Nanosecond discharges on water droplet: interface-driven formation of silver nanomaterials

Interactions between pulsed electrical discharges and liquid materials represent a growing research field with significant interests spanning fundamental discharge physics and diverse applications. These applications include water processing, material treatment (such as synthesis and functionalization), and healthcare. This study focuses on understanding and utilizing nanosecond pulsed discharges generated in air in the presence of millimeter-scale water droplets. We investigate how applied voltage (V_a) and the droplet's electrical conductivity (EC) influence the discharge dynamics, which can transition between streamer and spark modes. We found that while streamer ignition is relatively insensitive to EC, the transition to a spark can be finely controlled by it, and both V_a and EC significantly impact interfacial transport mechanisms. Drawing on this fundamental understanding, we applied streamer-spark discharges to synthesize silver nanostructures directly within silver nitrate solutions contained in the water droplets. Analysis reveals a strong correlation between the discharge regimes, the liquid's electrical properties, and the resulting nanostructure morphology and concentration. Notably, we successfully synthesized silver nanosheets without relying on substrates or stabilizing agents, suggesting that the air-liquid interface itself can act as a dynamic medium facilitating 2D nanostructure self-assembly. This work highlights pulsed plasma-liquid processes as a versatile and environmentally friendly approach for synthesizing advanced nanomaterials with precisely controlled properties.