  Water electrolysis performed in microsystems with a fast change of voltage
polarity produces optically invisible nanobubbles containing H2 and O2 gases.
In this form the gases are able to the reverse reaction of water formation.
Here we report extreme phenomena observed in a millimeter-sized open system.
Under a frequency of driving pulses above 100 kHz the process is accompanied by
clicking sounds repeated every 50 ms or so. Fast video reveals that
synchronously with the click a bubble is growing between the electrodes which
reaches a size of 300 um in 50 us. Detailed dynamics of the system is monitored
by means of a vibrometer by observing a piece of silicon floating above the
electrodes. The energy of a single event is estimated as 0.3 uJ and a
significant part of this energy is transformed into mechanical work moving the
piece. The observations are explained by the combustion of hydrogen and oxygen
mixture in the initial bubble with a diameter of about 40 um. Unusual
combustion mechanism supporting spontaneous ignition at room temperature is
responsible for the process. The observed effect demonstrates a principal
possibility to build a microscopic internal combustion engine.
