius in the 10 to 50 micrometer range with a monodispersity of one part in 10,000 to 100,000 and spacing within a linear stream of approx. 100 micrometer. The evaporation of droplets within a linear stream can be readily determined by measuring the wavelength shift of specific MDRs from successive droplets further downstream from the orifice, since the droplet decreases in size monotonically with time after exiting the orifice. The results demonstrate that the wake of preceding droplets significantly decreases the evaporation rate of a droplet within the linear stream.