Hot Solid Surface Research Articles

Prediction of Protein-DNA Interface Hot Spots Based on Empirical Mode Decomposition and Machine Learning.

Protein-DNA complex interactivity plays a crucial role in biological activities such as gene expression, modification, replication and transcription. Understanding the physiological significance of protein-DNA binding interfacial hot spots, as well as the development of computational biology, depends on the precise identification of these regions. In this paper, a hot spot prediction method called EC-PDH is proposed. First, we extracted features of these hot spots' solid solvent-accessible surface area (ASA) and secondary structure, and then the mean, variance, energy and autocorrelation function values of the first three intrinsic modal components (IMFs) of these conventional features were extracted as new features via the empirical modal decomposition algorithm (EMD). A total of 218 dimensional features were obtained. For feature selection, we used the maximum correlation minimum redundancy sequence forward selection method (mRMR-SFS) to obtain an optimal 11-dimensional-feature subset. To address the issue of data imbalance, we used the SMOTE-Tomek algorithm to balance positive and negative samples and finally used cat gradient boosting (CatBoost) to construct our hot spot prediction model for protein-DNA binding interfaces. Our method performs well on the test set, with AUC, MCC and F1 score values of 0.847, 0.543 and 0.772, respectively. After a comparative evaluation, EC-PDH outperforms the existing state-of-the-art methods in identifying hot spots.

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.

Tailoring vapor film beneath a Leidenfrost drop

For a drop on a very hot solid surface, a vapor film will form beneath the drop, which has been discovered by Leidenfrost in 1756. The vapor escaping from the Leidenfrost film causes uncontrollable flows, and actuates the drop to move around. Recently, although numerous strategies have been used to regulate the Leidenfrost vapor, the understanding of surface chemistry for modulating the phase-change vapor dynamics remains incomplete. Here, we report how to rectify vapor by “cutting” the Leidenfrost film using chemically heterogeneous surfaces. We demonstrate that the segmented film cut by a Z-shaped pattern can spin a drop, since the superhydrophilic region directly contacts the drop and vaporizes the water, while a vapor film is formed on the superhydrophobic surrounding to jet vapor and reduce heat transfer. Furthermore, we reveal the general principle between the pattern symmetry design and the drop dynamics. This finding provides new insights into the Leidenfrost dynamics modulation, and opens a promising avenue for vapor-driven miniature devices.

The Low-Temperature Ring during Droplet Impact on a Superhydrophilic Surface

Droplet impact on the solid surfaces is widespread in nature, daily life, and industrial applications. The spreading characteristics and temperature evolution in the inertial spreading regime are critical for the heat and mass transfer process on the solid-liquid interface. This work investigated the spreading characteristics and temperature distribution of the thin liquid film in the inertial rapid spreading regime of droplet impact on the heated superhydrophilic surfaces. Driven by the inertial and capillary force, the droplet rapidly spreads on the superhydrophilic surface, resulting in a high temperature center in the impact center surrounded by a the low-temperature ring. The formation of the unique the low-temperature ring on the heated superhydrophilic surface is due to the much smaller time scale of rapid spreading than that of heat transfer from the hot solid surface to the liquid film surface. CFD numerical simulation shows that the impacting droplet spreads and congests in the front of liquid film, leading to the formation of vortex velocity distribution in the liquid film. Increasing We number and wall temperature can accelerate the heat transfer rate of liquid film and shorten the existence time of the low-temperature ring. The findings of the the low-temperature ring on the superhydrophilic surface provide the guidelines to optimization of surface structures and functional coatings for enhancing heat transfer in various energy systems.

Contact angle dynamics during the impact of single water droplet onto a hot flat practical stainless steel surface under medium Weber numbers

An experimental study on the contact angle dynamic during the impact of single water droplet onto a hot flat practical stainless steel surface under medium weber number was performed. The tested solid material was practical stainless steel of SUS 304. The surface roughness of the hot surface, the static contact angle at ambient pressure, and the diameter of the solid materials were respectively Ra = 0.2 µm, 85.7o, and 30 mm. The surface temperatures were varied from (60–200) oC. The liquid droplet diameter was 2.4 mm. The Weber numbers were 52.1, 57.6, and 63.1. The dynamic contact angles of single droplet during the impact were determined by implementing the developed image processing technique of the obtained video images from experiments. As a result, it was found that (1) the time needed to reach a maximum diameter of single droplet on a flat hot solid surface decreases with the increase of surface temperature, (2) the increase of the Weber number will postpone the spreading of the droplet from its receding, (3) there are three regions of the dynamic contact angle during the liquid evaporation, those are transient spreading region (region 1), a quasi-mechanical equilibrium region (region 2), and transient evaporation region (region 3), and (4) The contact angle in quasi-mechanical equilibrium region the under the nucleate boiling is insensitive to the surface temperature.

Electric resistence hysteresis of platinum filament in chilled air/hydrogen mixtures

Ignition of gaseous combustible mixtures on catalytically active hot solid surfaces has numerous applications in many industrial processes and is a complex process that involves close interaction between surface processes and transfer processes in the gas mixture. In this paper, stable and critical states catalytic oxidation of hydrogen impurities in air on a platinum filament are considered. It is shown that filament temperature and its resistance depending on the mixture temperature and hydrogen concentration are of the hysteresis features. Within this hysteresis region, it is possible to achieve the catalytic combustion mode of hydrogen as a result preheating the catalyst filament above a certain critical value. The dependence of the limiting hydrogen's concentration on catalyst filament's diameter, above which is observed in the cold gas mixture self-sustaining catalytic combustion without electric current.

Size-dependent spontaneous oscillations of Leidenfrost droplets

Abstract

The effect of ethylene glycol concentration on the interfacial dynamics of the successive droplets impacting onto a horizontal hot solid surface

The present work aimed to conduct the visualization study on the effect of the ethylene glycol concentration on the interfacial behavior of successive droplets during the impacting onto a horizontal hot solid surface. The tested liquid droplets were pure water and two ethylene glycol-water mixtures, which contain 10% and 20% of ethylene glycol. A polished cylindrical plate of stainless steel (SUS 304), with the surface roughness of 0.06 μm, was utilized as a solid surface. The surface varies for temperatures of about 110 °C–240 °C. The detailed phenomena during the droplet impact were recorded by using a high-speed video camera with the frame speed of 2000 fps and the resolution of the 1024 × 768. In addition, an own developed image processing technique was also implemented to obtain the quantitative parameters, such as the spreading ratio and the apex height during the droplet impacting onto a hot solid surface. As a result, the interfacial dynamics of the successive droplets were clarified. The addition of the concentration of ethylene glycol leads to the decrease of droplet oscillation during the spreading phase and delays both the bubble formation and the presence of secondary droplets. In addition, the research also reveals that the presence of the second droplet successfully improves the wetting area of the droplet.

Impact dynamics and heat transfer characteristics of liquid nitrogen drops on a sapphire prism

Drops close to a hot solid surface can be prevented from making contact by the vapour generation in between them. This so-called Leidenfrost effect occurs at a minimal plate temperature which is referred to as the Leidenfrost temperature. In spray cooling, were one uses impacting drops to cool down the hot solid, this effect is very undesirable: the vapour layer forms an isolating layer and prevents effective heat transfer between the drop and the solid. We study this phenomenon by impacting a single liquid nitrogen drop on a smooth sapphire prism using high-speed frustrated total internal reflection imaging. In these cryogenic conditions, the prism behaves as a perfect thermal conductor, while its transparency enables us to study the contact behaviour during the impact and the spreading phase of the drop. By varying the prism temperature and impact velocity of the drops we obtain a phase diagram of the impact characteristics. Using the Stokes number for the vapour flow, we find good agreement with previous studies for non-cryogenic liquids. The phase diagram is then compared with a second type of experiment in which a stream of drops cools the prism over time. The results of the two different type of measurements agree well, from which we conclude that the cooling power of a drop is strongly related to the wetting behaviour of the impacting drops. Finally, by comparing the wetted area with the contact line length we show that heat transfer in contact and transition boiling is dominated by conduction rather than evaporation.

Jet quenching phenomena during emergency cooling of high temperature solid surface

In the case of loss of coolant accident, LOCA in nuclear reactor, heat removal can be successful only when water rewets the surface. Jet impingement is a highly potential technique of heat removal from high temperature solid surface. After the impingement of liquid jet on hot solid surface it is not capable to wet the surface immediately, it takes time (wetting delay) to get a favorable condition by the liquid to wet and then propagate over the surface. Generation of vapor and its explosion creates the barrier for the liquid front to move. An experimental investigation has been conducted during sub-cooled water jet impingement quenching of three different cylindrical blocks of copper, brass and steel. The jet velocity varied from 3 to15 m/s, jet sub-cooling was 5-80 K and initial block temperature was 250 to 600 ºC. The study was conducted to determine the dominating parameters of maximum heat flux, wetting delay, quench front movement, boiling region size and finally developed a correlation of maximum heat flux for steel. After elapsing the wetting delay period, a visible smaller boiling region was observed to move radially from the impinged centre. The leading edge of this boiling region is designated as the quench front. The boiling region is a vital region as the maximum heat flux occurred at this region during cooling. The boiling region increases with the movement of the quench front and therefore the position of maximum heat flux moves accordingly. The boiling region width increases with material conductivity and decreases with liquid sub-cooling and liquid velocity. The maximum heat flux increases with jet velocity, liquid sub-cooling, solid material conductivity and it is almost independent of solid initial temperature. Wetting delay increases with block initial temperature and decreases with jet velocity and liquid sub-cooling.

The effect of surface roughness on dynamic behaviour of the successive multiple droplets impacting onto aluminium hot surfaces

The effects of hot surface roughness on dynamic behaviour of multiple droplets impacting hot solid surfaces were investigated in this research. The test material is Aluminium 6061 with surface roughness variation of 0.08 μm, 0.18 μm and 0.52 μm. The surface temperatures were 100 °C, 120 °C and 140 °C. The droplet dynamics during the impact have been observed using high-speed video camera then the results were characterized by using image processing technique. It could be concluded that each temperature shows different thermodynamic impact regime. The results also reveal that at surface temperature lower than 140 °C, the increase of surface roughness leads the increase of spreading diameter. In contrast, the spreading factor decreases with the increasing of surface roughness at 140 °C due to incomplete wetting of droplet on the surface. Meanwhile, the maximum spreading factor is relatively independent to the change of surface roughness.

Heat flux reconstruction by inversion of experimental infrared temperature measurements – Application to the impact of a droplet in the film boiling regime

An Inverse Heat Conduction Problem (IHCP) is considered in order to estimate the transient heat flux extracted from a hot solid surface by an impinging droplet. The resolution of the IHCP is made with the so-called quadrupole method, which provides an analytical expression of the temperature and the heat flux at the front surface of the solid wall, where the drop impact takes place. In the experiments, the thermal response of the front surface is recorded using IR thermography. For that, sapphire is chosen as the material of the solid wall, and the front surface is coated with a thin TiAlN ceramic layer (thickness of 300 nm). The latter is highly emissive and opaque in the IR while sapphire is transparent at the same wavelengths. This feature allows the surface impacted by the droplet to be viewed from the bottom by the IR camera. This approach has been implemented to gain some insights into the heat transfer from the solid surface as well as the formation and growth of the vapor film, which appears under the droplet in the regime of film boiling, when the solid temperature is much higher than the boiling temperature of the liquid. Due to the small thickness of the vapor film, heat conduction is predominant in the vapor layer. Hence, the thickness of the vapor film can be deduced from the value of the reconstructed local heat flux, assuming a linear profile of temperature between the liquid interface of the droplet at the saturation temperature and the solid surface measured by IR thermometry.

Coupled Catalytic/Gas Phase Ignition of Propane-Oxygen-Inert Mixtures on an Isothermally Heated Platinum Filament Supported on Quartz Bar

The ignition of gaseous flammable mixtures on catalytically active hot solid surfaces is an intricate process implying the tight interaction between surface and gas phase reactions. The paper presents the results obtained from the study of coupled catalytic-gas phase ignition of propane-oxygen-inert mixtures on an isothermally heated platinum filament stabilized on a thin quartz bar, measuring the induction periods, ti, rates of heat release, (dQr/dt)t and minimum ignition energies, Qmin. The experiments were conducted at different initial pressures between 73 and 101 kPa and filament temperatures between 1080 and 1184 K for 2.68% propane-air mixture and 4.02% propane-air mixture diluted with Ar or N2 in ratio 2:1. For any total pressure, the fuel concentration is the same in each mixture, the complementary composition containing the necessary oxygen for complete combustion and inerts or/and excess oxygen. The measured properties were rationalized using Arrhenius-type kinetic equations.

Leidenfrost drops on micro/nanostructured surfaces

In the Leidenfrost state, the liquid drop is levitated above a hot solid surface by a vapor layer generated via evaporation from the drop. The vapor layer thermally insulates the drop from the heating surface, causing deteriorated heat transfer in a myriad of important engineering applications. Thus, it is highly desirable to suppress the Leidenfrost effect and elevate the Leidenfrost temperature. This paper presents a comprehensive review of recent literature concerning the Leidenfrost drops on micro/nanostructured surfaces with an emphasis on the enhancement of the Leidenfrost temperature. The basic physical processes of the Leidenfrost effect and the key characteristics of the Leidenfrost drops were first introduced. Then, the major findings of the influence of various micro/nanoscale surface structures on the Leidenfrost temperature were presented in detail, and the underlying enhancement mechanism for each specific surface topology was also discussed. It was concluded that multiscale hierarchical surfaces hold the best promise to significantly boost the Leidenfrost temperature by combining the advantages of both micro- and nanoscale structures.

The effects of surface tension on the spreading ratio during the impact of multiple droplets onto a hot solid surface

The impact between multiple droplets onto hot surface is an important process in a spray cooling. The present study was conducted to investigate the dynamics of multiple droplet impact under various surface tensions. Here, the ethylene glycol with compositions of 0%, 5%, and 15% was injected through a nozzle onto stainless steel surface as the multiple droplet. The solid surface was heated at the temperatures of 100 °C, 150 °C, and 200 °C. To observe the dynamics of multiple droplets, a high speed camera with the frame rate of 2000 fps was used. A technique of image processing was developed to determine the maximum droplet spreading ratio. As the result, the surface tension contributes significantly to maximum spreading ratio. As the droplet surface tension decreases, the maximum spreading ratio increases. The maximum spreading ratio appears when the percentage of the ethylene glycol is 15% at the temperature of 150°C. From the visual observation, it is shown that a slower emergence of secondary droplets (droplet splashing) is carried out under a lower surface tension. Hence, surface tension plays an important role on the behavior of emerging secondary droplets. Furthermore, results of the experiments are useful for the validation of available previous CFD models.

Investigation of droplet behaviors for spray cooling using level set method

The droplet dynamics and heat removal is one of the popular research streams in spray cooling applications. The increase in the water droplet temperature as a result of its impact on the hot solid surface would determine the efficiency of the heat removal that is crucial in some sensitive applications such as spray cooling in depressurization systems in commercial nuclear power plants. Computer modelling of the underlying mechanism of the liquid droplet interaction with the hot solid surface would be necessary. The accuracy and the reliability of these models are important in simulating the multiphysics phenomena such as droplet dynamics and heat removal. In present work, the level set method coupled with heat transfer was used to simulate the water droplet impact on an isothermal solid surface. The changes in the temperature of water droplet were found to be highly dependent upon its size and the impingement speed. Topological variations in the droplet shape as a result of its impact onto the solid wall would cause abrupt changes in the temperature of the water droplet. In addition, the droplet detachment, coalescence and flattening were found to strongly influence the temperature of the droplet which would evidently affect the heat removal efficiency in the spray cooling systems employed in the commercial nuclear power plants. Furthermore, we demonstrated that the level set method had the ability to produce more accurate estimations of the water droplet dynamics when compared to the Volume-Of-Fluid (VOF) method in simulating the droplet behavior.

Spontaneous jumping, bouncing and trampolining of hydrogel drops on a heated plate

The contact between liquid drops and hot solid surfaces is of practical importance for industrial processes, such as thermal spraying and spray cooling. The contact and bouncing of solid spheres is also an important event encountered in ball milling, powder processing, and everyday activities, such as ball sports. Using high speed video microscopy, we demonstrate that hydrogel drops, initially at rest on a surface, spontaneously jump upon rapid heating and continue to bounce with increasing amplitudes. Jumping is governed by the surface wettability, surface temperature, hydrogel elasticity, and adhesion. A combination of low-adhesion impact behavior and fast water vapor formation supports continuous bouncing and trampolining. Our results illustrate how the interplay between solid and liquid characteristics of hydrogels results in intriguing dynamics, as reflected by spontaneous jumping, bouncing, trampolining, and extremely short contact times.

Spatially and temporally resolved measurements of the temperature inside droplets impinging on a hot solid surface

Heat transfers at the impact of a droplet on a hot solid surface are investigated experimentally. Millimeter-sized water droplets impinge a flat sapphire window heated at 600 °C. The time evolution of the droplet temperature is characterized using the two-color laser-induced fluorescence technique. For that, a Q-switched Nd:YAG laser is used for the excitation of the fluorescence to obtain instantaneous images of the droplet temperature. Water is seeded with two fluorescent dyes, one sensitive to temperature (fluorescein disodium) and the other not (sulforhodamine 640). Owing to a wavelength shift between the dyes’ emissions, the fluorescence signal of the dyes can be detected separately by two cameras. The liquid temperature is determined with a good accuracy by doing the ratio of the images of the dyes’ fluorescence. A critical feature of the method is that the image ratio is not disturbed by the deformation of the impacting droplet, which affects the signals of the dyes almost identically. Experiments are performed in the conditions of film boiling. A thin vapor film at the interface between the droplet and the solid surface prevents the deposition of liquid on the hot solid surface. Measurements highlight some differences in the rate of heat transfers and in the temperature distribution within the droplet between the bouncing and splashing regimes of impact.

The dynamics of the water droplet impacting onto hot solid surfaces at medium Weber numbers

The effects of the wettability of a droplet impacting onto a hot solid surface under medium Weber numbers were studied experimentally. The Weber numbers used in the present experiment were 52.1, 57.6, and 63.1. Three kinds of solid surfaces with different wettability were used. These were normal stainless steel (NSS), TiO2 coated NSS, and TiO2 coated NSS radiated with ultraviolet rays. The surface temperatures were varied from 60 to 200 °C. The image of side the view and 30° from horizontal were taken to explain the spreading and the interfacial behavior of a single droplet during impact the hot solid surfaces. It was found that under medium Weber numbers, the surface wettability plays an important role on the droplet spreading and evaporation time during the impact on the hot solid surfaces. The higher the wettability, the larger the droplet spreading on the hot surface, and the lower the evaporation time.

Deformation behavior of two droplets successively impinging obliquely on hot solid surface

We investigated the successive oblique collision of two droplets with a hot solid surface using flash photography. A pair of water droplets at room temperature was vertically dropped to impact a tilted smooth sapphire substrate one after the other. The diameter of the droplets was approximately 0.6mm and the impact velocity was varied between 1.6 and 2.1m/s. The spacing between the centers of the two falling droplets was also varied between 0.8 and 1.5mm. The substrate was titled at ⩽45° relative to the horizontal and its temperature was varied between 170 and 500°C. The leading droplet impacted the substrate and slid downwards over the surface, and this was followed by the off-centered collision of the trailing droplet relative to the deformed leading droplet. The subsequent motion of the “combined liquid” was observed to be essentially three-dimensional. For substrate temperatures of 200–300°C, the combined liquid was considerably distorted by the bursting of boiling vapor bubbles at the free surface. At a substrate temperature of 500°C, the liquid motion was roughly linearly symmetric. Rebounding phenomena were also observed in the liquid. The residence time of the droplets was measured for different conditions, and the results were used to derive an expression for predicting this parameter.