Leidenfrost drop impact on inclined superheated substrates

Abstract

In real applications, drops always impact on solid walls with various inclinations. For the oblique impact of a Leidenfrost drop, which has a vapor layer under its bottom surface to prevent its direct contact with the superheated substrate, the drop can nearly frictionlessly slide along the substrate accompanied by spreading and retracting. To individually study these processes, we experimentally observe the impact of ethanol drops on superheated inclined substrates using high-speed imaging from two different views synchronously. We first study the dynamic Leidenfrost temperature, which mainly depends on the normal Weber number We⊥. Then, the substrate temperature is set to be high enough to study the Leidenfrost drop behavior. During the spreading process, drops are always kept uniform, and the maximum spreading factor Dm/D0 follows a power-law dependence on the large normal Weber number We⊥ as Dm/D0=We⊥/12+2 for We⊥ ≥ 30. During the retracting process, drops with low impact velocities become non-uniform due to the gravity effect. For the sliding process, the residence time of all studied drops is nearly a constant, which is not affected by the inclination and the We number. The frictionless vapor layer resulting in the dimensionless sliding distance L/D0 follows a power-law dependence on the parallel Weber number We|| as L/D0∝We||1/2. Without direct contact with the substrate, the behaviors of drops can be separately determined by We⊥ and We||. When the impact velocity is too high, the drop fragments into many tiny droplets, which is called the splashing phenomenon. The critical splashing criterion is found to be We⊥*≃ 120 or K⊥=We⊥Re⊥1/2≃ 5300 in the current parameter regime.