פרופ' שמואל שפרן ממכון ויצמן למדע שלח את תשובתו של Dr Xinpeng Xu , אשר התמחה בנושא כשהיה פוסטדוקטורנט במעבדתו של פרופ' שפרן.
בנוסף מצורף מאמר בנושא
I have some suggestions about how to show the Leidenfrost effects to the young students. First, it is very easy to observe the Leidenfrost effects (Actually, I do this every week at home when I cook food.).
Question to the students: We put a water drop on a heated plate. Will the drop always evaporate faster for heater plate? Intuitively, the heater the plate is, the faster the drop will evaporate and hence the shorter the lifetime of drops (of the same size or volume). But this is not always right !
Physics: Physically, the origin of the phenomenon is the vapor film formed under the drop. The vapor film can not only thermally isolates the drop from the heat pan but also reduce friction between drop and the pan. Therefore, two distinctive properties of Leidenfrost drop are long lifetime and high mobility. The vapor film formed under the drop is, however, very thin, about 100 μm. It is impossible to see by eyes. Actually, it is because of this the phenomena was a long-standing puzzle after Leidenfrost saw it in 1785.
Experimental equipments and materials: a time-meter, a Pan (to be heated up. A heating plate with thermometer will be perfect), a graduated pipette.
Control for the experiments:
(1) For water drops, you need to heat the Pan up to 300 degree to see the Leidenfrost phenomena. Heat the pan to the desired temperature fist and then use the graduated pipette to put the drop on the heated pan.
(2) It will be quite impressive if the students can observe the the shape change of the Leidenfrost drop during its evaporation. If you put too large drops (about 5 mm in diameter), the drop will split into smaller drops due to the so-called Rayleigh-Taylor instability of the vapor film under the drop. If you put a small drop (about 2-3 mm in diameter), the drop will stably evaporate, become smaller and smaller and drop shape changes from puddle shape (due to the gravity) to quasi-spherical, to spherical (when the drop size smaller than 1 mm, difficult to see by eyes).
(3) Another interesting thing is to observe the high motility of Leidenfrost drops. When the plate is heat enough (above the Leidenfrost temperature) it moves around quite fast (because of small friction, a small force can give large velocity).
(4) Finally, a quantitative measure of this Leidenfrost phenomena is to plot the drop lifetime vs. Pan temperature. As shown in the following figure (Fig. 2 from the attached review paper).
For more details, you can read Wikipedia or a recent review about the phenomena.