Drop Evaporation Rate Research Articles

Effect of the angle of wedge-shaped hydrophilic patterns on drop evaporation and contact line behaviour: A lattice Boltzmann study

Hypothesis:Angular hydrophilic patterns can manipulate the drop morphology for applications like inkjet-printing. It is hypothesized that such patterns can also affect the contact line behaviour and evaporation rate of drops in a dynamic way, further affecting the quality of depositions. Here, the effect of the angular region is quantified using a wedge-shaped pattern. Experiments:The pseudo-potential multicomponent lattice Boltzmann (LB) model is used for the diffusion-limited (isothermal) evaporation of pinned liquid drops from wedge-shaped hydrophilic patterns. Findings:The contact line unpins the earliest at the apex region of the smallest wedge angles due to a pronounced difference between the local and overall contact angles. The difference reduces linearly with increasing wedge angles, leading to their delayed unpinning. The evaporative flux along the contact line is initially independent of the contact angle but reduces sharply near the unpinning angle. The variation of flux across wedge angles depends more on unpinning timings, as maximum flux values did not differ significantly. The evaporation rate is dynamic, with the smaller wedge angles initially exhibiting a higher evaporation rate because of larger drop surface areas, while the larger wedge angles dominate later owing to the delayed unpinning and retaining larger drop surface areas.

Water Drop Evaporation on Slippery Liquid-Infused Porous Surfaces (SLIPS): Effect of Lubricant Thickness, Viscosity, Ridge Height, and Pattern Geometry.

Sessile drop evaporation and condensation on slippery liquid-infused porous surfaces (SLIPS) is crucial for many applications. However, its modeling is complex since the infused lubricant forms a wetting ridge around the drop close to the contact line, which partially blocks the free surface area and decreases the drop evaporation rate. Although a good model was available after 2015, the effects of initial lubricant heights (hoil)i above the pattern, and the corresponding initial ridge heights (hr)i, lubricant viscosity, and solid pattern type were not well studied. This work fills this gap where water drop evaporations from SLIPS, which are obtained by infusing silicone oils (20 and 350 cSt) onto hydrophobized Si wafer micropatterns having both cylindrical and square prism pillars, are investigated under constant relative humidity and temperature conditions. With the increase of (hoil)i, the corresponding (hr)i increased almost linearly on lower parts of the drops for all SLIPS samples, resulting in slower drop evaporation rates. A novel diffusion-limited evaporation equation from SLIPS is derived depending on the available free liquid-air interfacial area, ALV, which represents the unblocked part of the total drop surface. The calculation of the diffusion constant, D, of water vapor in air from (dALV/dt) values obtained by drop evaporation was successful up to a threshold value of (hoil)i = 8 μm within ±7%, and large deviations (13-27%) were obtained when (hoil)i > 8 μm, possibly due to the formation of thin silicone oil cloaking layers on drop surfaces, which partially blocked evaporation. The increase of infused silicone oil viscosity caused only a slight increase (12-17%) in drop lifetimes. The effects of the geometry and size of the pillars on the drop evaporation rates were minimal. These findings may help optimize the lubricant oil layer thickness and viscosity used for SLIPS to achieve low operational costs in the future.

Sessile volatile drop evaporation under microgravity

The evaporation of sessile drops of various volatile and non-volatile liquids, and their internal flow patterns with or without instabilities have been the subject of many investigations. The current experiment is a preparatory one for a space experiment planned to be installed in the European Drawer Rack 2 (EDR-2) of the International Space Station (ISS), to investigate drop evaporation in weightlessness. In this work, we concentrate on preliminary experimental results for the evaporation of hydrofluoroether (HFE-7100) sessile drops in a sounding rocket that has been performed in the frame of the MASER-14 Sounding Rocket Campaign, providing the science team with the opportunity to test the module and perform the experiment in microgravity for six consecutive minutes. The focus is on the evaporation rate, experimentally observed thermo-capillary instabilities, and the de-pinning process. The experimental results provide evidence for the relationship between thermo-capillary instabilities and the measured critical height of the sessile drop interface. There is also evidence of the effects of microgravity and Earth conditions on the sessile drop evaporation rate, and the shape of the sessile drop interface and its influence on the de-pinning process.

The dependence of drop evaporation rate and wettability on corrosion kinetics

The effect of corrosion of aluminum alloy on the droplets evaporation kinetics both for superhydrophilic and superhydrophobic surfaces is studied experimentally. The characteristic stages of the drop diameter changes due to corrosion correlate with the evaporation stages. Corrosion controls the drop wettability and the evaporation rate. At high rates of corrosion and with intense gas release, the evaporation rate is shown to decrease due to a reduction in the rate of heat supply to the droplet surface. The performed analysis of limiting factors illustrates that evaluating only electrochemical factors is not sufficient. Modeling the corrosion kinetics also requires taking into account the statistical nature of the corrosion process, which depends on the wettability and diameter of the drop. It has been for the first time demonstrated that the corrosion kinetics is determined by the modes of wetting. Expressions relating the corrosive current to the modes for the contact line of the drop are presented. Pre-temperature annealing of the material leads to an increase in the rate of corrosion and the appearance of large pits.

Superhydrophobic behavior of lauryl methacrylate copolymers on the cotton fabric surface

The authors propose to use glycidyl methacrylate and lauryl methacrylate copolymers to provide the cotton fabric surface with superhydrophobic properties. The modification was carried out by immersion of cotton fabric in solutions of copolymers in methylethylketone and following heat treatment at 140°C. Stability of the superhydrophobic state of polymer coatings is studied during long-term contact with droplets of water and aqueous acidic, alkaline and salt solutions under saturated vapor conditions. Under the conditions of high humidity and absence of contact with the external environment, a low drop evaporation rate is ensured, which allows to trace the long-term change of the contact angle of the sessile drop. The mechanism of stability of superhydrophobic properties is discussed. Polymer coatings that are based on the glycidyl methacrylate and lauryl methacrylate block copolymer provide better stability of superhydrophobic properties compared with the random copolymer. It is shown that modification of the cotton fabric surface by glycidyl methacrylate and lauryl methacrylate copolymers does not affect its vapor permeability.

Locking of water balance of SDW polygons by means of filtrate recycling

To prevent underflooding of SDW polygons with geomembranes in the foundation, a design procedure of admissible water ingress into the exploited polygon has been developed. The admissible annual settled sludges is within 12 – 50 mm. To remove surcharge water, it is offered to take away snow from the polygon in the winter time and to raise the filtrate evaporation during its recycling in the summer season. The mathematical model is developed and design dependences for drop evaporation rate are obtained. The sufficient filtrate evaporation degree is reached in its spraying with drop diameter of 0.12 mm from height of 5 m. Recycling allows reducing the filtrate contamination as well. It is denoted the necessity of the upper waterproofing of the polygon after its filling. The developed procedures will be beneficial in designing SDW polygons.

INFLUENCE OF DROP SPACING ON BURNING OF AN EMULSIFIED-DROP STRING

Combustion characteristics of water-in-dodecane emulsion drops with various initial spacings were stud ied experimentally by using a free-falling drop burning apparatus. The initial drop spacings ( Si) were 2.5, 5, 10, 40, 75 (70), 100. Si (s/d i) was defined as the ratio of the drop center-to-ce nter distance ( s) to the initial drop diameter (di). The water content (β) and the oxygen concentration (ΩO2 ) were fixed at 5% and 21%, while two drop sizes 550 µm and 450 µm were compared. The results showed that the transi tion of the drop flame occurred for all cases in the experiment. For Si > 10 along the flow direction, the flame around the drops would change from a blue spherical flame to a yellow flame and a wake f lame, and the drop flame extinguished later in the downstream region. Soot particles was generated and drops collision and merging occurred to form a flame tube for Si =2.5 in both cases of di = 550 µm and 450 µm. Besides, drop expansion was observed in both cases of di = 550 µm and di = 450 µm, while micro-explosion only occurred in the far d ownstream region for Si = 40, di = 450 µm. It was also shown that the emulsion drop evapora tion rate was not a constant, and the trend of the drop evaporation rate was strongly influenced by changin g the initial drop size.

Influence of the spray pyrolysis seeding and growth parameters on the structure and optical properties of ZnO nanorod arrays

ZnO nanorods (NRs) were grown on fluorine doped tin oxide (FTO) substrates at low temperatures (90 °C) from Zn2+ precursors in alkaline media previously seeded with ZnO nanoparticles. These were deposited onto the FTO substrate heated at 350 °C by spray pyrolysis of a Zn acetate solution in a water ethanol mixture. The structure of seeds was tuned by the ethanol to water ratio, Γ, which controls the solvent evaporation rate of drops impinging the substrate. From a detailed characterization using a combination of scanning electron microscopy, X-ray diffraction, UV–visible absorption and cathodoluminescence spectroscopies, the dependence of the morphology and optical properties of the ZnO NRs on the seeding conditions was demonstrated. NRs grown on seeds deposited from solutions with Γ in the 0.03–0.06 range – i.e. when the surface excess of ethanol in the water–ethanol mixture has a maximum – show thinner average diameters and stacking faults due to the presence of zinc blende domains embedded into an overall wurtzite NR. They furthermore exhibit blue-shifted near band edge emission peak and a high deep level emission in cathodoluminescence. All these findings support the use of spray pyrolysis as a simple and reproducible way to control the seeds deposition, influencing the growth, the structure and the optical properties of the final ZnO NRs.

Influence of pyrolytic seeds on ZnO nanorod growth onto rigid substrates for photocatalytic abatement of Escherichia coli in water

ZnO nanorods (ZnO NRs) were grown on ZnO seeded fluorine doped tin oxide (FTO) substrates at low temperatures (90 °C) from Zn2+ precursors in alkaline aqueous solution. The ZnO seeds were deposited on the FTO substrate heated at 350 °C by spray pyrolysis of a zinc acetate solution in a water ethanol mixture. The structure of seeds was tuned by the ethanol water ratio, Γ, which controls the solvent evaporation rate of drops impinging the substrate. The relationship between the microstructure and optical properties of the ZnO NR films and the photocatalytic antibacterial activity for Escherichia coli abatement, was determined through a detailed characterization of the material. The higher photocatalytic antibacterial activity was performed by ZnO NR films grown on seeds deposited from solutions with Γ in the 0.0–0.03 range. With these films, the population of viable E. coli dropped more than six orders, from 8 × 108 to 4 × 102 CFU. These results show the potential of these materials in water disinfection.

Streamwise interaction of burning drops

The spatial distribution of drops and their interactions are influential parameters in spray combustion. Most available researches on this subject were about lateral spacing or were performed in micro-gravity. Studies about upstream/downstream convective interaction of burning drops are scarce. In this study, drop strings of different spacing were investigated in a high-temperature oxidizing environment for their flame transition, flame width variation and drop evaporation rate. The flame transition showed that along the flow direction, the drop flame initially located ahead of the drop, became a spherical envelope flame, then moved behind the drop, and finally burned as a wake flame. It was found that a drop string with an initial drop spacing (Si) of 2.5 or 5 was surrounded by a bulk flame tube, exhibited local group burning and soot layer. In addition, for Si=2.5, spacing instability and collision merging of the burning drops occurred; the wake flame stretched away from the drop could attach to and stabilize on the rear drop. In the experiment, for all cases, most of drops in the string were not surrounded by the flame. For Si<30, the drop evaporation rate was lower than that of a single drop. For 30<Si<75, the drop evaporation rate was higher than that of a single drop. The interaction of drops diminished if Si was more than 75.

Diffusion-controlled evaporation of sodium dodecyl sulfate solution drops placed on a hydrophobic substrate

In this work, the effect of SDS anionic surfactant on the diffusion-controlled evaporation rate of aqueous solution drops placed on TEFLON–FEP substrate was investigated with 11 different SDS concentrations. Drop evaporation was monitored in a closed chamber having a constant RH of 54–57% by a video camera. The initial contact angle, θi decreased from 104±2° down to 68±1° due to the adsorption of SDS both at the water–air and the solid–water interfaces. The adsorption of SDS on the solid surface was found to be 76% of that of its adsorption at the water–air interface by applying Lucassen-Reynders approach. An equation was developed for the comparison of the evaporation rates of drops having different θi on the same substrate. It was found that the addition of SDS did not alter the drop evaporation rate considerably for the first 1200s for all the SDS concentrations. The main difference was found to be the change of the mode of drop evaporation by varying the SDS concentration. The constant θ mode was operative up to 80mM SDS concentration, whereas constant contact area mode was operative after 200mM SDS concentrations due to rapid drop pining on the substrate.

Experimental investigation of the effect of thermal properties of the substrate in the wetting and evaporation of sessile drops

An experimental investigation on the evaporation of sessile drops is reported in this paper. The effect of thermal properties of the substrate is investigated by studying the evaporation of sessile drops on substrates with various thermal conductivities. The results show that the evaporation rate of drops is limited by thermal properties of the substrate especially for high evaporation rates. Furthermore the temperature of the evaporating drop is measured to show that it is dependent on both the evaporation rate and the thermal conductivity of the substrate. Preliminary results on the evaporation of water droplets under a reduced pressure environment show clearly that as evaporation is increased the gap between substrates having different thermal conductivities is widened. The results of this investigation demonstrate the crucial role played by the nature of the substrate in the process of spontaneous evaporation of sessile drops.

Freely Receding Evaporating Droplets

The evaporation rate of drops of pure, completely wetting liquids is investigated experimentally and theoretically in the situation where no pinning of the contact line is occurring. It is shown that the observed dynamics is controlled by the volatility of the liquid, as expected, but also by the properties of the wetting film left on the substrate. Contact line instabilities, and nonmonotonic behavior of the receding contact angle may be observed and are accounted for. However, the water still needs further investigations.

Heat and mass transfer of evaporating tin chloride droplets into chemically active atmosphere

The results of studying of the tin chloride droplets evaporation rate in air containing different chemically active additions have been presented. The model of drop evaporation rate increasing has been suggested in frame of the diffusional theory, when the chemical reaction took place essentially in gas phase.

Isolation of crystallizing droplets by electrostatic levitation

The principles of electrostatic levitation where the positioning and stabilization of a sample are accomplished by applying appropriate electrostatic forces to a charged sample are outlined, and attention is focused on a feedback control algorithm, drop-launching method, and four-drop levitator. Drop levitation in 1-g is discussed, and crystal-growth experiments are presented. An experiment in which the protein concentration of a levitated drop is controlled by a feedback system is described. During levitation, the drop evaporation rate is controlled in a programmed way in order to acquire proper protein concentration levels for both nucleation and growth. The containerless approach of protein crystal growth when applied in the space environment is assessed.

The Effect of Drop Evaporation Rate on Containment Pressure Transients

The effect of a mechanistic drop evaporation model on the pressure-temperature transient of a containment under loss-of-coolant accident (LOCA) conditions has been investigated. To implement the model, the traditional two-node lumped parameter (atmosphere and sump) had to be expanded to encompass additional open thermodynamic systems. The calculations were compared against results obtained by a widely employed containment analysis code using the instantaneous evaporation model. The mechanistic drop evaporation model was found to produce higher peak pressures and substantially higher degrees of superheat for a steam line break LOCA.The dependence of pressure in both saturated and superheated air-steam atmospheres was generalized in terms of normalized pressure-energy derivatives. For superheated atmospheres, these derivatives were found to depend on the mode of energy removal. Two idealized energy removal modes were defined (purely condensing and purely noncondensing). The normalized pressure-energy deriva...

Evaporation rates of drops in forced convection with superposed transverse sound field

Evaporation rates for single drops of distilled water suspended in dry air in upwards motion and subject to a horizontal standing wave sound field have been measured from photographic records. The range of drop diameter (0.8–2mm), sound pressure (132–152dB) and frequency (82–734 Hz) yielded parameter ranges of oscillating Reynolds number 16 < Re < 180 and Strouhal number 0.3 < S < 2.8 at convective Reynolds numbers in the range 5 < Re c < 45. Empirical correlations are presented for S < 1 and S > 1, respectively. The data for these cases show the increase in Nusselt number owing to the sound field to be 5–90% and 8–30%, respectively.

A Wind Tunnel Investigation of the Rate of Evaporation of Small Water Drops Falling at Terminal Velocity in Air

An experimental study of the effect of ventilation on the rate of evaporation of small water drops falling at terminal velocity in air has been carried out in a wind tunnel where water drops could he suspended freely in the tunnel airstream. For Reynolds numbers NRe≳2 it was found that the Sherwood number NSh was linearly related to NRe1/2NI, in a manner which could best be expressed by the equation 1/2 Nsh=0.78 + 0.308 Nsc1/2NRe1/2, where NSc is the Schmidt number. For NRe≲ the Sherwood number was found to smoothly approach 1/2 NSh=1 at NRe=0, with d(1/2NSh/d(NSc1/2)=0 according to the relationship 1/2 NSh=1.00 + 0.108 (NSc1/2NRe1/2)2. The experimental results are discussed in the light of previous experimental and theoretical work reported in the literature. Excellent agreement was found between our experimental results and the numerical results of Woo and Hamielec presented in an accompanying article. Two tables are given that easily allow computing the evaporation rates for cloud drops and small rain drops of sizes between 20 and 600μ radius for a wide range of atmospheric temperatures from 0–40C and relative humidities from 0–100%. As an example, the evaporation rate of drops failing in a NACA standard atmosphere is calculated and compared to similar computations reported in the literature.

On the Evaporation of Raindrops in the Presence of Vertical Gradients of Temperature and Relative Humidity

Abstract By use of the time-dependent diffusion equation it is shown that a heat-balance equation for an evaporating raindrop based on the “quasi-stationary” assumption (∂//t = 0) is accurate. This equation is then used to obtain the expression T = θw + zRdθw/dz for the steady-state temperature T of an evaporating drop in an atmosphere having linear vertical gradients of temperature and relative humidity, where θw is the equilibrium drop temperature in a uniform atmosphere (approximately the environmental wet-bulb temperature), dθw/dz is its vertical gradient, and zR is the thermal relaxation distance of the drop (falling at terminal velocity). The term zRdθw/dz is the thermal lag in the drop's response to the varying environment. Computations show that the ratio of the drop evaporation rate with the tag term to the rate without the tag departs significantly from unity for large drops and high humilities.