Droplet collision mixing diagnostics using single fluorophore LIF

A novel droplet mixing measurement technique is presented that employs single fluorophore laser-induced fluorescence, custom image processing, and statistical analysis for monitoring and quantifying mixing in confined, high-speed droplet collisions. The diagnostic procedure captures time-varying fluorescent signals following binary droplet collisions and reconstructs the spatial concentration field by relating fluorophore intensity to relative concentration. Mixing information is revealed through two governing statistics that separate the roles of convective rearrangement and molecular diffusion during the mixing process. The end result is a viewing window into the rich dynamics of droplet collisions and a diagnostic tool that differentiates between poor and effective mixing. The technique has proved invaluable in the laboratory by allowing direct comparison of different hydrodynamic conditions, such as collision Reynolds and Peclet number, and collision geometries, such as T and Y-junctions. Experiments indicate improved mixing rates and degree of homogenization as the convective timescale for the collision is decreased. Visualization of mixing residuals using pseudo color mapping also identifies areas that are largely segregated from the mixing process, resulting in islands where mixing is poor and stirring has proved ineffective. As the collision velocity is increased, vortical flow fields become apparent and mixing is greatly improved.