Inter-Droplet Electric Fields Drive Water Redox Chemistry in Microdroplets.

The chemistry of water microdroplets is often interpreted from the perspective of isolated droplet interfaces, although practical microdroplet systems more commonly exist as dense ensembles of interacting droplets. Here, we show that a closed reactor based on ultrasonic nebulization enables water microdroplets to generate hydrogen, oxygen, hydroxyl radicals, and hydrogen peroxide under ambient conditions. These products appear only during active nebulization, and isotopic labeling confirms that water directly contributes to the formation of the detected hydrogen and oxygen products. The generated droplets carry net charge and repeatedly approach one another within the confined reactor, creating transient interdroplet gaps throughout the droplet ensemble. Finite-element simulations indicate that these regions can host electric fields strong enough to initiate water redox chemistry. Complementary spectroscopic and mass spectrometric analyses further verify the formation and accumulation of oxidative intermediates during operation. These results show that the reactivity of water microdroplets is shaped not only by the properties of individual droplet interfaces but also by the local electric fields that arise from collective droplet behavior.