Scientific Achievement

A new experimental approach is developed for studying surface chemistry in micron-sized droplets. An electrodynamic balance is used to collide two droplets, which produces a pH jump and a chemical reaction. Reaction kinetics are accelerated relative to macroscale beakers. Using a kinetic model the reaction acceleration mechanism is attributed to the droplet interface.

Significance and Impact

Recent studies report the dramatic acceleration of chemical reactions in micron-sized compartments. In the majority of these studies the exact acceleration mechanism is unknown, but the droplet interface is thought to play a significant role. We show that the accelerated synthesis of azamonardine in droplets originates from the fast diffusion of O2 and the enrichments of reagents at the droplet interface.

Research Details

Dopamine reacts with resorcinol to form a fluorescent product azamonardine and is used as a model system to examine how droplet interfaces accelerate reaction kinetics. The reaction is initiated by colliding two droplets levitated in a branched quadrupole trap, which allows the reaction to be observed in individual droplets where the size, concentration, and charge are carefully controlled. The collision of two droplets produces a pH jump and the reaction kinetics are quantified optically and in situ by measuring the formation of azamonardine. The reaction was observed to occur 1.5 to 7.4 times faster in 9-35 micron droplets compared to the same reaction conducted in a macroscale container. A kinetic model of the experimental results suggests that the acceleration mechanism arises from both the more rapid diffusion of oxygen into the droplet, as well as increased reagent concentrations at the air-water interface.

Publication Details

E. K. Brown, G. Rovelli and K. R. Wilson, “pH jump kinetics in colliding microdroplets: accelerated synthesis of azamonardine from dopamine and resorcinol,” Chemical Science, 2023.
DOI: 10.1039/D3SC01576A