In a heated and oiled frying pan, a drop of water quickly boils and moves from side to side thanks to the jet steam.
A drop of water easily and quickly "slides" over the hot surface of the pan. This behavior is well known and studied. It is associated with the Leidenfrost effect: if the surface is heated above the boiling point of the liquid, a vapor layer instantly forms under the drop. It slows down the evaporation of the drop itself and reduces friction between it and the surface, allowing it to move along it, as if on ice. It is curious that the Leidenfrost effect is also manifested in such a distant region as the appearance of araniiforms - "spider-like" geological formations on Mars.
However, if the pan is covered with oil, the boiling point of which is much higher, no vapor layer will appear. Nevertheless, droplets slide on such a surface much easier than on a clean one. Observations show that their speed can be 10 or even 100 times higher. This effect has only recently been explained. This is covered in an article by Kripa Varanasi and colleagues at the Massachusetts Institute of Technology (MIT) published in Physical Review Letters.
Scientists conducted observations using a microscope and a high-speed video camera capable of shooting up to 100 thousand frames per second. It turned out that at the corresponding values of temperature, viscosity and thickness of the fat layer, the oil forms a thin coating on the drop. The boiling point of this coating is much higher than that of water. Therefore, when it begins to evaporate - primarily from the lower side, which is closer to the hot surface - tiny bubbles form under the layer of oil.
Irregular bumps reduce the adhesion of the droplet to the surface. In addition, these vapor-filled cavities impede the transfer of heat and also create temperature irregularities that cause the droplet to vibrate and facilitate its movement. At the same time, the pressure inside such bubbles rapidly increases until the oil layer ruptures. Escaping outward, the steam pushes the drop forward with the help of jet thrust, like air escaping from a perforated balloon.
The authors hope that the discovered effect will find application in microfluidics, making it possible to accelerate the movement of microscopic liquid droplets. The boiling of water and the formation of vapor bubbles begin in certain areas of the droplet - nucleation centers. Scientists believe that they will soon learn to control the position of these centers and, as a result, will be able to precisely control the movement of boiling droplets on the oil surface.