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.

Low-Frequency Raman Scattering in Colliding Benzene Droplets

Low-frequency intermolecular motions are of importance for understanding the structure of molecular liquids, which can be elucidated by Raman spectroscopy. A liquid droplet provides a field where stimulated Raman scattering (SRS) readily proceeds. Our previous study showed that multiorder SRS is more effectively generated in colliding droplets than a single droplet. Here we report that the multiorder SRS generated in colliding benzene droplets included low-frequency bands of the liquid benzene, which is considered to represent a temporal state of liquid emerging in the course of the droplet collision. This method enables measurements of the low-frequency intermolecular motions of the novel liquid state emerging at the mixed region of the colliding droplet without suffering from background of Rayleigh scattering.

Multiorder Stimulated Raman Scattering in Colliding Droplets

Nonlinear Raman spectroscopy is a versatile method to enhance the intensities of Raman scattering. It requires an intense light field that can be provided by a liquid droplet acting as a high-quality optical cavity. Here, colliding droplets were used as a novel optical cavity to enhance the intensity of Raman scattering. Specifically, multiorder stimulated Raman-scattered light was generated with significant intensity from colliding droplets of carbon tetrachloride (CCl₄). The intensities of the Raman bands were analyzed with a simple theory that roughly reproduced the experimental spectrum. Overall, the method facilitates Raman spectroscopy of molecules in liquids because of its high sensitivity and resolution.

Initial Collision Process of Two Miscible Droplets

Dynamic properties of the metastable interface between two miscible solutions are investigated by the collision of two droplets. A clear interface is observed between the two colliding droplets. The interface moves in the colliding droplet toward the side where the original droplet has a lower surface tension. The interface is set to the middle of the colliding droplet by controlling the surface tension of the droplets to observe the chemical reactions at the droplet interface by cavity-enhanced Raman spectroscopy. This study provides a foundation for further research on the initial process of the chemical reactions of two miscible solutions.

Evaporation and Subsequent Adsorption of Alcohol Molecules at Aqueous Droplet Surface Observed by Cavity-Enhanced Raman Spectroscopy

Mass transfer toward and across liquid surfaces is important for the interpretation of various interfacial phenomena, such as evaporation, adsorption, and mass accommodation, which have been investigated by the use of various methods. These studies, however, have focused on only one of the mass-transfer processes occurring at the surface. We investigate the surface concentration of alcohol molecules at aqueous droplet surfaces on the several-millisecond time scale using cavity-enhanced droplet Raman spectroscopy. A decrease and subsequent increase of the alcohol concentration are observed in a set of measurements, which arise from an evaporation and subsequent adsorption of the alcohol molecules at the surface. This facilitates an understanding of the surface kinetics of molecules at the liquid surfaces.

Development of a Tandem Electrodynamic Trap Apparatus for Merging Charged Droplets and Spectroscopic Characterization of Resultant Dried Particles

Materials work in multicomponent forms. A wide range of compositions must be tested to obtain the optimum composition for a specific application. We propose optimization using a series of small levitated single particles. We describe a tandem-trap apparatus for merging liquid droplets and analyzing the merged droplets and/or dried particles that are produced from the merged droplets under levitation conditions. Droplet merging was confirmed by Raman spectroscopic studies of the levitated particles. The tandem-trap apparatus enables the synthesis of a particle and spectroscopic investigation of its properties. This provides a basis for future investigation of the properties of levitated single particles.