Leidenfrost Phenomenon and its Impact on Sessile Drop Evaporation for Different Liquids and Surfaces

Recent study of the applications of the Leidenfrost effect has sparked renewed interest in this phenomenon. Due to Leidenfrost effect, a droplet can be levitated by its own vapor layer on a sufficiently heated surface. In this work an experimental investigation has been performed to determine the Leidenfrost point (LFP) of different liquids on different surfaces. Copper, aluminum and brass plates were used with water, methanol, and ethanol as different liquids to study the variation of LFP. The experiment was conducted with incident droplets of water, methanol, and ethanol on the three different metal blocks which were heated by cartridge heaters. The input powers in the heaters were varied by a voltage variac. At each temperature, droplet evaporation time was recorded where the highest evaporation time corresponded with the LFP. For the ranges of parameters considered in this study, the Leidenfrost temperature for water varied from 180°C to 200°C. For organic fluids like methanol it varied from 160°C to 200°C while for ethanol it varied from 160°C to 200°C. These findings are important because accurate modeling of Leidenfrost phenomenon is crucial for the design of many cooling devices. If a vapor layer is present between the liquid and the surface to be cooled, then the cooling becomes inefficient. This is especially important for the rapid cooling of overheated components in high power density systems, such as nuclear reactors and other devices.

Final fate of a Leidenfrost droplet: Explosion or takeoff

When a liquid droplet is placed on a very hot solid, it levitates on its own vapor layer, a phenomenon called the Leidenfrost effect. Although the mechanisms governing the droplet’s levitation have been explored, not much is known about the fate of the Leidenfrost droplet. Here we report on the final stages of evaporation of Leidenfrost droplets. While initially small droplets tend to take off, unexpectedly, the initially large ones explode with a crack sound. We interpret these in the context of unavoidable droplet contaminants, which accumulate at the droplet-air interface, resulting in reduced evaporation rate, and contact with the substrate. We validate this hypothesis by introducing controlled amounts of microparticles and reveal a universal 1/3-scaling law for the dimensionless explosion radius versus contaminant fraction. Our findings open up new opportunities for controlling the duration and rate of Leidenfrost heat transfer and propulsion by tuning the droplet’s size and contamination.

Modeling the Influence of Marangoni Flows on the Leidenfrost State on Solid and Liquid Substrates

Boiling heat transfer affects various processes related to energy, water and manufacturing. In the film boiling regime, heat transfer is substantially lower than in the nucleate boiling regime, due to the formation of a vapor layer at the solid-liquid interface (Leidenfrost effect). In this work, we present analytical modeling of the Leidenfrost state of droplets on solid and liquid substrates. A key aspect of this study is the focus on surface tension gradients on the surface of a liquid (Leidenfrost droplet or liquid substrate), which actuate thermo-capillary driven Marangoni flows. It is noted that this work develops a first-order simplified model, which assumes a uniform vapor layer thickness. The presence of Marangoni flows has non-trivial implications on the resulting thickness of the Leidenfrost vapor layer. Our analysis shows that the pumping effect generated in the vapor layer due to Marangoni flows can significantly reduce the Leidenfrost vapor layer thickness.