Equivalent capacitive thickness of the vapor layer below Leidenfrost drops

Abstract

For the first time, a capacitive method is reported to provide information about the thickness of the vapor layer below Leidenfrost drops. Contrarily to the other available techniques, the present approach is fast and simple which allows a continuous monitoring of the thickness over time. The electrostatic system can be represented by a parallel-plate capacitor. The heated substrate and the conductive liquid water are the plates of the capacitor while the insulating vapor constitutes the dielectric layer. The concept of equivalent capacitive thickness (hcapacitance) is introduced using electrostatic calculations taking into account the real shape of the drops. Experimentally, hcapacitance has been obtained from the simultaneous measurement of the drop radius and the capacitance. The temporal evolution of hcapacitance has been evaluated for various initial drop volumes. For all the drop volumes, a linear diminution with time is obtained. The results evidence that the major parameter that controls the value of hcapacitance is the instantaneous drop volume. The comparison between hcapacitance and the thicknesses obtained by laser diffraction and literature data reveals that the value of the thicknesses seems to depend on the measurement method. In fact, the thickness is variable from the center to the edge of the drop and each method is more or less sensitive to different positions. At the same time, hcapacitance remains larger than the resulting thickness using light diffraction and is included between the thicknesses at the center (hcenter) and at the neck (hneck) of the drop.