Final Stages Of Evaporation Research Articles

Black hole remnants are not too fast to be dark matter

We comment on recent claims that recoil in the final stages of Hawking evaporation gives black hole remnants large velocities, rendering them inviable as a dark matter candidate. We point out that due to cosmic expansion, such large velocities at the final stages of evaporation are not in tension with the cold dark matter paradigm so long as they are attained at sufficiently early times. In particular, the predicted recoil velocities are robustly compatible with observations if the remnants form before the epoch of big bang nucleosynthesis, a requirement which is already imposed by the physics of nucleosynthesis itself.

Marangoni effect of surfactant droplet in transition boiling and formation of secondary droplet

The dynamic processes of surfactant droplets impacting onto substrates of varied temperatures have been widely studied in heat transfer, cooling and printing. In this work, we observe the impacting process of aqueous droplets of surfactants SDS, CTAB, and Triton X-100 on a hot aluminum plate via a high-speed camera, in order to study the dynamics of different surfactant droplets impacting on a hot aluminum substrate. Experimentally, it is discovered that the surfactant droplets in transition boiling produce a secondary droplet of non-wetting state in the final stage of evaporation. The analysis demonstrates that after the droplet impacts the substrate, a temperature gradient is created between the top of the droplet and the triple-phase contact line, increasing the surfactant concentration near the triple-phase contact line as compared with that of the top. The top liquid is maintained by the Marangoni effect, which is caused by the concentration gradient. In the final stage of the evaporation process, the residual droplet gradually shrinks into a sphere. It is detached from the substrate and taken off under the impulse force of the bubble explosion at the bottom, generating the secondary droplet. The radius of the secondary drop increases with the raising of initial concentration of the drop, but ultimately reaches the saturation size. This work explains the role of surfactants in forming secondary droplets. Additionally, this work provides a reference for understanding the physical mechanism of Leidenfrost effect and the controlling of boiling heat transmission.

Interaction and motion of two neighboring Leidenfrost droplets on oil surface

Evaporation of droplets on a hot oil surface is a natural phenomenon. However, most of existing studies focus on the evaporation of a single droplet, and the evaporation of multiple droplets is insufficiently understood. Here, we explore the Leidenfrost evaporation of two identical FC-72 droplets on the surface of a hot oil bath. The oil temperature ranges from 73.6 to 126.6 ℃, and the evaporation of droplets each with an initial diameter of 1.5 mm is recorded by an infrared thermographer and a high-speed camera. The shallow oil depth keeps the oil temperature uniform relatively in the slot compared with that in the deep liquid pool due to the larger ratio of the surface area for copper-oil contact to the slot volume. We find that the neighboring droplets evaporate in three stages: non-coalescing, bouncing, and separating. The radius of neighboring Leidenfrost droplets follows the power law <i>R</i>(<i>t</i>)~(1−<i>t</i>/<i>τ</i>)<sup><i>n</i></sup>, where <i>τ</i> is the characteristic droplet lifetime and <i>n</i> is an exponent factor. Moreover, the diffusion-mediated interaction between the neighboring droplets slows down the evaporation process compared with the action of isolated Leidenfrost droplet and leads to an asymmetric temperature field on the droplet surface, thereby breaking the balance of the forces acting on the droplets. A simple dual-droplet evaporation model is developed which considers four forces acting horizontally on the droplet, namely, the Marangoni force resulting from the non-uniform droplet temperature, the gravity component, the lubrication-propulsion force, and the viscous drag force. Scale analysis shows that the Marangoni force and gravity component dominate dual-droplet evaporation dynamics. In the non-coalescence stage, the gravity component induces the droplets to attract each other, while the vapor film trapped between droplets prevents them from directly contacting. When the droplets turn smaller, the gravity component is insufficient to overcome the Marangoni force. Hence, the droplets separate in the final evaporation stage. Finally, we conclude that the competition between Marangoni force and gravitational force is the origin of the bounce evaporation by comparing the theoretical and experimental transition times at distinct stages. This study contributes to explaining the complex Leidenfrost droplet dynamics and evaporation mechanism.

Time-Resolved SAXS Study of Polarity- and Surfactant-Controlled Superlattice Transformations of Oleate-Capped Nanocubes During Solvent Removal.

Structural transformations and lattice expansion of oleate-capped iron oxide nanocube superlattices are studied by time-resolved small-angle X-ray scattering (SAXS) during solvent removal. The combination of conductor-like screening model for real solvents (COSMO-RS) theory with computational fluid dynamics (CFD) modeling provides information on the solvent composition and polarity during droplet evaporation. Evaporation-driven poor-solvent enrichment in the presence of free oleic acid results in the formation of superlattices with a tilted face-centered cubic (fcc) structure when the polarity reaches its maximum. The tilted fcc lattice expands subsequently during the removal of the poor solvent and eventually transforms to a regular simple cubic (sc) lattice during the final evaporation stage when only free oleic acid remains. Comparative studies show that both the increase in polarity as the poor solvent is enriched and the presence of a sufficient amount of added oleic acid is required to promote the formation of structurally diverse superlattices with large domain sizes.

Hawking-radiation recoil of microscopic black holes

Hawking radiation would make microscopic black holes evaporate rapidly, which excludes them from many astrophysical considerations. However, it has been argued that the quantum nature of space would alter this behaviour: the temperature of a Planck-size black hole vanishes, and what is left behind is a Planck-mass remnant with the cross-section of ≈ 1 0 − 70 m 2 , which makes a direct observation nearly impossible. Such black hole remnants have been identified as possible dark matter candidates. Here, we argue that the final stage of evaporation has a recoil effect that gives them a velocity up to ≈ 1 0 − 1 c . This would lead to a disagreement with the cold dark matter cosmological model unless the primordial black hole formation ceased shortly after the inflation era.

Analysis of Circulation Reversal and Particle Transport in Evaporating Drops

We report experimental evidence of circulation reversal in evaporating drops. The internal flow patterns and particle distribution inside the evaporating water droplet with suspended polystyrene particles was studied using the Particle Image Velocimetry (PIV) technique. Droplet placed on externally heated substrate showed two symmetric and counter rotating Marangoni convection cells at the beginning of evaporation process. Towards the final stages of evaporation, the two counter rotating Marangoni convection cells reversed the direction of circulation. Interestingly, it was observed that between these two stages four symmetric and counter rotating convection cells appeared inside the evaporating droplet. Our results are in support of recent theoretical investigations on circulation reversal in evaporating drops. The flow patterns were driven by the surface tension gradient caused by the surface temperature distribution along the droplet-air interface. The surface temperature distribution is not only dependent on the non-uniform evaporation flux but also on the conduction path from the substrate to the droplet surface. Towards the end of evaporation, the fluid flow becomes radially outward from the center to the pinned edge of the droplet. We have also investigated the evaporating droplets placed on a curved substrate. Unlike the case of flat substrate, the evaporation from curved surface resulted monotonic temperature profile. The particle distribution and resulting deposition patterns was strongly influenced by the internal fluid flow pattern and was observed to be different from that of a flat substrate.

The Black Hole Entropy Distance Conjecture and Black Hole Evaporation

AbstractWe extend the recently proposed Black Hole Entropy Distance Conjecture to the case of charged black holes in de Sitter space. By systematically studying distances in the space of black hole geometries with multiple horizons, we find that the distance is generically related to the logarithm of the entropy. From the infinite distance conjecture this predicts the appearance of a massless tower of modes in the limit of infinite entropy. Further, we study the evaporation of these black holes and relate it to the geometric distance. We find that the corresponding distance to the final stage of evaporation is finite. We conclude that evaporation does not lead to the appearance of a light tower of black hole microstates.

Thermodynamics of non-commutative scalar-tensor-vector gravity black holes

In this paper, we analyze the thermodynamic stability of Schwarzschild Modified Gravity (MOG) black holes in a non-commutative framework. We show that, unlike a commutative MOG black hole, in the coherent state picture of non-commutativity MOG black holes are thermodynamically stable. At the final stage of evaporation a stable remnant with zero temperatures and finite entropy is left in this non-commutative framework. Also, we consider the Parikh–Wilczek tunneling mechanism of massive particles from non-commutative MOG black holes and demonstrate that information leaks out of non-commutative MOG black holes in the form of some non-thermal correlations.

Innovative air conditioning system with rational distribution of thermal load

The efficiency of air conditioning (AC) systems depends on the operation of their air coolers at varying heat loads in response to current changeable climatic conditions. The intensity of heat transfer of refrigerant, evaporated inside air coils, drops at the final stage of evaporation, that is caused by drying out the inner wall surface. This results in lowering the overall heat transfer coefficient and reduction of air cooler efficiency in the whole. The concept of overfilling air coils that leads to excluding a dry-out of their inner surface and falling the overall heat transfer intensity at variation of refrigerant flows in response to change of current thermal load on air coolers is developed.

Revisiting Primordial Black Hole Evolution

Primordial black holes (PBHs) are the sensitive probe for physics and cosmology of very early Universe. The observable effect of their existence depends on the PBH mass. Mini PBHs evaporate and do not survive to the present time, leaving only background effect of products of their evaporation, while PBHs evaporating now can be new exotic sources of energetic particles and gamma rays in the modern Universe. Here we revisit the history of evolution of mini PBHs. We follow the aspects associated with growth versus evaporation rate of “a mini PBH being trapped inside intense local cosmological matter inhomogeneity”. We show that the existence of baryon accretion forbidden black hole regime enables constraints on mini PBHs with the mass M ≤ 5.5 × 10 13 g. On the other hand, we propose the mechanism of delay of evaporation of primordial population of PBHs of primordial mass range 5.5 × 10 13 g ≤ M ≤ 5.1 × 10 14 g. It can provide their evaporation to be the main contributor to γ -ray flux distribution in the current Universe. At the final stage of evaporation these PBHs can be the source of ultrahigh energy cosmic rays and gamma radiation challenging probe for their existence in the LHAASO experiment.

Rainbow gravity corrections to the information flux of a black hole and the sparsity of Hawking radiation

In this paper, we investigated the information flux of Schwarzschild black hole and the sparsity of Hawking radiation in the presence of rainbow gravity. The results demonstrate that the rainbow gravity has a very significant effect on the information flux. When the mass of rainbow black hole approaches to the order of Planck scale, the Bekenstein entropy loss per emitted quanta in terms of the mass of black hole reduces to zero. Furthermore, we also find the sparsity of Hawking radiation in rainbow gravity is no longer a constant; instead, it monotonically decreases as the mass of black hole decrease. At the final stages of evaporation, contrary to those of GUP case, the modified sparsity becomes infinity, which indicates the effect of quantum gravity stops Hawking radiation and leads to remnant.

A divergent volume for black holes calls for no ‘firewall’

The presumption that Hawking radiations are thermally distributed can be considered to result from their entanglement with the internal degrees of freedom for a black hole. This leads to the ‘firewall’ paradox if unitary evolution continues into Page’s time when a black hole evaporates away half of its initial entropy. However, if the interior of a black hole houses sufficient degrees of freedom to maintain entanglement with the outside at all times, unitarity can be preserved during the complete radiation process and no firewall will be required. This paper proposes a scenario that rescinds the firewall by introducing the concept of volume for a black hole. Based on the operational definition by Christodoulou and Rovelli (Christodoulou and Rovelli 2015 Phys. Rev. D 91 064046), we show that the volume and its associated entropy for a collapsed black hole diverges if the final evaporation stage is treated using noncommutative space. This implicates the interior of a black hole possesses adequate space to store information for a black hole of any mass, like the inside of a ‘magician’s bag’.

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.

Production of a degenerate Fermi-Fermi mixture of dysprosium and potassium atoms

We report on the realization of a mixture of fermionic $^{161}$Dy and fermionic $^{40}$K where both species are deep in the quantum-degenerate regime. Both components are spin-polarized in their absolute ground states, and the low temperatures are achieved by means of evaporative cooling of the dipolar dysprosium atoms together with sympathetic cooling of the potassium atoms. We describe the trapping and cooling methods, in particular the final evaporation stage, which leads to Fermi degeneracy of both species. Analyzing cross-species thermalization we obtain an estimate of the magnitude of the inter-species $s$-wave scattering length at low magnetic field. We demonstrate magnetic levitation of the mixture as a tool to ensure spatial overlap of the two components. The properties of the Dy-K mixture make it a very promising candidate to explore the physics of strongly interacting mass-imbalanced Fermi-Fermi mixtures.

Nonisothermal Evaporation of Layers of Aqueous Salt Solutions

Evaporation of layers of aqueous solutions of salts (LiBr, CaCl2, NaCl, MgCl2, BaCl2, and CsCl) is studied experimentally. Experimental data are compared with evaporation of the water layer. The solution is placed on a horizontal surface of a cylindrical heating section. Experiments on surface crystallization of salts are carried out. For aqueous solutions of salts LiBr, LiCl, and CaCl2, there is an extremum for the heat transfer coefficient αl. For water and for solutions of salts NaCl and CsCl, the extremum is absent. The first factor is a decreasing function of time, and the second factor is an increasing function of time. For the water layer, both factors continuously increase with time, and the maximum evaporation rate corresponds to the final stage of evaporation. The heat balance for interface layer is made up. The role of the free gas convection in the heat balance strongly depends on the salt concentration and varies with the rise of evaporation time. For low salt concentrations the influence of free convection in the gas phase on heat transfer in the liquid phase can be neglected; however, for high concentrations this effect is comparable with other factors. The curves for the rate of crystallization have been built. More than two time differences between the experiment and the calculation are associated with the kinetics of dendritic structures.

On the viability of regular black holes

The evaporation of black holes raises a number of conceptual issues, most of them related to the final stages of evaporation, where the interplay between the central singularity and Hawking radiation cannot be ignored. Regular models of black holes replace the central singularity with a nonsingular spacetime region, in which an effective classical geometric description is available. It has been argued that these models provide an effective, but complete, description of the evaporation of black holes at all times up to their eventual disappearance. However, here we point out that known models fail to be self-consistent: the regular core is exponentially unstable against perturbations with a finite timescale, while the evaporation time is infinite, therefore making the instability impossible to prevent. We also discuss how to overcome these difficulties, highlighting that this can be done only at the price of accepting that these models cannot be fully predictive regarding the final stages of evaporation.

Heat transfer of aqueous salt solutions during evaporation on a structured heated wall

Evaporation and heat transfer of the layers of water and aqueous salt solutions (H2O/LiBr, H2O/CaCl2, H2O/LiCl) have been studied experimentally. Evaporation rate is greater for a structured wall than for a smooth one. When simulating droplet evaporation and heat transfer, it is necessary to take into account free convection of air and vapor. There is a change of the heat transfer coefficient for salt solutions layers on the structured surface with the presence of an extremum. The largest value of the heat transfer coefficient α on the structured surface corresponds to water for the final stage of evaporation. For salt solutions, α is lower than that of water. The maximum excess of heat transfer coefficient of micro-structured wall above the smooth wall (20–25%) corresponds to the maximum height of the liquid layer for the initial period of evaporation. With increasing time, the excess is reduced.

Non-isothermal Evaporation of Salt Solutions on a Microstructured Surface

ABSTRACTHeat transfer of a droplet and layer during evaporation of aqueous solutions of salts has been studied. The behavior of salt solutions on a smooth and microstructured surface is compared here. Evaporation rate of aqueous salt solutions is greater for a microstructured surface than for a smooth wall. The behavior of heat transfer coefficient α can be described by two time regimes: quasi-constant values of α and significant increase in heat transfer at a multiple decrease in the liquid layer height. Measurements made with application of the particle image velocimetry showed that the structured surface increases liquid speed inside the sessile drop. The largest value of the heat transfer coefficient α on the structured surface corresponds to water for the final stage of evaporation. For salt solutions, the heat transfer coefficient is lower than that for water in the entire period of evaporation on the structured surface. The maximal excess (20–30%) of α of the structured wall above the smooth surface corresponds to the maximal height of the liquid layer at the beginning of evaporation. With increasing time, the excess is reduced. A drop of heat transfer intensification with a decrease in the layer height relates to suppression of free convection (a multiple decrease in the average velocity in the drop).

Evaporation of layers of salt solutions

Nonisothermal evaporation of layers of water and aqueous salts solutions of H2O/LiBr, H2O/CaCl2 and H2O/LiCl was studied experimentally. The liquid layer was placed on a horizontal heated wall. The initial concentration of salt C0 was 10 %. The wall temperature Tw = 75 °C and ambient air pressure was 1 bar. It was shown that the heat flux q increases for water for the final evaporation stage and falls for salt solutions due to the increase in salt concentration C and due to a significant drop in the equilibrium partial pressure of water vapor.

Extremal noncommutative black holes as dark matter furnaces

In this paper, we consider dark matter annihilation in the gravitational field of noncommutative black holes. Instead of a violent fate predicted in the usual Hawking radiation, we propose a thermal equilibrium state where a mildly burning black hole relic is fueled by dark matter accretion at the final stage of evaporation.