Triple Leidenfrost Effect: Preventing Coalescence of Drops on a Hot Plate (original) (raw)

High jump of impinged droplets before Leidenfrost state

Physical Review E

Unlike the traditionally reported Leidenfrost droplet which only floats on a thin film of vapor, we observe a prominent jump of the impinged droplets in the transition from the contact boiling to the Leidenfrost state. The vapor generation between the droplet and the substrate is vigorous enough to propel the spreading droplet pancake to an anomalous height. The maximum repellent height can be treated as an index of the total transferred energy. Counterintuitively, a stronger vaporization and a higher jump can be generated in the conditions normally considered to be unfavorable to heat transfer, such as a lower substrate temperature, a lower droplet impact velocity, a higher droplet temperature, or a lower thermal conductivity of the deposition on the substrate. Since the total transferred energy is the accumulation of the instantaneous heat flux during the droplet contacting with the substrate, it can be deduced that a longer contact time period is secured in the case of a lower instantaneous heat flux. The inference is supported by our experimental observations. Moreover, the phase diagrams describe the characteristics of the high repellency under different substrate temperatures, droplet subcooling temperatures, and Weber numbers. It allows us to manipulate the droplet jump for the relative applications.

Dynamics of thermal and wetting footprint of a volatile droplet during Leidenfrost transition

2021

The levitation of a volatile droplet on a highly superheated surface is known as the Leidenfrost effect. Wetting state during transition from full wetting of a surface by a droplet at room temperature to Leidenfrost bouncing, i.e., zero-wetting at high superheating, is not fully understood. Here, visualizations of droplet thermal and wetting footprint in the Leidenfrost transition state are presented using two optical techniques: mid-infrared thermography and wetting sensitive total internal reflection imaging under carefully selected experimental conditions, impact Weber number < 10 and droplet diameter < capillary length, using an indium-tin-oxide coated sapphire heater. The experimental regime was designed to create relatively stable droplet dynamics, where the effects of oscillatory and capillary instabilities were minimized. The thermography for ethanol droplet in Leidenfrost transition state (superheat range of 82K-97K) revealed thermal footprint with a central hot zone ...

The Leidenfrost transition of liquid droplets impinging onto a superheated surface

International Conference on Liquid Atomization and Spray Systems (ICLASS), 2021

Heat transfer during the impact of a droplet on a sapphire substrate is investigated by means of infrared thermography. For that, the sapphire is coated with a very thin layer of TiAlN having a high emissivity in the IR domain. Spatially and time resolved measurements of the temperature at the front face of the solid wall are obtained. Results obtained for water droplets show that the dynamic Leidenfrost point (LFP) is close to 450°C and coincides with the onset of a fingering pattern. Approaching the dynamic LFP, despite the cooling by the droplet, the wall surface temperature never decreases below 310°C which is about the temperature of the spinodal for water, i.e. the maximum temperature at which water can still exist in the liquid state. Considering that a wetting contact is taking place below the dynamic LFP, wall temperature measurements demonstrate that the drop impact comes with a very strong superheating of the liquid. The liquid touching the wall is heated up to the spinodal temperature. Based on the idea that the dynamic LFP could correspond to the wall temperature, for which the contact temperature at a solid/liquid interface is equal to the spinodal temperature, a model is proposed for the dynamic LFP. This model considers the thermal effusivities of the liquid and the wall, as well as the liquid flow in the spreading lamella.

Coalescence of drops on the free-surface of a liquid pool at elevated temperatures

Physics of Fluids, 2020

The coalescence dynamics of ethanol drops injected from a needle on the free-surface of an ethanol pool maintained at a higher temperature than the drop is experimentally studied using a high-speed imaging system. The drop is always kept at 25 ○ C, and the temperature of the ethanol pool is varied using a heater. The coalescence behavior depends on the size of the drop, the height of the needle tip from the freesurface, and the temperature of the ethanol pool. A parametric study is carried out by varying these parameters. The drop exhibits a residence period at low impact velocity, when it floats on the free-surface before the coalescence begins. Subsequently, the complete coalescence and partial coalescence dynamics are observed for different sets of parameters considered. It is found that increasing the temperature of the ethanol pool reduces the residence time of the drop. This phenomenon is explained by analyzing the forces acting on the drop and the capillary waves generated due to the temperature gradient between the drop and the ethanol pool. During partial coalescence, we also observed that the diameter of the daughter droplet decreases as the size of the primary drop and pool temperature are increased. As expected, due to the gravity effect, increasing the size of the drop also decreases the residence time. A regime map designating the complete coalescence and partial coalescence dynamics is plotted in the pool temperature and drop impact height space.

Thermal Antibubbles: When Thermalization of Encapsulated Leidenfrost Drops Matters

Physical Review Letters

Antibubbles are ephemeral objects composed of a liquid drop encapsulated by a thin gas shell immersed in a liquid medium. When the drop is made of a volatile liquid and the medium is superheated, the gas shell inflates at a rate governed by the evaporation flux from the drop. This thermal process represents an alternate strategy for delaying the antibubble collapse. We model the dynamics of such "thermal" antibubbles by incorporating to the film drainage equation the heat-transfer-limited evaporation of the drop, which nourishes the gas shell with vapor, as for Leidenfrost drops. We demonstrate that the inflation of the gas shell is drastically inhibited by the thermalization of the initially colder drop. Because of this thermalization effect, smaller drops evaporate much faster than larger ones.

Morphed inception of dynamic Leidenfrost regime in colloidal dispersion droplets

Physics of Fluids

Droplet impact on a heated substrate is an important area of study in spray cooling applications. On substrates significantly hotter than the saturation temperature, droplets immediately hover on its vapor cushion, exhibiting the Leidenfrost phenomenon. Here, we report the phenomena wherein addition of Al2O3 nanoparticles to water significantly increases the onset of dynamic Leidenfrost temperature ( TDL) and suppresses the overall Leidenfrost regime. We experimentally revealed that the onset of TDL delays with increasing the nanoparticle concentration of the colloidal dispersions at a particular Weber number ( We). But, for a constant concentration, the onset of TDL decreases with an increase of impact We. In contrast to water droplets, the colloid droplets exhibit vigorous spraying behavior due to the nanoparticulate residue deposition during the spreading and retraction stages. Further, the residue on the heated substrate changes the departure diameter of the vapor bubbles during...

Heat transfer for Leidenfrost drops bouncing onto a hot surface

Experimental Thermal and Fluid Science, 2013

When droplets impinge onto a hot wall, different regimes can be observed depending on the wall temperature and Weber number. In the case of interest, the temperature of the wall is more than the Leidenfrost temperature and the Weber number based on normal velocity of the droplet is less than the threshold value leading to the splashing regime (We < 80). We particularly focused on the perfect bouncing regime (We < 30) for which an impinging droplet levitates on a thin layer of its own vapour. This vapour is instantaneously created between the base of the deforming droplet and the heated surface so that direct contact with the hot solid is avoided. For these low Weber numbers, the droplet surface energy is high enough compared to its kinetic energy to permit the rebound, so that the droplet recovers its initial shape without breaking up after the bounce. Although the generated vapour insulates the droplet, some heat is exchanged with the wall during the interaction (i.e. during the resident time). In this paper, we report on experimental measurements of heat transfer due to droplet impact in the Leidenfrost regime. The energy released by the wall and measured using an inverse conduction method leads to an estimation of the heat transfer coefficient during impact in the Leidenfrost regime. For that, the time evolution of the droplet base surface is estimated using a simple modelling validated on experimental data. Finally, the energy measured is compared to existing models.

Suppressed Leidenfrost phenomenon during impact of elastic fluid droplets

arXiv: Fluid Dynamics, 2020

The present article highlights the role of non-Newtonian (elastic) effects on the droplet impact phenomenology at temperatures considerably higher than the boiling point, especially at or above the Leidenfrost regime. The Leidenfrost point (LFP) was found to decrease with increase in the impact Weber number (based on velocity just before the impact) for fixed polymer (Polyacrylamide, PAAM) concentrations. Water droplets fragmented at very low Weber numbers (~22), whereas the polymer droplets resisted fragmentation at much higher Weber numbers (~155). We also varied the polymer concentration and observed that till 1000 ppm, the LFP was higher compared to water. This signifies that the effect can be delayed by the use of elastic fluids. We have showed the possible role of elastic effects (manifested by the formation of long lasting filaments) during retraction in the improvement of the LFP. However for 1500 ppm, LFP was lower than water, but with similar residence time during initial ...

The Leidenfrost effect: From quasi-spherical droplets to puddles

Comptes Rendus Mécanique, 2012

In the framework of the lubrication approximation, we derive a set of equations describing the steady bottom profile of Leidenfrost drops coupled with the vapor pressure. This allows to derive scaling laws for the geometry of the concave bubble encapsulated between the drop and the hot plate under it. The results agree with experimental observations in the case of droplets with radii smaller than the capillary length R c as well as in the case of puddles with radii larger than R c .

Delayed Leidenfrost phenomenon during impact of elastic fluid droplets

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences

This article highlights the role of non-Newtonian (elastic) effects on the droplet impact phenomenon at temperatures considerably higher than the boiling point, especially at or above the Leidenfrost regime. The Leidenfrost point (LFP) was found to decrease with an increase in the impact Weber number (based on the velocity just before the impact) for fixed polymer (polyacrylamide) concentrations. Water droplets fragmented at very low Weber numbers (approx. 22), whereas the polymer droplets resisted fragmentation at much higher Weber numbers (approx. 155). We also varied the polymer concentration and observed that, up to 1000 ppm, the LFP was higher than that for water. This signifies that the effect can be delayed by the use of elastic fluids. We have shown the possible role of elastic effects (manifested by the formation of long lasting filaments) during retraction in the increase of the LFP. However, for 1500 ppm, the LFP was lower than that for water, but had a similar residence ...