Leidenfrost drops cooling surfaces: theory and interferometric measurement

Abstract

When a liquid drop is placed on a highly superheated surface, it can be levitated by its own vapour. This remarkable phenomenon is referred to as the Leidenfrost effect. The thermally insulating vapour film results in a severe reduction of the heat transfer rate compared to experiments at lower surface temperatures, where the drop is in direct contact with the solid surface. A commonly made assumption is that this solid surface is isothermal, which is at least questionable for materials of low thermal conductivity, resulting in an overestimation of the surface temperature and heat transfer for such systems. Here we aim to obtain more quantitative insight into how surface cooling affects the Leidenfrost effect. We develop a technique based on Mach-Zehnder interferometry to investigate the surface cooling of a quartz plate by a Leidenfrost drop. The three-dimensional plate temperature field is reconstructed from interferometric data by an Abel inversion method using a basis function expansion of the underlying temperature field. By this method we are able to quantitatively measure the local cooling inside the plate, which can be as strong as 80 K. We develop a numerical model which shows good agreement with experiments and enables extending the analysis beyond the experimental parameter space. Based on the numerical and experimental results we quantify the effect of surface cooling on the Leidenfrost phenomenon. By focusing on the role of the solid surface we provide new insights into the Leidenfrost effect and demonstrate how to adjust current models to account for non-isothermal solids and use previously obtained isothermal scaling laws for the neck thickness and evaporation rate.

Self-induced flows enhance the levitation of Leidenfrost drops on liquid baths

“…is mainly transferred by conduction across the vapor gap from the substrate, hence cooling down the underlying liquid bath. Such a local cooling, exacerbated by poor thermal conductivity of the substrate [33], could be responsible for the direction of circulation observed in the case of an ethanol drop both by potentially giving rise to a sinking buoyant plume of cooler (hence heavier) liquid beneath the drop, and by means of a surface-tension gradient (Marangoni stresses) directed inwards along the bath surface (towards a cooler spot under the drop, where the surface tension, a decreasing function of temperature, is higher). Although the direction of the Marangoni stresses immediately underneath the drop is a priori less obvious (due to a stronger cooling expected at the neck, the location of the most intense evaporation [33]), we shall confirm later on that the aforementioned inward tendency is overall predominant for the Marangoni contribution.…”

“…A Leidenfrost drop was then generated above the bath at the tip of a needle continuously fed by a syringe pump, enabling both to immobilize the drop and to study a quasisteady situation with a drop of constant size, similarly to what accomplished over a solid substrate in Refs. [32,33].…”

Double Contact During Drop Impact on a Solid Under Reduced Air Pressure

“…Total internal reflection (TIR) imaging can pin down the actual contacts [23], of particular importance in studying Leidenfrost impacts [24,25]. Here, the exact surface temperature [26] and the heat transfer between the drop and substrate and its cooling can be of primary importance [27].…”

How ambient conditions affect the Leidenfrost temperature

“…To avoid the influence of roughness and cooling effects in the solid, we use a silicon plate (ThorLabs WG81050), which is optically smooth and has good thermal conductivity and sufficient thickness of 5 mm to avoid any cooling effects. 21,25,27 The plate is resting on a brass heater block, whose temperature is controlled by a PID controller. The surface temperature T of the silicon plate was measured prior to the experiment by a Pt-100 sensor to calibrate the set point of the controller.…”

“…The prediction of the Leidenfrost temperature T L is however still an unsolved problem. It is known that T L depends on the type of liquid 21 as well as the roughness 22,23 and thermal conductivity 21,[24][25][26][27] of the plate. We also know that it increases with increasing impact velocity of the drop.…”