Azimuthal rotation induced by the Marangoni force makes small Leidenfrost droplets move in random zig-zag directions

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.