Large Water Droplets Research Articles

Extremely large water droplet impact onto a deep liquid pool.

Most studies of droplet impact on liquid pools focus on droplet diameters up to the capillary length (0.27cm). We break from convention and study extremely large water droplets (1 to 6cm diameter) falling into a pool of water. We demonstrate that the depth and width of the cavity formed by large droplet impact is greatly influenced by the deformed shape of the droplet at impact (i.e., prolate, spherical, and oblate), and larger droplets amplify this behavior by flattening before impact. In particular, the maximum cavity depth is a function of the Froude number and axis ratio of the droplet just before impact. Further, the cavity depth is more dependent on the droplet height than width, and the maximum cavity diameter is independent of the droplet height. In general, we observe that more oblate droplets result in decreasing cavity depths for a fixed liquid volume. This is because an increase in horizontal droplet diameter results in a reduced impact energy flux and therefore reduced cavity depth.

Coarsening droplets for frosting delay on hydrophilic slippery liquid‐infused porous surfaces

AbstractFrosting occurs due to the freezing of condensed water droplets on a supercooled surface. The nucleated frost propagates through interdroplet bridges and covers the entire surface, resulting from the deposition of highly supersaturated vapor surrounding tiny droplets. While inhibition of the formation of frost bridges is not possible, the propagation of frost can be delayed by effectively removing tiny droplets. Passive technologies, such as superhydrophobic surfaces (SHS) and hydrophobic slippery liquid‐infused porous surfaces (SLIPS), rely on static growth and direct contact with densely distributed droplets. However, use of these approaches in delaying frost propagation involves challenges, as the interdroplet distance remains small. Here, we report a new approach of spontaneous droplet movement on hydrophilic SLIPS to delay the formation of interdroplet frost bridges. Surface tension forces generated by the hydrophilic oil meniscus of a large water droplet efficiently pull neighboring droplets with a diameter of less than 20 μm from all directions. This causes a dynamic separation between water droplets and an adjacent frozen droplet. Such a process delays the formation and propagation of interdroplet frost bridges. Consequently, there is significant delay in frosting on hydrophilic SLIPS compared to those on SHS and hydrophobic SLIPS.

Research on Icing Classification Method Based on Wind Tunnel Test with Double Impact Surface Probe

Facing supercooled large water droplet environment, an effective ice detection method is a prerequisite to implement the avoidance strategy and get out of the icing environment of SLD as soon as possible. Fiber-optic icing sensors were arranged on the double impact surface probe. The probe was used for icing wind tunnel test. Different machine learning algorithms were used to establish the classification method of icing conditions based on multi-sensor ice thickness information fusion. An appropriate algorithm was selected for the classification method to detect icing conditions. The icing classification method based on SVM could effectively distinguish the conventional water droplet icing condition from the SLD icing condition, and it has significant potential on aviation industry application.

Exergy and energy analysis of a novel solar powered fog desalination system

A novel system that applied fogging as a desalination technique was recently published, which could be considered a further development of the humidification-dehumidification desalination system (HDH), that provides an advantage through its ability to desalinate high salinity levels. This involves spraying hot saline water at high pressure vertically upwards, creating a mist instead of large water droplets. This increases productivity since the fine mist facilitates evaporation and the low-saline mist can go upwards more than the large high-salinity droplets. However, experiments showed that this methodology, in terms of productivity, performance and cost, was between other technologies. It was noted that the performance of this system peaked at ~39 %. Therefore, in this paper, an exergy analysis of the system is presented to determine the location of the highest exergy destruction and also highlight its comparative feasibility. The results indicated that the exergetic efficiency of the system reached 17.3 % while the main source of irreversibility is the fogging process itself (~75 %). Additionally, increasing the water temperature from 33 to 76 °C increased the exergetic efficiency of the fogger from 4 % to 20 %. Therefore, the authors recommend using a combination of humidification and atomization via nozzles of larger orifices with different spraying angles.

Temperature-responsive peristome-structured smart surface for the unidirectional controllable motion of large droplets

The manipulation of fast, unidirectional motion for large droplets shows important applications in the fields of fog collection and biochemical reactions. However, driving large droplets (>5 μL) to move directionally and quickly remains challenging due to the nonnegligible volume force. Herein, we fabricated a scalable, bionic peristome substrate with a microcavity width of 180 μm using a 3D printing method, which could unidirectionally drive a large water droplet (~8 μL) at a speed reaching 12.5 mm/s by temperature-responsive wettability. The substrate surface was grafted with PNIPAAm, which could reversibly change its wettability in response to temperature, thereby enabling a temperature-responsive smart surface that could regulate droplet movement in real-time by changing the temperature. A series of temperature-responsive smart patterns were designed to induce water transport along specific paths to further realize controllable droplet motion with the antibacterial treatment of predesignated areas. The ability to achieve temperature-responsive unidirectional motion and dynamic control of droplet movement could allow programmable fluidic biosensors and precision medical devices.A temperature-responsive smart surface was produced to control the unidirectional motion of large droplets between spreading and pinning movement by changing the surface wettability.

A feasibility Study on the exhaust-gas treatment efficiency of a wet-scrubber system equipped in a diesel engine: A focus on smoke emission

A feasibility examination was conducted on a wet-scrubber system designed in this study to investigate the exhaust gas treatment efficiency of a wet-scrubber system equipped with a diesel engine. This preliminary investigation limits to evaluate the feasibility of the wet scrubber system in decreasing smoke. The influence of multiphase flow configuration in wet-scrubbing aftertreatment systems on emission formation is an interesting topic in the field of exhaust gas aftertreatment. It is our initial attempt to investigate the influence of multiphase flow configuration in a wet-scrubbing hybrid aftertreatment system on key pollutants emitted from Diesel engines in the future. This wet scrubber works by spraying water into the exhaust gas stream. The engine was tested at three-speed conditions (1,500 rpm, 1,800 rpm, and 2,000 rpm, respectively) while varying its torque from 15 to 30 N.m. The results show that under these operating conditions, the smoke level after the scrubber decreases by up to 50%. Please note that only fresh water was used in this study, and the impact of catalysts that could further decrease exhaust gas pollution will be investigated in future studies. The water droplets serve as a medium for capturing and collecting the soot particles present in the exhaust gas. Through collisions, the water droplets cause the soot particles to adhere to their surfaces, a process known as impaction. Additionally, the larger water droplets can facilitate coagulation or agglomeration of the particles, resulting in their enlargement and enhanced capture efficiency.

Production of small powder-stabilized droplets using superhydrophobic mesh

Massive production of tiny droplets is critical for industrial processes and biomedical applications. Such droplets are formed by splitting the bulk liquid (top-down) or condensation to grow the droplet (bottom-up). Various generation methods have been reported; however, the generated droplets stick to the contacting substrate and/or coalesce into a larger droplet, limiting handling flexibility and/or size uniformity. Herein, tiny powder-stabilized droplets were massively generated by applying pulse pressure to a “powdered” superhydrophobic mesh on a water pool. The generated droplets were stabilized by a fine hydrophobic powder, namely, liquid marble, which does not coalesce or stick to the contact materials. The effects of mesh size, wettability, powdering, and applied pressure on the droplet formation dynamics were investigated. The generated droplet size depended on the mesh size and droplets were not formed on the hydrophilic mesh. When the superhydrophobic mesh gradually sank into the water pool to apply quasi-static hydro-pressure, the penetrated water did not split, and a continuous large water droplet was formed. However, tapping the superhydrophobic mesh to apply pulsed hydro-pressure induced multiple droplets to split through the mesh penetration. Without powder, the formed droplets coalesced to form large droplets. However, when the superhydrophobic mesh was powdered, the formed droplets were coated with the hydrophobic powder to form a liquid marble, preventing their coalescence.

Water repellency by volcanic ash interacting with organic matter: Incubation response and effect on infiltration

Volcanic ash deposition disrupts soil surface hydrology. Our previous study showed that soil infiltration was reduced eightfold after a volcanic eruption in various land-use systems adjacent to Mount Kelud (Indonesia). Yet, soil macroporosity was relatively unchanged, indicating soil hydrophobicity. We tested the hypothesis that hydrophobicity or water repellency (WR) can be induced by volcanic ash interacting with organic matter and quantified its effect on surface water infiltration.We combined volcanic ash with leaf litter from coffee, durian, pine and mixed sources and tested for WR as a function of incubation time (0–16 weeks) and soil water content (θ, %), with Water Drop Penetration Time (WDPT, s) and Contact Angle (CA, o) as WR metrics. Lipids content (%) and pH were also analysed during incubation. Water droplets were placed onto a material tested for WR in a slope-adjustable stage to determine the critical angle for droplet runoff before penetration. Finally, to quantify the effect of WR on water flow, we layered 5 and 10 cm of ash with organic matter additions on top of a control soil column and performed infiltration (hydraulic conductivity) measurements.Among litter sources, pine litter induced the highest WR. There were significant relationships between WR, lipid, and pH. However, these relationships became weaker with material water contents > 7%. A significant relationship was also found between CA and critical slope of both small and large drops. A higher CA lowered the minimum critical slope for droplet runoff to 7° and 10° for the diameters of large and small water droplets, respectively. The mixture of volcanic ash and organic matter layered into the soil surface resulted in a five-times lower average hydraulic conductivity, with indications of air entrapment limiting water infiltration in a thicker ash layer. A higher CA was strongly associated with a lower hydraulic conductivity ratio, particularly under the 5 cm organic matter treatment. The WR effects on hydraulic conductivity are equivalent to at least a ten-fold reduction in effective soil porosity. By reducing infiltration, WR may contribute to ash movement in the volcanic landscape.

Bioinspired flexible, breathable, waterproof and self-cleaning iontronic tactile sensors for special underwater sensing applications

Flexible tactile sensors are under high pursuit for wearable healthcare devices. However, challenges still exist in achieving both wearing comfortability through air permeability and hydrophobicity to resist water splash or even to be used in the special underwater circumstance. Inspired by the smart bio-structure of lotus leaf, we develop a flexible, breathable, waterproof and self-cleaning tactile sensor by sandwiching one ionogel electrolyte between two polydopamine (PDA)/MXene/stearic acid (STA) fabric electrodes. The micro-channels in the fabric and the through-holes in the gel endow the sensor with a high air breathability of 723.5 mm s−1 and the STA micro-sheets decorated on the outside surface of the sensor provide hydrophobicity with a large water droplet contact angle of 140.7° for the unique properties of self-cleaning and washability. By taking advantage of the supercapacitive sensing mechanism, the microstructures of MXene nanosheets on microfibers and the using of an interlayer with an internal aperture to initially separate electrodes and electrolyte, an extremely high sensitivity of up to 1677.79 kPa−1 is achieved. Practical sensing applications of the developed flexible sensors worn on different parts of human body for physiological signal monitoring, motion detection and silent information communication through Morse code in the special underwater circumstance are demonstrated.

Evaluation of flow field design effects on proton exchange membrane fuel cell performance

Fluid distribution, conduction, and heat control are important phenomenon in the fuel cell fraternity, therefore it is crucial to develop a state-of-the-art bipolar plate (BP) to attain optimum cell performance. Metal foam (MF) and fine mesh have attracted a lot of attention in mitigating some of the challenges associated with straight, and serpentine channels. In this study, MF, 3D fine mesh, fine wire mesh (FWM) flow fields are compared with triple serpentine flow field to develop an optimum design for improved PEMFC performance. Two different foam designs are studied to attenuate the existing drawback associated with MF, mainly caused by high water retention. The 3D fine mesh is leading in performance under anodic and cathodic stoichiometry of 1 and 3 respectively. On increasing the anodic and cathodic stoichiometry to 1.2 and 3.5 respectively, the FWM took the lead. This is brought by the improved water drainage under high stoichiometry. Because FWM is already in mass production, although for other purposes, it is cost competitive over the other designs. The fine mesh and the MF have the potential to break down large water droplet making them easy to drain. They also showed symmetric fluid flow, compared to the serpentine design.

Trajectory Surface Hopping for a Polarizable Embedding QM/MM Formulation.

We present the implementation of trajectory surface-hopping nonadiabatic dynamics for a polarizable embedding QM/MM formulation. Time-dependent density functional theory was used at the quantum mechanical level of theory, whereas the molecular mechanics description involved the polarizable AMOEBA force field. This implementation has been obtained by integrating the surface-hopping program Newton-X NS with an interface between the Gaussian 16 and the Tinker suites of codes to calculate QM/AMOEBA energies and forces. The implementation has been tested on a photoinduced electron-driven proton-transfer reaction involving pyrimidine and a hydrogen-bonded water surrounded by a small cluster of water molecules and within a large water droplet.

TiO2-based slippery liquid-infused porous surfaces with excellent ice-phobic performance

Ice formation and accretion on surfaces of instruments such as infrastructures, power lines and aircrafts can trigger serious social danger, economic loss and threaten the safety of lives. It is of practical significance to develop functionalized passive anti-icing surfaces. Herein, we synthesized titanium dioxide nanotube arrays on titanium substrate through anodic oxidation method. The TiO2 nanotube arrays were further modified and infused with perfluoropolyether (PFPE) lubricant to form slippery liquid-infused porous surfaces (SLIPS). The relationship of the nanostructures and the slippery performance, the liquid holding property and anti-icing property of TiO2 based SLIPS was investigated in details. The contact angle hysteresis (CAH) and sliding angles of water droplets on the TiO2 SLIPS are as low as 1° and 4°, respectively, indicating the excellent slippery property of the SLIPS. The CAH of water droplets on TiO2 SLIPS could be still as low as 5° after being treated with 100 mL of flowing water, which show the long term stability and superior liquid holding property of the TiO2 SLIPS. We propose that the excellent slippery property and long term stability for holding liquid of TiO2 SLIPS attribute to the uniform nanostructures, the small diameter of the nanotubes, the smooth surface and high capillary force of TiO2 nanotube arrays. Both the icing delay tests and the ice adhesion force tests conducted on TiO2 SLIPS indicate that the long-term ice-phobic stability is strongly correlated with the surface smoothness and uniformity of nanostructures. The results show that large water droplets with size about 15 μL on TiO2 NT based SLIPS can keep as liquid phase at very low temperature (–15 ℃) for up to 24 h, which exhibits much longer icing delay time at –15 ℃ than most of the reported superhydrophobic surfaces and SLIPS. And the TiO2 SLIPS exhibit very good icing delay effect even after being reused for 17 times at –15 ℃, indicating the superior long-term stability and anti-icing effect of TiO2 SLIPS as passive ice-phobic surfaces. The ice adhesion force of icicles on TiO2 SLIPS was as low as ∼50 kPa after the tenth test, further indicating their long-term stability as anti-icing surfaces. In summary, the TiO2 nanotube arrays based SLIPS obtained in this work exhibit superior slippery property, anti-biofouling, excellent anti-icing property and long term stability after being treated with a large amount of flowing water or reused for many times under very low temperature. This work shows that SLIPS can be applied as ice-phobic surfaces with long term stability at low temperature, potentially useful for their practical applications as anti-icing and ice-phobic surfaces.

A diffusion-enhancing icing theory for the freezing transition of supercooled large water droplet in impact

The freezing behavior of supercooled large droplet(SLD) in impact causes severe threats to civil aircraft. However, the mechanism by which the impact dynamics of SLD accelerates the icing still remains unclear. In this paper the impact and freezing of water droplets with different supercooling on inclined surfaces of different wettability were investigated experimentally and theoretically. Compared to one/two freezing morphologies and linear supercooling-freezing time relation reported previously, four freezing morphologies were observed in different supercoolings, and the transition of freezing morphologies resulted in the piecewise nonlinear supercooling-freezing time relation. By comparing the characteristic times of droplet dynamics and horizontal icing, it was found that the dynamics-icing interaction could lead to higher heat conduction area in higher supercooling. Theoretical analysis shows that the unsteady heat conduction enhances the freezing rate in a short time initially. The coupled effect of large contact area and initial heat flux greatly enhances the diffusion of latent heat in freezing of droplet with high supercooling. Based on this machanism, a diffusion-enhancing icing theory is proposed. Compared with previous icing theories, this theory can give much better predictions about the freezing rate of impinging supercooled water droplets, especially in conditions of high supercooling and hydrophobic surface.

Impingement and freezing of a supercooled large droplet on an ice surface

In the present study, we carried out an experimental investigation of the impinging and freezing processes of a supercooled large water droplet on an ice surface. One high speed camera was used to measure the dynamic motions of the water droplet while two charge coupled device (CCD) cameras were adopted to obtain the images of the freezing process and the freezing morphologies, respectively. The effects of the water droplet temperature and the ice surface temperature on the impact and freezing processes of the water droplet were carefully evaluated. The results showed that the subcooling degree of the water droplet had an apparent influence not only on the spreading process but on the freezing morphology as well. When the subcooling temperature of the water droplet was high (e.g., Tw = –3.0 °C), a triangle cross-section profile formed. However, once the subcooling temperature of the water droplet was relatively low (e.g., Tw = –9.0 °C), the final ice morphology consisted of two parts: one part was a cone-like bead in the center while the other part was an ice ring at the periphery. Moreover, at the same water droplet temperature, raising the ice surface temperature led to an increase in the maximum spreading factor.

Uncertainty and sensitivity analysis of boundary conditions during water droplet phase change regimes

This paper presents the heat and mass transfer model of a liquid droplet in the consistently changing condensation, transit and equilibrium evaporation regimes, and iterative scheme of numerical simulation of the instantaneous temperature of a semi-transparent droplet in the case of complex radiative-convective heating. The whole cycle of the phase transition regimes of medium and large water droplets flowing in the flue gas was modeled, taking into account the application of water spraying into the flue gas of a biofuel furnace. Simulation results expressed as thermal, energy and phase transformation parameters of water droplets are analyzed in the universal Fourier time scale. The factors defining the change of heat transfer regime of a water droplet are termed as the suppression of droplet slipping in flue gas flow at initial phase transformation stage, and the decrease of radiation absorption at the final stage. Particular attention is paid to the evaluation of the influence of parameters defining the interaction of complex transfer processes on the droplet's phase transformations performing the sensitivity analysis of the heat and mass transfer parameters for the “hypothetical” droplet case (the diameter and the slipping velocity in to gas did not change). It is substantiated, that the main parameters defining the sensitivity of water droplets equilibrium evaporation temperature are flue gas humidity, droplets dispersity, flue gas temperature and droplets slipping velocity.The performed statistical uncertainty and sensitivity analysis showed that the uncertainty of heat and mass transfer parameters describing the phase transformations of a water droplet until equilibrium evaporation regime is affected mostly by droplet dispersity, slipping velocity, flue gas temperature and pressure. The size of droplets and their slip velocity in the flue gas flow were identified as very important parameters for the interaction of complex heat and mass transfer processes of a sprayed water. The main uncertainties were defined at the transition from condensing to transit evaporation regime, and it was defined that with a confidence interval of at least 95 % and accepting a tolerance limit with a probability of at least 95 %, the transit evaporation regime starts at the range Fo ≈ (0.02–0.1). The droplet surface temperature, i.e. parameter determining the droplet phase transformations and its dynamics, could be assessed with a mean error of ±10 %, accepting 95 % probability with 95 % confidence level.

Pt/Fe/Ni decorated CVD grown CNTs on carbon paper as electrocatalytic electrodes in polymer fuel cells: An investigation on H2 gas on the growth of CNTs and reduction of electrocatalysts

In the present study, the in-situ synthesis of CNTs on carbon paper (CNT/CP), to be used as electrocatalyst support at PEM fuel cell, was experimentally studied with the aim of investigating the role of hydrogen gas. In this regard, the microstructural characterization along with TGA analysis revealed that the structure of the CNTs and the density of defects are highly affected by the absence or presence of hydrogen gas during the synthesis process. According to the experiments, an optimal flow ratio of Ar/H2/C2H2: 200/30/20 was obtained for the synthesis of CNTs on CP impregnated in nickel catalyst. The unique CNT/CP structure presents high specific surface area (159.24 m2/g), large water droplet contact angle (157/2 ± 0.3°) indicating superhydrophobicity, and high hydrophobic stability, therefore, they can be considered as a promising candidate for many technological applications. In this work, CNT/CP was used for supported Pt/Fe/Ni catalysts as PEM fuel cell cathode. The results of the Half-cell testing showed that not only the application of CNTs instead of carbon black has a particular role in the performance improvement of the polymer fuel cathode but also the presence of hydrogen gas in the metal catalyst reduction stage significantly improves the efficiency to 91%. Considering that the high efficiency was achieved without the requirement of excess catalysts loading and the corresponding additional expenses in PEMFCs.

WELL SCREEN FOR THE TECHNOLOGY OF WATER- CUT OIL DOWNHOLE SEPARATION

Background According to a number of studies, the efficiency of the preliminary water discharge units in terms of the quality of the discharged water and the amount of residual water in oil depends on the degree of destruction. With an increase in the latter, the efficiency of the pre-dumping installations increases significantly. In oil-gathering reservoirs, due to the relatively low flow rates, emulsification of liquids practically does not occur and, moreover, conditions are created for the deposition of large water droplets along the cross-section of the pipeline and the formation of a continuous underlying layer of water, which can be captured by ground filters. The most intensive emulsification of formation fluids occurs in the working bodies of electric centrifugal pumps in production wells. The emulsions formed in these pumps have the greatest stability and interface. Aims and Objectives To prepare liquids for stratification in pre-discharge units, it is most expedient to install filters in separator wells before electric centrifugal pumps. The authors consider that the disadvantage of these installations is the insufficient quality of water treatment, since water is not filtered from solid suspended particles and oil globules. To solve this problem, it is proposed to install a volumetric filtration filter made from a package of brush disks and located on a tubing string in a downhole water discharge unit. Results The results of bench tests of the structure of the downhole volumetric filtration filter for the downhole separation technology are presented, the layouts of the main submersible equipment are shown. It has been substantiated that the use of this filter will allow: o to increase the productivity of wells due to the efficiency of gravitational stratification of water-oil liquid on the developed surface of the filter brush discs; o reduce the cost of the filter cleaning operation in comparison with known filters; o to increase the degree of purification of the product (water) from impurities; o prevent erosional destruction of the inner surface of the tubing string.

SENECA, QVAESTIONES NATVRALES 4B.4.2: AERIS OR TEMPORIS? REMARKS ON THE MEANING OF TEMPVS

AbstractIn Quaestiones naturales 4b.4.2 Seneca states that in early spring the weather drastically changes: in the warmer sky larger water droplets are formed and cause rain. The description of this ‘greater change’ (maior inclinatio) is linked in the manuscript tradition to two different controversial readings, temporis and aeris, which are irregularly distributed. Most recent editors have printed the first reading, but H.M. Hine is probably right to accept aeris. A careful linguistic, stemmatic and stylistic examination shows that temporis is likely to be a Medieval Latin gloss of aeris: the equivalence of both words would be difficult to justify in Classical Latin, but in Late Latin and in Medieval Latin tempus developed a climatological meaning which is explicitly found in medieval writers and glossaries and is also very widespread in Romance languages. The presence of this gloss in the hyparchetype Ψ, which is ultimately the source for most medieval copies, accounts for the irregular distribution of both readings in the manuscript tradition; this hypothesis is particularly consistent with Hine's suggestion that Ψ probably had interlinear or marginal readings. This historical investigation on the meaning of tempus is also relevant to the end of the same passage, where stylistic and linguistic evidence supports the reading tepore rather than tempore.

The mechanical effect of moisturization on airborne COVID-19 transmission and its potential use as control technique

Mounting evidence from scientific community seems to suggest that COVID-19 virus can potentially spread by airborne transmission. As a result, methods and techniques for preventing environmental contagious, such as ventilation or air filtration have been proposed. Here, it is investigated the effect of moisturization on airborne COVID-19 transmission from a mechanical point of view in which comparatively large water droplets promote the growth -by collision and coalescence, of suspended airborne COVID-19 and then accelerating its gravitational settling. Utilizing a classical raindrop collisional model from cloud science and the available experimental data an expression for the removal time of suspended airborne COVID-19 as function of the relative humidity was derived. The mechanical model is in good agreement with the recent reported experimental research in which high temperature and high relative humidity reduce COVID-19 contagious and then is a point in favor of the mechanic model of the effect of moisture in the COVID-19 airborne transmission. The results encourage further research on the deliberate moisturization of room air (by using ceiling mounted humidifiers) as a potential technique for control of airborne COVID-19 transmission.

Peculiarities in Leidenfrost water droplet evaporation

The investigations involved a large water droplet deposited on the heating surface, the temperature of which was higher than the Leidenfrost point. The main element of the experimental setup was the heating cylinder with K-type shielded thermocouple located in its centre just below the surface. The measuring system was located on highly sensitive scales. The analysis of the droplet behaviour in time was conducted based on measured droplet mass changes over time and also photographic data recorded with high resolution digital camera. The energy balance equation is given for the assumption that evaporation from the droplet upper surface is small compared with the amount of heat dissipated from the bottom surface. The formula for the heat transfer coefficient depends on two slope values and an orthogonal projection of the drop onto the heating surface. The slopes are estimated based on the droplet diameter linear time dependence and mass versus the contact zone relationship. The solution provides a good representation of droplet evaporation under Leidenfrost conditions. The investigations, reported in the study, which concern water droplet at atmospheric pressure deposited on a hot surface with the temperature higher than the Leidenfrost point, indicate the following regularities: droplet orthogonal projection onto the heating surface changes linearly with the droplet mass, evaporation of the same amount of mass decreases linearly with an increase in the heating surface temperature, slope of the graph showing mass loss versus the heating surface temperature successively decreases.