Droplet slicing measurements of internal circulation

Measurement of internal circulation using laser-induced oxygen-quenched fluorescence are being pursued under known initial conditions. Decane doped with naphthalene was used to form droplets from either a droplet-on-demand generator, or an aerodynamic droplet generator which fall a short distance in a chamber filled with a carefully controlled shear flow of nitrogen and a variable amount of oxygen. A thin sheet of ultra-violet light from the fourth harmonic of a Nd:YAG laser illuminates single droplets. A highly-magnified image of the naphthalene fluorescence is digitally recorded using an intensified two-dimensional CCD detector interfaced to a laboratory computer. Since oxygen is a strong fluorescence quencher, any liquid volume element which has been exposed to it by surface contact or diffusion will suffer a reduction in fluorescence intensity. Convection from the surface due to internal circulation as well as diffusion cause image regions to appear dark. Oxygen free experiments, variation of droplet initial internal flow patterns, and varied shear flow conditions are being used to examine shear-induced internal circulation.