Leidenfrost State Research Articles

Critical radius deviated from Leidenfrost state of droplets on liquid layer

The levitation of Leidenfrost droplets on liquid pool is fascinating, but its final stage is lack of understanding. Here, we found that a droplet levitated on liquid layer eventually deviated from Leidenfrost state once its radius was lower than a critical radius due to evaporation. The critical radius of ethanol droplet deviated from Leidenfrost state on silicone oils with a thickness ranging from 2.0 to 15.0 mm was determined by experiment. The influences of the initial radius of droplet, viscosity, and thickness of liquid layer on critical radius were analyzed. In addition, the critical radius decreases with increase in superheat for ΔT lower than 25.0 °C, whereas it does not significantly vary after ΔT exceeding 25.0 °C. The bottom temperature Tb of droplet does not approach to saturation temperature even under a high superheat. The experiment found that Marangoni convection existed inside droplet. Based on a theoretical model considering Marangoni convection, the reason for droplet deviated from Leidenfrost state was explained. These findings are helpful for understanding the final state of Leidenfrost droplet on liquid layer and would provide a potential practical application such as extinction of oil pool fires with liquids.

Leidenfrost Effect-Induced Chaotic Vortex Flow for Efficient Mixing of Highly Viscous Droplets.

Efficiently mixing highly viscous liquids in microfluidic systems is appealing for green chemistry such as chemical synthesis and catalysis, but it is a long-standing challenge owing to the unfavorable diffusion kinetics. In this work, a new strategy is explored for mixing viscous droplets by harnessing a peculiar Leidenfrost state, where the substrate temperature is above the boiling point of the liquid without apparent liquid evaporation. Compared to the control experiment where the droplet stays at a similar temperature but in the contact boiling regime, the mixing time can be reduced significantly. Moreover, it is demonstrated that the liquid mixing originates from the chaotic convection flow in the Leidenfrost droplet, characterized by the internal vortex motion evidenced by the microscale visualization. A correlation between mixing time and droplet volume is also proposed, showing a good agreement with experimental results. It is further shown that Leidenfrost droplets can be used to synthesize nanoparticles of the desired morphology, and it is anticipated that this simple and scalable fabrication approach will find applications in the biological, pharmaceutical, and chemical industries.

Experimental study on the dynamics of a liquid nitrogen droplet impacting a surface under Leidenfrost conditions

Liquid nitrogen droplet impacting a superheated surface is a fundamental phenomenon of liquid nitrogen spray cooling, whereas the mechanisms behind which are still unclear. We designed and developed a visual experimental system to investigate the dynamics of a liquid nitrogen droplet impacting a superheated surface under cryogenic conditions. The impact dynamics of the liquid nitrogen droplet at the Leidenfrost state are captured, and the effects of Weber number (We) and surface temperature on the spreading and rebound characteristics of the droplet are analyzed. The findings show that the droplet exhibits spreading, retraction and rebound at a low We. Droplet spreading and rebound characteristics are mainly affected by We while insensitive to surface temperature. The maximum spreading coefficient exhibits a power-law increase with We, while the maximum rebound coefficient shows an upward and then downward trend with We. The dimensionless maximum spreading time, dimensionless residence time, and dimensionless maximum rebound time show power-law increase with We. Corresponding fitting correlations for these factors for liquid nitrogen droplets are also proposed. This study contributes to an in-depth understanding of the impact dynamics of cryogenic liquid droplet under cryogenic conditions.

Numerical investigation of droplet impact dynamics on Janus-textured heated substrates

Janus-textured substrates refer to surfaces with heterogeneous topographies, which have received particular attention recently due to their potential application in manipulating droplet-bouncing behaviors [Li et al., Nat. Phys. 12, 606–612 (2016)]. In this paper, the droplet impact dynamics on the Janus-textured heated substrates are numerically investigated with an improved thermal lattice Boltzmann method. A comprehensive parametric study is conducted by varying the wettability, the Jakob number, the Weber number, and the surface topographies. With different control parameters, three distinct boiling regimes are observed, i.e., the contact boiling regime, the transition boiling regime, and the film boiling regime (Leidenfrost state). To reveal the underlying physics, the distributions of the unbalance Young's force, the thermophoretic force, and the vapor pressure difference in the system are theoretically analyzed. As for the self-propulsion behaviors, it is find that the droplet tends to move toward the denser side (area with more pillar arrays) for the contact boiling regime. However, when the droplet is under the Leidenfrost state, its bouncing dynamics depend on the combined effects of the Weber number and the wettability, and a decrease in wettability induces the droplet to migrate toward the sparser side (area with fewer pillar arrays). These physical insights enrich the fundamental understanding of the droplet-bouncing dynamics on heated substrates and also provide guidelines for designing advanced surfaces to manipulate the droplet-bouncing behavior.

Local Solvability of a Boundary Value Problem for One-Dimensional Motion of a Granular Matter

This paper investigates the motion of a granular medium for a shallow, vertically shaken bed. Granular matter is one of the most common in nature, and its study has received much attention in recent decades. On the one hand, such a matter behaves like a fluid and has the ability to take the form of its container and to leak away. On the other hand, its behavior is similar to a solid. This work assumes the Leidenfrost state as an initial state with granular matter resembling a fluid heated up from below. The goal is to establish the theorem on the local solvability of the initial-boundary value problem for the one-dimensional motion of a granular medium with consideration to vibrations and scopes of the hydrodynamical approach. The introduction gives brief overviews of the problem and related studies. The Section 1 discusses the one-dimensional isothermal problem for the motion of a granular matter, which is treated like a continuous medium, within the scopes of the hydrodynamical model. The original set of equations is rearranged, and the theorem on the existence of a generalized solution is established. The Section 2 proves the local temporal solvability of the initial-boundary problem Sobolev's and Holder's spaces.

A standing Leidenfrost drop with Sufi whirling

When a water drop is placed on a hot solid surface, it either undergoes explosive contact boiling or exhibits a stable state. In the latter case, the drop floats over an insulating layer of vapor generated by rapid vaporization of water at the surface/drop interface; this is known as the Leidenfrost state. Here, we discuss a previously unrecognized steady state in which a water drop "stands" on a hot smooth surface. In this state, the drop stabilizes itself with partial adhesion on the hot surface, leading to unique deformation and rotation behavior reminiscent of Sufi whirling-a form of spinning dance. Our analysis of this standing Leidenfrost state reveals the underlying mechanisms that drive the drop's stable partial adhesion and subsequent deformation with rotation. The heat-transfer efficiency of this standing state is up to 390% greater than that of the traditional floating Leidenfrost state.

Transient boiling heat transfer mechanism of droplet impacting heated cylinder

The spray on the heated cylindrical surface for boiling widely exists in the industrial application. In order to discover the transient boiling heat transfer mechanism, the cases of droplet impacting on the cylindrical surface were experimentally carried out. In this experiment, the droplet hydrodynamics were captured by high-speed camera. The effects of impact velocity and wall temperature on the hydrodynamics and boiling heat transfer performance were studied in detail. The results show that, for the droplet impacting on the heated cylindrical surface, four types of phase change heat transfer regimes, i.e., film evaporation regime, nucleate boiling regime, transition boiling regime and film boiling regime are found. From the built map of relationship between wall temperature and Weber number, it is demonstrated that when the wall superheat is low, increasing the droplet impact velocity enlarges the solid-liquid contact area, boosting the heat transfer performance in the film evaporation and nucleate boiling regimes. However, when the wall superheat is high, increasing the droplet impact velocity or Weber number makes the Leidenfrost point be lower and is unfavorable for heat transfer due to the easy generation of vapor layer, which strongly differs from the conclusion of droplet impacting on the heated flat surface according to others’ investigations. The underlying mechanism can be revealed from two aspects, i.e., the formation mechanism of Leidenfrost state and the effect of surface geometry. Finally, the quantitative relationship between Leidenfrost point and Weber number is given.

Explosive boiling induced fast transportation of Leidenfrost droplet to target location

Leidenfrost droplet possesses ultra-low flow resistance, but it is challenging to obtain large thrust force for fast transportation and regulate the direction of droplet motion. Here, for the first time, we demonstrate a novel mechanism for the control of droplet dynamics by explosive boiling. Our system consists of two surfaces that have different functions: a smooth surface running in the Leidenfrost state for droplet levitation and a skirt ring edge surface (SRES) as an explosive boiling trigger. For droplet-wall collision with SRES, micro/nanoscale roughness not only enhances energy harvesting from the skirt ring to the droplet due to increased radiation heat transfer but also provides nucleation sites to trigger explosive boiling. The symmetry breaking of explosive boiling creates a thrust force that is sufficient to propel the droplet. The suppression of the thrust force relative to the inertia force regulates the droplet trajectory as it passes through a target location. We show orbit lines passing through a focusing spot that is ∼1% of the Leidenfrost surface area around its center with a maximum traveling speed of ∼85 cm/s, which is ∼2 times of that reported in the literature. The scale law analysis explains the droplet size effect on the self-propelling droplet dynamics. Our work is attractive for applications under the conditions of the required traveling speed and direction of the droplet.

Boiling Transitions During Droplet Contact on Superheated Nano/Micro-Structured Surfaces.

Manipulating surface topography is one of the most promising strategies for increasing the efficiency of numerous industrial processes involving droplet contact with superheated surfaces. In such scenarios, the droplets may immediately boil upon contact, splash and boil, or could levitate on their own vapor in the Leidenfrost state. In this work, we report the outcomes of water droplets coming in gentle contact with designed nano/microtextured surfaces at a wide range of temperatures as observed using high-speed optical and X-ray imaging. We report a paradoxical increase in the Leidenfrost temperature (TLFP) as the texture spacing is reduced below a critical value (∼10 μm) that represents a minima in TLFP. Although droplets on such textured solids appear to boil upon contact, our studies suggest that their behavior is dominated by hydrodynamic instabilities implying that the increase in TLFP may not necessarily lead to enhanced heat transfer. On such surfaces, the droplets display a new regime characterized by splashing accompanied by a vapor jet penetrating through the droplets before they transition to the Leidenfrost state. We provide a comprehensive map of boiling behavior of droplets over a wide range of texture spacings that may have significant implications toward applications such as electronics cooling, spray cooling, nuclear reactor safety, and containment of fire calamities.

A paradigm shift in liquid cooling by multitextured surface design

A paradigm shift in liquid cooling by multitextured surface design

Sustaining Robust Cavities with Slippery Liquid-Liquid Interfaces.

The formation of a stable gas cavity on the surfaces of solid bodies is essential for many practical applications, such as drag reduction and energy savings, owing to the transformation of the originally sticky solid–liquid interface into a free‐slip liquid–vapor interface by the creation of either liquid repellency or a Leidenfrost state on the surfaces. Here, it is shown that the simple infusion of a textured sphere with a smooth, slippery liquid layer can more easily create and sustain a stable gas cavity in a liquid at lower impact velocities compared to a dry solid sphere with the same contact angle. With a key parameter of curvature ratio, the early lamella dynamics during water entry of spheres and drops impact on planes are first unified. With the perspective of wetting transition, the unforeseen phenomenon of prone to cavity formation are successfully explained, which is the preferential lamella detachment from a slippery surface due to the higher viscosity of the lubricant relative to air. It is envisioned that the findings will provide an important and fundamental contribution to the quest for energy‐efficient transport.

Dependence of Leidenfrost droplets behavior on the substrate surface state

In modern society where the words such as SDGs and carbon neutralization have become keywords, it is essential to study how to successfully reuse the unused heat energy, such as the factory heat removal, in the circular society. Therefore, we focused on studying the Leidenfrost phenomenon of liquid in order to convert the unused heat into energy that can be used again. When a liquid approaches the object whose temperature is much higher than the liquid's boiling point, the liquid droplets produce an insulating vapor layer that makes the droplets hover over the hot object. It is called Leidenfrost phenomenon. This non-contact state between the droplet and hot object hinders the heat transfer and increases the evaporation time dramatically. In general, temperature and object surface state will affect the evaporation time of the liquid in Leidenfrost state. In this research, we studied the relationship between the evaporation time and the releasability of object surface. The experimental results shown that the evaporation time was extended by subjecting the substrate to a releasable and non-adhesive treatment. Moreover, it is also necessary to investigate the effect of the change in the releasability of the substrate surface on the characteristic of droplets in the Leidenfrost state, which self-moves on a substrate having a serrated structure.

Dependence of Leidenfrost droplets behavior on the solvent species

The Leidenfrost phenomenon is a unique behavior of droplets, in which a liquid, close to a surface with hotter temperature than the liquid's boiling point, produces an insulating vapor layer that makes the droplets hover over the surface, rather than making physical contact with it. This non-contact state between the droplet and hot surface leads to the decrease in heat transfer and the increase in evaporation time. Due to its characteristics, this Leidenfrost phenomenon has been involved in various industrial technologies, such as liquid cooling methods, precision machinery and semiconductor fabrication. In this study, we conducted the basic research using different solvents for innovating an industrial technology by controlling the behavior of droplet in the Leidenfrost state. As a result, we found that the behavior of the mixed solvents exhibit different compared with that of each solvents, and the droplet behavior can be controlled by adjusting the mixing ratio of the solvents. In the future, it is necessary to investigate the behavior on the diffrent surface state and to observe the Leidenfrost phenomenon in the smaller droplets.

Triple Leidenfrost Effect: Preventing Coalescence of Drops on a Hot Plate.

We report on the collision-coalescence dynamics of drops in Leidenfrost state using liquids with different physicochemical properties. Drops of the same liquid deposited on a hot concave surface coalesce practically at contact, but when drops of different liquids collide, they can bounce several times before finally coalescing when the one that evaporates faster reaches a size similar to its capillary length. The bouncing dynamics is produced because the drops are not only in Leidenfrost state with the substrate, they also experience Leidenfrost effect between them at the moment of collision. This happens due to their different boiling temperatures, and therefore, the hotter drop works as a hot surface for the drop with lower boiling point, producing three contact zones of Leidenfrost state simultaneously. We called this scenario the triple Leidenfrost effect.

Leidenfrost Effect as a Directed Percolation Phase Transition.

Volatile drops deposited on a hot solid can levitate on a cushion of their own vapor, without contacting the surface. We propose to understand the onset of this so-called Leidenfrost effect through an analogy to nonequilibrium systems exhibiting a directed percolation phase transition. When performing impacts on superheated solids, we observe a regime of spatiotemporal intermittency in which localized wet patches coexist with dry regions on the substrate. We report a critical surface temperature, which marks the upper bound of a large range of temperatures in which levitation and contact coexist. In this range, with decreasing temperature, the equilibrium wet fraction increases continuously from zero to one. Also, the statistical properties of the spatiotemporally intermittent regime are in agreement with that of the directed percolation universality class. This analogy allows us to redefine the Leidenfrost temperature and shed light on the physical mechanisms governing the transition to the Leidenfrost state.

Influence of the substrate permeability on Leidenfrost temperature

Leidenfrost state of droplets compromises heat dissipation in cooling applications because of the formation of a thin insulating vapor layer over which the coolant droplet levitates. Here we report on the dependence of the Leidenfrost temperature (LFT) of a deionized water droplet on the surface morphology and propose a pressure-based model to explain the associated droplet dynamics. We observe that the LFT increases with the height of micropillars and spacing between them. The LFT increases by ~ 270 °C compared to a smooth surface and reaches ~ 507 °C for a micropillar array with an interpillar spacing of 100 µm and height of 63 µm. The wall heat flux at the Leidenfrost state of the droplet varies between ~ 15 W cm−2 and ~ 52 W cm−2 for different microtextured surfaces. We show that irrespective of the height of the textures, the effect of surface roughness diminishes beyond a certain critical interpillar spacing. We develop a semi-analytical model to show that the excess vapor gap between the top of the pillars and the base of the droplet is dependent on the permeability of the substrate and influences the vapor pressure under the droplet. The excess vapor gap is shown to play a crucial role in determining the range of temperatures that sustain transition boiling and affects the rate of evaporation of the droplet in Leidenfrost state.

An experimental and theoretical investigation of electrostatic suppression of the Leidenfrost state

The formation of a thermally insulating vapor layer at the Leidenfrost state poses a major concern for heat transfer performance and safety operation of many phase change thermal systems. It is desirable to develop effective means to suppress the Leidenfrost state and to elevate the Leidenfrost point (LFP). In this work, the electrostatic suppression of a Leidenfrost drop was investigated by using a combined experimental and analytical approach. The effects of the voltage and driving frequency of the electric field on the LFP were measured. A theoretical model based on the electromechanical force balance and the Kelvin-Helmholtz instability analysis was formulated to quantify the relationship between the LFP and the applied electric field. This approach overcomes the major drawbacks of the existing models in the literature by offering a direct prediction of the LFP for a wide range of fluid types (electrically conducting or dielectric) enhanced by either DC or AC field.

Leidenfrost temperature: Surface thermal diffusivity and effusivity effect

Droplet impact on hot surfaces results in either droplet-surface contact or droplet-surface non-contact, i.e., the Leidenfrost state. The Leidenfrost droplet is levitated upon its vapor, deteriorating the heat transfer. The Leidenfrost temperature depends on the thermal properties of the surface, which are usually characterized by two parameters: the thermal diffusivity and the thermal effusivity. In this paper, the effects of these two parameters on the Leidenfrost temperature are clarified experimentally by varying the one of interest while keeping the other one constant. The experimental results indicate that the Leidenfrost temperature is affected by the thermal effusivity rather than by the thermal diffusivity; the higher the thermal effusivity is, the lower the Leidenfrost temperature; and the increase of the Leidenfrost temperature with the droplet impact velocity is greater for the surface with the lower thermal effusivity. To further understand the experimental findings, a theoretical model is proposed, which considers the transient heat transfer in the surface. The theoretical analysis shows that the Leidenfrost temperature scales with the inverse of the thermal effusivity and the root square of the droplet impact velocity, in good agreement with the experimental results.

On explosive boiling of a multicomponent Leidenfrost drop

The gasification of multicomponent fuel drops is relevant in various energy-related technologies. An interesting phenomenon associated with this process is the self-induced explosion of the drop, producing a multitude of smaller secondary droplets, which promotes overall fuel atomization and, consequently, improves the combustion efficiency and reduces emissions of liquid-fueled engines. Here, we study a unique explosive gasification process of a tricomponent droplet consisting of water, ethanol, and oil ("ouzo"), by high-speed monitoring of the entire gasification event taking place in the well-controlled, levitated Leidenfrost state over a superheated plate. It is observed that the preferential evaporation of the most volatile component, ethanol, triggers nucleation of the oil microdroplets/nanodroplets in the remaining drop, which, consequently, becomes an opaque oil-in-water microemulsion. The tiny oil droplets subsequently coalesce into a large one, which, in turn, wraps around the remnant water. Because of the encapsulating oil layer, the droplet can no longer produce enough vapor for its levitation, and, thus, falls and contacts the superheated surface. The direct thermal contact leads to vapor bubble formation inside the drop and consequently drop explosion in the final stage.

Electrohydrodynamic analysis of bubble burst in large Leidenfrost droplets

A thin vapor gap forms underneath a liquid drop on a sufficiently hot surface, which prevents solid–liquid contact (the Leidenfrost effect). This vapor gap can be partly eliminated by applying an electrical potential difference across the vapor gap to electrostatically suppress the Leidenfrost state. An interesting hydrodynamics-related phenomenon that can occur in Leidenfrost droplets is the formation of a vapor dome and subsequent bubble burst at the center of the droplet. This work reports a comprehensive study of vapor dome formation and bubble burst in large Leidenfrost droplets under the influence of an electric field. First, a detailed numerical model (non-linear thin film lubrication equation) is developed to analyze the evolution of the vapor dome and bubble burst. Second, a simplified stability analysis is conducted to analytically estimate the critical droplet diameter (for bubble burst) under the influence of an electric field. Third, experiments are conducted to measure the critical diameter of Leidenfrost droplets for bubble burst under the influence of electric fields. The results from the numerical modeling and stability analysis show very good agreement with experimental measurements. The critical diameter for bubble burst and the time period between consecutive vapor bursts reduce with the applied electric field. Comparisons are made between the presently studied vapor burst and film boiling; similarity in the underlying hydrodynamic phenomena results in the length and time scales for bubble burst being similar to those encountered in film boiling.