When the temperature exceeds the Leidenfrost point, the Leidenfrost effect appears. As the temperature of the pan rises above 100 ☌ (212 ☏), the water droplets hiss when touching the pan, and these droplets evaporate quickly. Initially, as the temperature of the pan is just below 100 ☌ (212 ☏), the water flattens out and slowly evaporates, or if the temperature of the pan is well below 100 ☌ (212 ☏), the water stays liquid. The effect can be seen as drops of water are sprinkled onto a pan at various times as it heats up.
(The "gaseous" phase could be replaced by another immiscible liquid phase.Excitation of normal modes in a drop of water during the Leidenfrost effect The theoretical description of contact arises from the consideration of a thermodynamic equilibrium between the three phases: the liquid phase (L), the solid phase (S), and the gas or vapor phase (G) (which could be a mixture of ambient atmosphere and an equilibrium concentration of the liquid vapor). The shape of a liquid–vapor interface is determined by the Young–Dupré equation, with the contact angle playing the role of a boundary condition via the Young equation. Schematic of a liquid drop showing the quantities in the Young equation. It changes with surface tension and hence with the temperature and purity of the liquid. It is independent of the inclination of solid to the liquid surface. The contact angle depends upon the medium above the free surface of the liquid, and the nature of the liquid and solid in contact. The equilibrium contact angle reflects the relative strength of the liquid, solid, and vapour molecular interaction. The equilibrium contact is within those values, and can be calculated from them. However, in practice a dynamic phenomenon of contact angle hysteresis is often observed, ranging from the advancing (maximal) contact angle to the receding (minimal) contact angle. A given system of solid, liquid, and vapor at a given temperature and pressure has a unique equilibrium contact angle.
It quantifies the wettability of a solid surface by a liquid via the Young equation. The contact angle is the angle, conventionally measured through the liquid, where a liquid– vapor interface meets a solid surface. Cloth, treated to be hydrophobic, shows a high contact angle.
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