Dynamics of the vapor layer below a Leidenfrost drop

Minimum Leidenfrost Temperature on Smooth Surfaces

During the Leidenfrost effect, a thin insulating vapor layer separates an evaporating liquid from a hot solid. Here we demonstrate that Leidenfrost vapor layers can be sustained at much lower temperatures than those required for formation. Using a high-speed electrical technique to measure the thickness of water vapor layers over smooth, metallic surfaces, we find that the explosive failure point is nearly independent of material and fluid properties, suggesting a purely hydrodynamic mechanism determines this threshold. For water vapor layers of several millimeters in size, the minimum temperature for stability is ≈140 °C, corresponding to an average vapor layer thickness of 10-20  μm.

Self-excitation of Leidenfrost drops and consequences on their stability

Volatile liquids (water, alcohol, etc.) poured on hot solids levitate above a layer of vapor. Unexpectedly, these so-called Leidenfrost drops often suddenly start to oscillate with star shapes, a phenomenon first reported about 140 y ago. Similar shapes are known to be triggered when a liquid is subjected to an external periodic forcing, but the unforced Leidenfrost case remains unsolved. We show that the levitating drops are excited by an intrinsic periodic forcing arising from a vibration of the vapor cushion. We discuss the frequency of the vibrations and how they can excite surface standing waves possibly amplified under geometric conditions of resonance-an ensemble of observations that provide a plausible scenario for the origin, mode selection, and sporadic nature of the Leidenfrost stars.

Leidenfrost droplet trampolining

A liquid droplet dispensed over a sufficiently hot surface does not make contact but instead hovers on a cushion of its own self-generated vapor. Since its discovery in 1756, this so-called Leidenfrost effect has been intensively studied. Here we report a remarkable self-propulsion mechanism of Leidenfrost droplets against gravity, that we term Leidenfrost droplet trampolining. Leidenfrost droplets gently deposited on fully rigid surfaces experience self-induced spontaneous oscillations and start to gradually bounce from an initial resting altitude to increasing heights, thereby violating the traditionally accepted Leidenfrost equilibrium. We found that the continuously draining vapor cushion initiates and fuels Leidenfrost trampolining by inducing ripples on the droplet bottom surface, which translate into pressure oscillations and induce self-sustained periodic vertical droplet bouncing over a broad range of experimental conditions.

The classic Leidenfrost phenomenon is familiar, yet its physics is rather complex. Graeber et al. observe the unexpected development of repeated hopping of a droplet trampolining on its own vapor cushion on a hot plate and show under which conditions this self-initiated motion occurs.

Rectification of Mobile Leidenfrost Droplets by Planar Ratchets

The self-transportation of mobile Leidenfrost droplets with well-defined direction and velocity on millimetric ratchets is one of the most representative and spectacular phenomena in droplet dynamics. Despite extensive progress in the ability to control the spatiotemporal propagation of droplets, it remains elusive how the individual ratchet units, as well as the interactions within their arrays, are translated into the collective droplet dynamics. Here, simple planar ratchets characterized by uniform height normal to the surface are designed. It is revealed that on planar ratchets, the transport dynamics of Leidenfrost droplets is dependent not only on individual units, but also on the elegant coordination within their arrays dictated by their topography. The design of planar ratchets enriches the fundamental understanding of how the surface topography is translated into dynamic and collective droplet transport behaviors, and also imparts higher applicability in microelectromechanical system based fluidic devices.

Biological and Engineered Topological Droplet Rectifiers

The power of the directional and spontaneous transport of liquid droplets is revealed through ubiquitous biological processes and numerous practical applications, where droplets are rectified to achieve preferential functions. Despite extensive progress, the fundamental understanding and the ability to exploit new strategies to rectify droplet transport remain elusive. Here, the latest progress in the fundamental understanding as well as the development of engineered droplet rectifiers that impart superior performance in a wide variety of working conditions, ranging from low temperature, ambient temperature, to high temperature, is discussed. For the first time, a phase diagram is formulated that naturally connects the droplet dynamics, including droplet formation modes, length scales, and phase states, with environmental conditions. Parallel approaches are then taken to discuss the basic physical mechanisms underlying biological droplet rectifiers, and a variety of strategies and manufacturing routes for the development of robust artificial droplet rectifiers. Finally, perspectives on how to create novel man-made rectifiers with functionalities beyond natural counterparts are presented.

High-speed X-ray imaging of the Leidenfrost collapse

The Leidenfrost layer is characterized by an insulating vapor film between a heated surface and an ambient liquid. The collapse of this film has been canonically theorized to occur from an interfacial instability between the liquid and vapor phases. The interfacial instability alone, however, is insufficient to explain the known influence of the surface on the film collapse process. In this work, we provide visual evidence for two key mechanisms governing the film collapse: the interfacial instability, and the nucleation of vapor upon multiple non-terminal liquid-solid contacts. These results were obtained by implementing high-speed X-ray imaging of the film collapse on a heated sphere submerged in liquid-water. The X-ray images were synchronized with a second high-speed visible light camera and two thermocouples to provide insight into the film formation and film collapse processes. Lastly, the dynamic film thickness was quantified by analysis of the X-ray images. This helped assess the influence of surface roughness on the disruption of the film. The results of this work encourage further investigation into non-linear stability theory to consolidate the role of the surface on the liquid-vapor interface during the film collapse process.

Acoustic detection of electrostatic suppression of the Leidenfrost state

At high temperatures, a droplet can rest on a cushion of its vapor (the Leidenfrost effect). Application of an electric field across the vapor gap fundamentally eliminates the Leidenfrost state by attracting liquid towards the surface. This study uses acoustic signature tracking to study electrostatic suppression of the Leidenfrost state on solid and liquid surfaces. It is seen that the liquid-vapor instabilities that characterize suppression on solid surfaces can be detected acoustically. This can be the basis for objective measurements of the threshold voltage and frequency required for suppression. Acoustic analysis provides additional physical insights that would be challenging to obtain with other measurements. On liquid surfaces, the absence of an acoustic signal indicates a different suppression mechanism (instead of instabilities). Acoustic signature tracking can also detect various boiling patterns associated with electrostatically assisted quenching. Overall, this work highlights the benefits of acoustics as a tool to better understand electrostatic suppression of the Leidenfrost state, and the resulting heat transfer enhancement.

Effect of surface topography and wettability on the Leidenfrost effect

When deposited on a superheated surface, a droplet can be levitated by its own vapour layer, a phenomenon that is referred to as the Leidenfrost effect. This dynamic effect has attracted interest for many potential applications, such as cooling, drag reduction and drop transport. A lot of effort has been paid to this mechanism over the past two and half centuries. Herein, we not only review the classical theories but also present the most recent theoretical advances in understanding the Leidenfrost effect. We first review the basic theories of the Leidenfrost effect, which mainly focuses on the relationship between the drop shape, vapour layer and lifetime. Then, the shift in the Leidenfrost point realized by fabricating special surface textures is introduced and the mechanisms behind this are analyzed. Furthermore, we present the reasons for the droplet transport in both classical Leidenfrost and pseudo-Leidenfrost regimes. Finally, the promising breakthroughs of the Leidenfrost effect are briefly addressed.

Lotus-like effect for metal filings recovery and particle removal on heated metal surfaces using Leidenfrost water droplets

A "lotus-like" effect is applied to demonstrate the ability of the Leidenfrost water droplets to recover Cu particles on a heated Al substrate. Cu particles on the heated surface adhere to the rim of the Leidenfrost droplets and eventually coat the droplets' surface to form an aggregation. When Fe filings are added to the Cu particles, the aggregated mixture can then be collected using a strong rare earth magnet (NdFeB) upon evaporation of the water. We also show that the Leidenfrost effect can be effectively utilized to recover both hydrophobic (dust and activated carbon) and hydrophilic (SiO2 and MgO) particles from heated Al surfaces without any topographical modification or surfactant addition. Our results show that hydrophobic and hydrophilic materials can be collected with >92% and >96% effectiveness on grooved and smooth Al surfaces, respectively. Furthermore, we observed no significant differences in the amount of material collected above the Leidenfrost point within the tested temperature range (240 °C vs. 340 °C) as well as when the Al sheet was replaced with a Cu sheet as the substrate. However, we did observe that the Leidenfrost droplets were able to collect a greater amount of material when the working liquid was water than when it was ethanol. Our findings show promise in the development of an effective precious coinage metal filings recovery technology for application in the mint industry, as well as the self-cleaning of metallic and semiconductor surfaces where manual cleaning is not amenable.