World’s tiniest combustion chambers

Once initiated, a combustion reaction can typically sustain itself until it runs out of fuel: Reacting molecules generate heat, heat incites more molecules to react, and the cycle repeats. But in small spaces such as microscopic bubbles, surface interactions drain away heat faster than combustion can generate it. For that reason, many people consider microscale combustion a physical impossibility. Though microbubbles may indeed be too small to combust, a University of Twente team led by Miko Elwenspoek has now discovered that, counterintuitively, nanobubbles aren’t. Applying alternating positive and negative voltage pulses to electrodes immersed in water, the team generated bubbles just hundreds of nanometers in diameter and made up of a volatile mixture of hydrogen and oxygen. At pulse rates higher than 100 kHz, however, the bubbles were nowhere to be found. In their place were telltale signs of combustion, including wear on the electrode—as evidenced by this atomic force microscope image showing the postexperiment surface roughness of one electrode—and a temperature rise indicative of an exothermic reaction. The researchers suspect that the tiny explosions may be a result of fast dynamics and extreme internal pressures, estimated to be as high as 30 atmospheres in the experiment, that arise due to the surface tension of the highly curved nanobubbles. They anticipate that the breakthrough could find a wide range of applications, including engines for microelectromechanical devices. (V. B. Svetovoy et al., Phys. Rev. E, in press.)—Ashley G. Smart