Discharge-bubble luminescence

When a bubble in a liquid collapses, the gas inside it can get compressed and heated to the point that it spontaneously ionizes. The resulting plasma is short-lived, and when its atoms recombine it gives off a flash of light. (See Physics Today, April 2012, page 18 , and the article by Detlef Lohse, Physics Today, February 2003, page 36 .) The process was first studied using ultrasound-generated bubbles some 10–100 µm in size that lasted tens of microseconds; the picosecond light bursts from the collapsing bubbles earned the moniker sonoluminescence. Dielectric breakdown at the focus of a pulsed laser can also induce bubbles—an order of magnitude larger and lasting an order of magnitude longer than the acoustic bubbles, with flashes lasting nanoseconds. A new paper by Keping Yan and colleagues at China’s Zhejiang University examines a more recent source of luminescing bubbles: electric discharge. Connecting an underwater electrode to a pulsed power source, the team produced an oscillating bubble for a sufficiently strong voltage pulse. A high-speed camera, capturing a frame every 25 µs, recorded the bubble expansion and collapse. The discharge-induced bubbles grew up to a centimeter across and lasted for milliseconds, and the luminescence lasted some tens of microseconds. A key question of bubble collapse is the internal temperature, and the longer duration of discharge-induced luminescence should provide opportunities to accurately measure the emission spectrum. The researchers’ modeling suggests their bubbles reach peak temperatures of about 7000 K. That’s close to what’s been reported for ultrasound and laser bubbles; the temperature determination in all the systems, however, depends on mass transfer, chemical reactions, and other modeling details of the collapse dynamics. (Y. Huang et al., Appl. Phys. Lett. 107, 184104, 2015, doi:10.1175/JPO-D-14-0237.1 .)