Long-lived reactive species influence the rapid activation process of a water droplet by atmospheric-pressure pulsed air discharge

In this study, the atmospheric-pressure pulsed air discharge over the water droplet (APAD-WD) was used to product plasma-activated water (PAW). The activation efficacy of PAW was systematically evaluated by examining bactericidal activity against Escherichia coli (E. coli) as a function of discharge power and treatment time. The results indicate that PAW prepared by sealed processing at 3.5 W power exhibits the highest antimicrobial activity, achieving a bactericidal efficiency of 6.67 logs for E. coli within 4 s. Moreover, the concentrations of hydrogen peroxide (H2O2), nitrite ion (NO2-) and nitrate ion (NO3-) in the PAW treated with APAD-WD for 12 s are 1.71 mM, 0.90 mM and 4.95 mM, respectively. Water is quickly activated by the APAD-WD, which can be attributed to the large specific surface area of water droplet, thin water layer and high electron density of pulsed discharge. Compared to open treatment, PAW prepared by closed treatment (CT-PAW) exhibits higher chemical and biological reactivity. Experimental evidence demonstrates that closed treatment effectively increases the concentration of long-lived reactive species near water droplets, thereby rapidly activating the solution.