Final fate of a Leidenfrost droplet: Explosion or takeoff

Rapid and ultra-sensitive trace metals detection of water by partial Leidenfrost superhydrophobic array surface enhanced laser-induced breakdown spectroscopy

The rapid and ultra-sensitive detection of trace elements in liquid is a primary concern for researchers. In this study, a partial Leidenfrost effect superhydrophobic (PLSHB) array surface was used for rapid in situ evaporation enrichment of sample droplets. Within 4 min, a 50 μL droplet sample was completely evaporated, resulting in all solutes in it being concentrated within a circular range measuring approximately 350 μm in diameter, without the formation of a coffee ring structure. The limits of detection for six metals (Pb, Ba, Be, Mn, Cr, Cu) in water were determined to be as follows: 0.82 μgL-1, 0.27 μgL-1, 0.033 μgL-1, 0.136 μgL-1, 0.241 μgL-1, and 0.083 μgL-1. Furthermore, laser-induced breakdown spectroscopy (LIBS) was employed to detect the enriched solutes from ten liquid samples with identical concentrations on the PLSHB array surface; these measurements exhibited a relative standard deviation (RSD) of only 3.7%. Spike experiments involving the addition of the aforementioned six metals into drinking water demonstrated recovery rates ranging from 85.7% to 117.7%. Therefore, the application potential of PLSHB array surface enhanced LIBS for rapid, stable, and ultra-sensitive detection and analysis of trace metal elements across various fields such as industry, environmental science, and biomedicine might be highly promising.

Explosive Leidenfrost-Droplet-Mediated Synthesis of Monodispersed High-Entropy-Alloy Nanoparticles for Electrocatalysis

High-entropy-alloy nanoparticles (HEA NPs) exhibit promising potential in various catalytic applications, yet a robust synthesis strategy has been elusive. Here, we introduce a straightforward and universal method, involving the microexplosion of Leidenfrost droplets housing carbon black and metal salt precursors, to fabricate PtRhPdIrRu HEA NPs with a size of ∼2.3 nm. The accumulated pressure within the Leidenfrost droplet triggers an intense explosion within milliseconds, propelling the carbon support and metal salt rapidly into the hot solvent through explosive force. The exceptionally quick temperature rise ensures the coreduction of metal salts, and the dilute local concentration of metal ions limits the final size of the HEA NPs. Additionally, the explosion process can be fine-tuned by selecting different solvents, enabling the harvesting of diverse HEA NPs with superior electrocatalytic activity for alcohol electrooxidation and hydrogen electrocatalysis compared to commercial Pt (Pd) unitary catalysts.

Understanding and Utilizing Droplet Impact on Superhydrophobic Surfaces: Phenomena, Mechanisms, Regulations, Applications, and Beyond

Droplet impact is a ubiquitous liquid behavior that closely tied to human life and production, making indispensable impacts on the big world. Nature-inspired superhydrophobic surfaces provide a powerful platform for regulating droplet impact dynamics. The collision between classic phenomena of droplet impact and the advanced manufacture of superhydrophobic surfaces is lighting up the future. Accurately understanding, predicting, and tailoring droplet dynamic behaviors on superhydrophobic surfaces are progressive steps to integrate the droplet impact into versatile applications and further improve the efficiency. In this review, the progress on phenomena, mechanisms, regulations, and applications of droplet impact on superhydrophobic surfaces, bridging the gap between droplet impact, superhydrophobic surfaces, and engineering applications are comprehensively summarized. It is highlighted that droplet contact and rebound are two focal points, and their fundamentals and dynamic regulations on elaborately designed superhydrophobic surfaces are discussed in detail. For the first time, diverse applications are classified into four categories according to the requirements for droplet contact and rebound. The remaining challenges are also pointed out and future directions to trigger subsequent research on droplet impact from both scientific and applied perspectives are outlined. The review is expected to provide a general framework for understanding and utilizing droplet impact.

Leidenfrost droplet jet engine by bubble ejection

HYPOTHESIS

Despite the flourishing studies of Leidenfrost droplet motion in its boiling regime, the droplet motion across different boiling regimes has rarely been focused on, where bubbles are generated at the solid-liquid interface. These bubbles are probable to dramatically alter the dynamics of Leidenfrost droplets, creating some intriguing phenomena of droplet motion.

EXPERIMENTS

Hydrophilic, hydrophobic, and superhydrophobic substrates with a temperature gradient are designed, and Leidenfrost droplets with diverse fluid types, volumes, and velocities travel from the hot end to the cold end of the substrate. The behaviors of droplet motion across different boiling regimes are recorded and depicted in a phase diagram.

FINDINGS

A special phenomenon of Leidenfrost droplets that resembles a jet engine is witnessed on a hydrophilic substrate with a temperature gradient: the droplet traveling across boiling regimes repulsing itself backward. The mechanism of repulsive motion is the reverse thrust from fierce bubble ejection when droplets meet nucleate boiling regime, which cannot take place on hydrophobic and superhydrophobic substrates. We further demonstrate that conflicting droplet motions can occur in similar conditions, and a model is developed to predict the occurring criteria of this phenomenon for droplets in diverse working conditions, which agrees well with the experimental data.

A double rigidity transition rules the fate of drying colloidal drops

The evaporation of drops of colloidal suspensions plays an important role in numerous contexts, such as the production of powdered dairies, the synthesis of functional supraparticles, and virus and bacteria survival in aerosols or drops on surfaces. The presence of colloidal particles in the evaporating drop eventually leads to the formation of a dense shell that may undergo a shape instability. Previous works propose that, for drops evaporating very fast, the instability occurs when the particles form a rigid porous solid, constituted of permanently aggregated particles at random close packing. To date, however, no measurements could directly test this scenario and assess whether it also applies to drops drying at lower evaporation rates, severely limiting our understanding of this phenomenon and the possibility of harnessing it in applications. Here, we combine macroscopic imaging and space- and time-resolved measurements of the microscopic dynamics of colloidal nanoparticles in drying drops sitting on a hydrophobic surface, measuring the evolution of the thickness of the shell and the spatial distribution and mobility of the nanoparticles. We find that, above a threshold evaporation rate, the drop undergoes successively two distinct shape instabilities, invagination and cracking. While permanent aggregation of nanoparticles accompanies the second instability, as hypothesized in previous works on fast-evaporating drops, we show that the first one results from a reversible glass transition of the shell, unreported so far. We rationalize our findings and discuss their implications in the framework of a unified state diagram for the drying of colloidal drops sitting on a hydrophobic surface.

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.

Impact of a suspension drop onto a hot substrate: diminution of splash and prevention of film boiling

In the present study, the effect of graphite lubricant additives on the dynamics of a single drop impact onto a heated surface has been investigated in the nucleate boiling and thermal atomization regimes. In the nucleate boiling regime the drop impact is accompanied by the nucleation and expansion of multiple vapor bubbles. The drop residence time at the substrate is determined by the time of its mass loss due to splash and evaporation. At higher temperatures, above the Leidenfrost point, impact may lead to drop rebound. In this experimental and theoretical study the effect of additives on the outcome of drop impact, in particular, the addition of solid graphite particles, is investigated. The residence time of the drop has been measured for various initial drop temperatures and suspension concentrations. The addition of the particles leads to some increase of the residence time, while its dependence on the substrate temperature follows the scaling relation obtained in the theory. Moreover, the presence of the particles in the drop leads to suppression of splash and a significant increase of the drop rebound temperature, which is often associated with the Leidenfrost point. These effects are caused by the properties of the deposited layer, and pinning of the contact line of the entire drop and of each vapor bubble, preventing bubble coalescence and drop rebound. The phenomena are also explained by a significant increase of the liquid viscosity caused by the evaporation of the bulk liquid at high wall temperatures.

Spontaneous Takeoff of Single Sulfur Nanoparticles during Sublimation Studied by Dark-Field Microscopy

The Leidenfrost effect describes a fascinating phenomenon in which a liquid droplet, when deposited onto a very hot substrate, will levitate on its own vapor layer and undergo frictionless movements. Driven by the significant implications for heat transfer engineering and drag reduction, intensive efforts have been made to understand, manipulate, and utilize the Leidenfrost effect on macrosized objects with a typical size of millimeters. The Leidenfrost effect of nanosized objects, however, remains unexplored. Herein, we report on an unprecedented Leidenfrost effect of single nanosized sulfur particles at room temperature. It was discovered when advanced dark-field optical microscopy was employed to monitor the dynamic sublimation process of single sulfur nanoparticles sitting on a flat substrate. Despite the phenomenological similarity, including the vapor-cushion-induced levitation and the extended lifetime, the Leidenfrost effect at the nanoscale exhibited two extraordinary features that were obviously distinct from its macroscopic counterpart. First, there was a critical size below which single sulfur nanoparticles began to levitate. Second, levitation occurred in the absence of the temperature difference between the nanoparticle and the substrate, which was barely possible for macroscopic objects and underscored the value of bridging the gap connecting the Leidenfrost effect and nanoscience. The sublimation-triggered spontaneous takeoff of single sulfur nanoparticles shed new light on its further applications, such as nanoflight.

Theoretical model of the Leidenfrost temperature

The Leidenfrost effect is a phenomenon in which a liquid, poured onto a glowing surface significantly hotter than the liquid's boiling point, produces a layer of vapor that prevents the liquid from rapid evaporation. Rather than making physical contact, a drop of water levitates above the surface. The temperature above which the phenomenon occurs is called the Leidenfrost temperature. The reason for the existence of the Leidenfrost temperature, which is much higher than the boiling point of the liquid, is not fully understood and predicted. For water we prove that the Leidenfrost temperature corresponds to a bifurcation in the solutions of equations describing evaporation of a nonequilibrium liquid-vapor interface. For water, the theoretical values of obtained Leidenfrost temperature, and that of the liquid-vapor interface which is smaller than the boiling point of liquid, fit the experimental results found in the literature.

Thermophobic Leidenfrost

We report that a volatile liquid deposited on a hot substrate with a gradient of temperature does not only levitate (Leidenfrost effect), but also spontaneously accelerates to the cold. This thermophobic effect is also observed with sublimating solids, and we attribute it to the ability of temperature differences to tilt (slightly) the base of the "object", which induces a horizontal component to the levitating force. This scenario is tested by varying the drop size (with which the acceleration increases) and the substrate temperature (with which the acceleration decreases), showing that the effect can be used to control, guide and possibly trap the elusive Leidenfrost drops.

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.

The role of drop shape in impact and splash

The impact and splash of liquid drops on solid substrates are ubiquitous in many important fields. However, previous studies have mainly focused on spherical drops while the non-spherical situations, such as raindrops, charged drops, oscillating drops, and drops affected by electromagnetic field, remain largely unexplored. Using ferrofluid, we realize various drop shapes and illustrate the fundamental role of shape in impact and splash. Experiments show that different drop shapes produce large variations in spreading dynamics, splash onset, and splash amount. However, underlying all these variations we discover universal mechanisms across various drop shapes: the impact dynamics is governed by the superellipse model, the splash onset is triggered by the Kelvin-Helmholtz instability, and the amount of splash is determined by the energy dissipation before liquid taking off. Our study generalizes the drop impact research beyond the spherical geometry, and reveals the potential of using drop shape to control impact and splash.

The dynamics of droplet impact and splash is important in many applications, yet its analysis involves difficult intertwined aspects. Here Liu et al. make shape parametrically accessible to experiment, with ferrofluidic drops passing a magnetic field in a defined way.

How ambient conditions affect the Leidenfrost temperature

By sufficiently heating a solid, a sessile drop can be prevented from contacting the surface by floating on its own vapour. While certain aspects of the dynamics of this so-called Leidenfrost effect are understood, it is still unclear why a minimum temperature (the Leidenfrost temperature T_L) is required before the effect manifests itself, what properties affect this temperature, and what physical principles govern it. Here we investigate the dependence of the Leidenfrost temperature on the ambient conditions: first, by increasing (decreasing) the ambient pressure, we find an increase (decrease) in T_L. We propose a rescaling of the temperature which allows us to collapse the curves for various organic liquids and water onto a single master curve, which yields a powerful tool to predict T_L. Secondly, increasing the ambient temperature stabilizes meta-stable, levitating drops at increasingly lower temperatures below T_L. This observation reveals the importance of thermal Marangoni flow in describing the Leidenfrost effect accurately. Our results shed new light on the mechanisms playing a role in the Leidenfrost effect and may help to eventually predict the Leidenfrost temperature and achieve complete understanding of the phenomenon, however, many questions still remain open.

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.

Sailing Droplets in Superheated Granular Layer

We report instability of a superheated granular layer when a droplet is deposited on top of the layer. We find that the instability caused by evaporating vapor may trap or cause the droplet to sail away from the deposited position. The sailing motion is triggered by an unstable pressure distribution originated from fast fluidization of metallic grains. We provide a predictive model and experimental verification of the enabling conditions for sailing motion based on limiting criteria for fast fluidization.

Inverse leidenfrost drop manipulation using menisci

Drops deposited on an evaporating liquid bath can be maintained in an inverse Leidenfrost state by the vapor emanating from the bath, making them levitate and hover without effective friction. These perfectly non-wetting droplets create a depression in the liquid interface that sustains their weight, which generates repellent forces when they approach a meniscus rising against a wall. Here, we study this reflection in detail, and show that frictionless Leidenfrost drops are a simple and efficient tool to probe the shape of an unknown interface. We then use the menisci to control the motion of the otherwise elusive drops. We create waveguides to direct and accelerate them and use parabolic walls to reflect and focus them. This could be particularly beneficial in the scale up of droplet cryopreservation processes: capillary interactions can be used to transport, gather and collect vitrified biological samples in absence of contact and contamination.

Rebound of self-lubricating compound drops

Drop impact on solid surfaces is encountered in numerous natural and technological processes. Although the impact of single-phase drops has been widely explored, the impact of compound drops has received little attention. Here, we demonstrate a self-lubrication mechanism for water-in-oil compound drops impacting on a solid surface. Unexpectedly, the core water drop rebounds from the surface below a threshold impact velocity, irrespective of the substrate wettability. This is interpreted as the result of lubrication from the oil shell that prevents contact between the water core and the solid surface. We combine side and bottom view high-speed imaging to demonstrate the correlation between the water core rebound and the oil layer stability. A theoretical model is developed to explain the observed effect of compound drop geometry. This work sets the ground for precise complex drop deposition, with a strong impact on two- and three-dimensional printing technologies and liquid separation.

Nanoparticle-Mediated Cavitation via CO2 Laser Impacting on Water: Concentration Effect, Temperature Visualization, and Core-Shell Structures

By taking advantage of seeded polymer nanoparticles and strong photo energy absorption, we report CO₂ laser impacting on water to produce cavitation at the air/water interface. Using a high-speed camera, three regimes (no cavitation, cavitation, and pseudo-cavitation) are identified within a broad range of nanoparticles concentration and size. The underlying correlation among cavitation, nanoparticles and temperature is revealed by the direct observation of spatiotemporal evolution of temperature using a thermal cameral. These findings indicate that nanoparticles not only act as preexisted nuclei to promote nucleation for cavitation, but also likely affect temperature to change the nucleation rate as well. Moreover, by exploiting a compound hexane/water interface, a novel core-shell cavitation is demonstrated. This approach might be utilized to attain and control cavitations by choosing nanoparticles and designing interfaces while operating at a lower laser intensity, for versatile technological applications in material science and medical surgery.