Cavity-Enhanced Fluorescence in Colliding Droplets of Rhodamine 6G Aqueous Solutions.

A liquid droplet can act as a microscale high-Q optical cavity via a whispering gallery mode, where light resonates with enhanced intensity. A collision of two droplets temporarily exhibits a unique morphology, which provides a more effective optical cavity than a single droplet. We investigated the mechanisms of cavity-enhanced fluorescence in colliding droplets of aqueous rhodamine 6G. Laser-excited spectra and fluorescence generation times were acquired. The fluorescence spectra had two peaks: one attributed to amplified spontaneous emission (ASE) and the other to lasing. The lasing generation time had a longer delay relative to that of ASE, which indicated that it required a longer propagation distance for the positive feedback. Overall, this provides a basis for the development of a highly efficient dye laser using colliding droplets.

Chemical Reactions at the Interface Periphery of Colliding Droplets Studied by Raman Image Analysis.

Chemical reactions at the interface of reactive solutions are of importance for a full understanding of solution reactions. We investigate the chemical reaction induced by the collision of two droplets. The extent of the reaction is measured by analyzing spectra and images of the Raman scattered light emerging from the interface of the colliding droplets of H2SO4 and NH3 aqueous solutions. The obtained product concentration is lower than that expected from a simple diffusion model. The result indicates that a fresh interface is produced at the periphery of the mixing region of the colliding droplets. This study provides the basis to extend this method to measure rapid chemical reactions at the interface of colliding droplets.