  We observed the zig-zag motion of small Leidenfrost water droplets (radii
less than 0.6 mm) on a hot, flat substrate. To understand this motion, we
conducted an experiment using a glass capillary to fix a droplet at its edge
and control the droplet height. Thermographic and interferometric observations
reveal that the droplets rotated both vertically and azimuthally. Based on the
characteristic frequency of the azimuthal rotation depending on the substrate
temperature and droplet height, we developed a semi-empirical model considering
unsteady Marangoni convection and its relaxation. We confirmed that our model
can predict the characteristic time of the zig-zag motion of unfixed
Leidenfrost droplets.
