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We report the generation of radical droplets activated by an underwater electrical discharge and the sterilization of bacteria cells by spraying the radical droplets to open air. The plasma formed by electrical discharge in a narrow dielectric capillary tube produces atomic oxygen, atomic hydrogen, and hydroxyl radicals from the dissociation of water vapor, which is achieved through Joule heating in the narrow capillary tube. The capillary discharge electrode is placed in a water flow line, and the plasma-activated water is sprayed as fine droplets through a nozzle at 0.08 L/min. In a bactericidal test, the droplets show a sterilization effect of more than 99.99% after an exposure time of 3 h. The proposed radical droplet generator may also be useful for the elimination of airborne microorganisms. [ABSTRACT FROM AUTHOR]