Review on modelling of corrosion under droplet electrolyte for predicting atmospheric corrosion rate

Atmospheric corrosion of metals is the most common type of corrosion which has a significant impact on the environment and operational safety in various situations of everyday life.Some of the common examples can be observed in land,water and air transportation systems,electronic circuit boards,urban and offshore infrastructures.The dew drops formed on metal surface due to condensation of atmospheric moisture facilitates corrosion as an electrolyte.The corrosion mechanisms under these droplets are different from classically known bulk electrolyte corrosion.Due to thin and non-uniform geometric thickness of the droplet electrolyte,the atmospheric oxygen requires a shorter diffusion path to reach the metal surface.The corrosion under a droplet is driven by the depletion of oxygen in the center of the droplet compared to the edge,known as differential aeration.In case of a larger droplet,differential aeration leads to preferential cathodic activity at the edge and is controlled by the droplet geometry.Whereas,for a smaller droplet,the oxygen concentration remains uniform and hence cathodic activity is not controlled by droplet geometry.The geometry of condensed droplets varies dynamically with changing environmental parameters,influencing corrosion mechanisms as the droplets evolve in size.In this review,various modelling approaches used to simulate the corrosion under droplet electrolytes are presented.In the efforts of developing a comprehensive model to estimate corrosion rates,it has been noted from this review that the influence of geometric evolution of the droplet due to condensation/evaporation processes on corrosion mechanisms are yet to be modelled.Dynamically varying external factors like environmental temperature,relative humidity,presence of hygroscopic salts and pollutants influence the evolution of droplet electrolyte,making it a complex phenomenon to investigate.Therefore,an overview of available dropwise condensation and evaporation models which describes the formation and the evolution of droplet geometry are also presented from an atmo s pheric corrosion viewpoint.     

Experimental Investigation on Flash Evaporation of Saltwater Droplets Released into Vacuum

In this paper, the flash evaporation process of saltwater droplets released into vacuum is experimentally investigated. During the experiment, a saltwater (NaCl) droplet was suspended on a thermocouple junction, which was used to measure the temperature evolution. The droplet surface temperature was captured by an infrared thermal imager, and the shape variation was recorded by a high speed camera. According to the experimental results, the component and solution concentration has great influence on the evaporation process. With a rise of salt concentration in water, the evaporation rate decreases. The shape of temperature transition curve also depends on the salt concentration in solution, no matter whether it is higher or lower than the eutectic point (22.4%). The effects of environmental pressure, initial droplet temperature and initial droplet diameter on the temperature transition of droplets were also summarized based on the experimental data.     

Demonstration of droplet size and vaporization rate measurements in the near field of a two-phase jet with droplet lasing spectroscopy

Droplet lasing spectroscopy has been applied to the measurement of droplet size and evaporation rate in a spray. A single droplet, doped with laser dye, was injected along the centerline of a liquid spray. Filters were used to block the strong elastic-scattering signal. The lasing emission from the doped droplet could be detected against the background with mass loadings of liquid in the spray as high as 20%. An analysis of the spectrum of droplet lasing was used to evaluate the droplet diameter. The evaporation rate of the droplet was obtained from consecutive lasing spectra that were obtained from the same droplet. An error analysis of the drop size and drop evaporation measurements was carried out and showed that accurate measurements of evaporation rates were feasible.     

Experimental and Theoretical Investigation on Rapid Evaporation of Ethanol Droplets and Kerosene Droplets During Depressurization

This paper reports an experimental and theoretical study of rapid evaporation of ethanol droplets and kerosene droplets during depressurization. For experimental method, an ethanol droplet or a kerosene droplet was suspended on a thermocouple, which was also used to measure the droplet center temperature transition. And the droplet shape variation was recorded by a high speed camera. A theoretical analysis was developed based on the heat balance to estimate the droplet center temperature transition, and the evaporation model proposed by Abramzon and Sirignano was used to describe the droplet vaporization. According to the experimental data and theoretical analysis, both of the environmental pressure and the initial droplet diameter have a prominent influence on the droplet temperature transition. Comparing the evaporation processes of ethanol droplets and kerosene droplets with water droplets, the ethanol droplets have the fastest evaporation rate, followed by water, and the evaporation rates of kerosene droplets are the slowest. Also it was found that a bubble can easily emerge within kerosene droplet, and its lifetime is more than 1 s.     

Analysis of time-dependent scattering spectra for studying processes associated with microdroplets

Techniques are presented for analysis of time-dependent scattering spectra from single droplets undergoing physical changes. Times of appearance of resonances in experimental spectra are aligned with theoretical resonances, and the size and refractive index of a droplet as functions of time are determined from the minimum errors in alignment between observed and theoretical resonances. The techniques have been applied to time-dependent elastic scattering spectra obtained from single droplets evaporating under quasi-steady conditions and during unsteady growth. The results of quasi-steady evaporation data show that size and refractive index can be determined with relative errors of 1 x 10(-4). The quasi-steady evaporation data of a droplet are used to identify the resonances observed during the unsteady growth of the same droplet, and the size and refractive index at each resonance are calculated from the identity of the resonance.     

The Image Processing of Droplet for Evaporation Experiment in SJ-10

We have completed an experiment for droplet evaporation processing using Young-Laplace fitting, exponent fitting, polynomial fitting and ellipse fitting, which could be used for multiple shapes of droplets. The droplet evaporation experiment test was an important science experiment in SJ-10. In order to get the change process of the physical parameter, such as the touching edges and the droplet evaporation rate, we had gained the contour edge image of the droplet and used mathematic method to do the fitting analysis. The accuracy of the physical parameter was depended on the accuracy of the mathematic fitting. Using the original Young-Laplace fitting method could not process all the images of evaporation and liquid interface from the space experiment facility of SJ-10, especially the smaller droplet images. We could get more accurate contour fitting and result using the new method described in this article. This article proposes a complete solution, including edge detecting and contour fitting. In edge detecting, Canny detector was applied to extract droplet edge. In contour fitting, Young-Laplace fitting, exponent fitting, polynomial fitting and ellipse fitting are designed to fit the contour of droplets, which make the solution apply to all of droplets in SJ-10.     

The inclusion of the effect of the evaporation coefficient on the rate of variation of droplet radius

The paper deals with the investigation of the effect of the evaporation (condensation) coefficient of droplet substance on the rate of unsteady variation of the radius of a spherical aerosol droplet in the cases of direct and indirect inclusion of this coefficient. It is found in both cases that the effect of evaporation coefficient is most significant at the initial instant of unsteady-state process of evaporation and of condensation growth of the droplet. At this instant, the size of spherical droplet has hardly any impact on the rate of variation of its radius. As the unsteady-state process continues, the effect of the evaporation coefficient on the rate of variation of the droplet radius depends significantly on the droplet size. The larger the droplet under consideration, the lower the effect of the evaporation coefficient on the rate of variation of its radius. The rates of variation, calculated for the same values of the evaporation coefficient but for different ways of inclusion of this coefficient, differ less for larger aerosol droplets. These methods of inclusion of the evaporation coefficient are considered for the process of slow evaporation of a droplet.     

Numerical simulation of gas-droplets mixing and spray evaporation in rotary spray desulfurization tower

Motion and evaporation of droplets significantly affect the semidry flue gas desulfurization efficiency and long-term operation. Both the flow field distribution and the heat and mass transfer in the spray towers are studied by numerical simulation, and the process of droplet motion and evaporation is analyzed in detail. Then, two indices, mixing variance and droplet mass-weighted life, are provided to quantify gas droplet mixing and the droplet group evaporation time. The simulation results show that the radial penetration distance of the droplets is longer with the diameter increase, and the appropriate swirl number improves the mixing between the flue gas and droplets. With the increase of droplet diameter and velocity, the droplet distribution in the tower is more widely, obtaining the optimum mixing variance. The droplet mass-weighted life is promoted linearly with the increase of average droplet diameter and the decrease of flue gas temperature. With flue gas temperature increase from 458 k to 488 K, the droplet mass-weighted life decreases linearly by 31%. In comparison, the initial droplet velocity and spray angle have a slight effect on the droplet mass-weighted life.     

Evaporation and Settling Behavior of Metalworking Fluid Aerosols

Environmental and industrial hygiene issues have been receiving ever increasing attention. One important issue is metalworking fluid (MWF) mist formation since it can have a negative effect on workplace air quality. A model is presented to characterize the evaporation and settling behavior of a diluted water-soluble MWF mist under ambient conditions. The model is based on the Langmuir-Knudsen law and Stoke' law. The Langmuir-Knudsen law is employed to describe the evaporation process of each MWF droplet and allows for nonequilibrium effects associated with small droplet size. Stoke' law is applied to characterize the droplet settling mechanism at small Reynolds numbers. Experiments are performed to validate the model. Experiments show that the evaporation rate decreases to zero as the water in a droplet completely evaporates to produce a small nonvolatile oil drop. It is found that the evaporation and settling of a collection of MWF droplets can be predicted by the proposed model.     

Droplet Image Feedback Control System in Evaporation Experiment

In order to realize the steady-state droplet evaporation, image feedback control system is designed based on DSP. The system has three main functions: to capture and store droplet images during the experiment; to calculate droplet geometrical and physical parameters such as volume, surface area, surface tension and evaporation velocity at a high-precision level; to keep the droplet volume constant. The DSP can drive an injection controller with the PID control to inject liquid so as to keep the droplet volume constant. The evaporation velocity of droplet can be calculated by measuring the injected volume during the evaporation. The structure of hardware and software of the control system, key processing methods such as contour fitting and experimental results are described.     

Theoretical Investigation on Rapid Evaporation of a Saline Droplet During Depressurization

This paper reports a theoretical investigation on rapid evaporation of a saline droplet during depressurization. A mathematical model was developed to simulate the droplet temperature variation by considering the ambient pressure change, the heat transfers due to evaporation and convection at the droplet surface, accompanying the heat and mass transfer inside the droplet. The component diffusion and the temperature gradient inside the droplet were mainly discussed by comparing the numerical droplet temperature with the experimental data. The result shows that, the variation of internal concentration is small, while the temperature gradient within the droplet is significant during the evaporation process. In addition, the influencing factors of the droplet temperature variation were analyzed, such as: the final ambient pressure, theinitial salt concentration and the initial droplet temperature. The present model calculations help to understand the thermodynamic process of rapid evaporation of a saline droplet during depressurization.     

Mixing of vaporizing droplets on liquid substrates caused by festoon instability

A new configuration for a microfluid mixer, in which a mixed volume in the form of a volatile liquid droplet rests on a layer of water, is proposed. The mixing occurs owing to the injection of festoons emerging in a droplet under the influence of evaporation themocapillary convection from the periphery of the droplet to its central part. A linear relationship between the injection rate and the injection layer temperature is established, and it is shown that a droplet has time to repeatedly mix during its lifetime.     

Mosaic Patterned Surfaces toward Generating Hardly-Volatile Capsular Droplet Arrays for High-Precision Droplet-Based Storage and Detection (Small 14/2023)

Precise detection involving droplets based on functional surfaces is promising for the parallelization and miniaturization of platforms and is significant in epidemic investigation, analyte recognition, environmental simulation, combinatorial chemistry, etc. However, a challenging and considerable task is obtaining mutually independent droplet arrays without cross-contamination and simultaneously avoiding droplet evaporation-caused quick reagent loss, inaccuracy, and failure. Herein, a strategy to generate mutually independent and hardly-volatile capsular droplet arrays using innovative mosaic patterned surfaces is developed. The evaporation suppression of the capsular droplet arrays is 1712 times higher than the naked droplet. The high evaporation suppression of the capsular droplet arrays on the surfaces is attributed to synergistic blocking of the upper oil and bottom mosaic gasproof layer. The scale-up of the capsular droplet arrays, the flexibility in shape, size, component (including aqueous, colloidal, acid, and alkali solutions), liquid volume, and the high-precision hazardous substance testing proves the concept's high compatibility and practicability. The mutually independent capsular droplet arrays with amazingly high evaporation suppression are essential for the new generation of high-performance open-surface microfluidic chips used in COVID-19 diagnosis and investigation, primary screening, in vitro enzyme reactions, environmental monitoring, nanomaterial synthesis, etc.     

Influence of the liquid layer thickness on the growth of droplets controlled by the thermal action of laser radiation

We have studied the growth of droplets in a thin layer of aqueous ethanol solution under conditions of the solutocapillary convective flow controlled by a laser beam. It is established that, with increasing layer thickness, the time necessary for the formation of a droplet separated from the layer increases, while the time required for the droplet to reach its maximum possible diameter (determined by the criteria of stability) decreases. In the initial stage of droplet formation, the diameter exhibits some decrease related to a short-term increase in the solvent evaporation rate.     

Evaporation of a picoliter droplet on a wetted substrate at reduced pressure

The results of theoretical and experimental investigation of the evaporation of picoliter water droplets on a substrate at reduced pressure (20–80 Torr) have been given. The substrate temperature varied in the range 25–40°C. The calculations have been carried out in a free-molecular approximation. It has been shown that the evaporation time sharply decreases if the average droplet height is less than 10 μm and is a few milliseconds for a 5-μm-high droplet. It has been experimentally and theoretically shown that for droplets higher than 10 μm, the evaporation time is a few seconds in the investigated pressure range.     

Modelling aerosol processes related to the atmospheric dispersion of sarin

We have developed mathematical models for evaluating the atmospheric dispersion of selected chemical warfare agents (CWA), including the evaporation and settling of contaminant liquid droplets. The models and numerical results presented may be utilised for designing protection and control measures against the conceivable use of CWA's. The model AERCLOUD (AERosol CLOUD) was extended to treat two nerve agents, sarin and VX, and the mustard agent. This model evaluates the thermodynamical evolution of a five-component aerosol mixture, consisting of two-component droplets together with the surrounding three-component gas. We have performed numerical computations with this model on the evaporation and settling of airborne sarin droplets in characteristic dispersal and atmospheric conditions. In particular, we have evaluated the maximum radii (r(M)) of a totally evaporating droplet, in terms of the ambient temperature and contaminant vapour concentration. The radii r(M) range from approximately 15-80 microm for sarin droplets for the selected ambient conditions and initial heights. We have also evaluated deposition fractions in terms of the initial droplet size.     

First integrals of equations of motion of a droplet in the presence of mass transfer to the carrier medium

Analysis is performed of first integrals in the limiting Stokes and Newtonian modes of motion of a droplet for two cases, namely, that of the droplet moving in its own vapor in the presence of phase transitions and that of the droplet moving in dusty gas without phase transformations but in view of the capture of dust particles. Expressions are found for the lengths over which the processes of condensation, complete evaporation of droplet, and deposition of particles occur. Comparison is made of the lengths and times of complete evaporation of droplet in the Stokes and Newtonian modes of droplet motion. Asymptotic formulas are obtained for these quantities, which correspond to low and high superheating of vapor, as well as formulas for the limiting diameters of droplet in the cases of condensation of vapor or capture of particles.     

Determination of Diffusion Coefficient in Droplet Evaporation Experiment Using Response Surface Method

Evaporation of a liquid droplet resting on a heated substrate is a complex free-surface advection-diffusion problem, in which the main driving force of the evaporation is the vapor concentration gradient across the droplet surface. Given the uncertainty associated with the diffusion coefficient of the vapor in the atmosphere during space evaporation experiments due to the environmental conditions, a simple and accurate determination of its value is of paramount importance for a better understanding of the evaporation process. Here we present a novel approach combining numerical simulations and experimental results to address this issue. Specifically, we construct a continuous function of output using a Kriging-based response surface method, which allows to use the numerical results as a black-box with a limited number of inputs and outputs. Relevant values of the diffusion coefficient can then be determined by solving an inverse problem which is based on accessible experimental data and the proposed response surface. In addition, on the basis of our numerical simulation results, we revisit a widely used formula for the prediction of the evaporation rate in the literature and propose a refined expression for the droplets evaporating on a heated substrate.     

Capillary Origami Inspired Fabrication of Complex 3D Hydrogel Constructs

Hydrogels have found broad applications in various engineering and biomedical fields, where the shape and size of hydrogels can profoundly influence their functions. Although numerous methods have been developed to tailor 3D hydrogel structures, it is still challenging to fabricate complex 3D hydrogel constructs. Inspired by the capillary origami phenomenon where surface tension of a droplet on an elastic membrane can induce spontaneous folding of the membrane into 3D structures along with droplet evaporation, a facile strategy is established for the fabrication of complex 3D hydrogel constructs with programmable shapes and sizes by crosslinking hydrogels during the folding process. A mathematical model is further proposed to predict the temporal structure evolution of the folded 3D hydrogel constructs. Using this model, precise control is achieved over the 3D shapes (e.g., pyramid, pentahedron, and cube) and sizes (ranging from hundreds of micrometers to millimeters) through tuning membrane shape, dimensionless parameter of the process (elastocapillary number C_e), and evaporation time. This work would be favorable to multiple areas, such as flexible electronics, tissue regeneration, and drug delivery.     

Explosive boiling of water droplets irradiated with intense CO(2)-laser radiation: numerical experiments

The results of numerical calculations of water-droplet explosions initiated by intense CO(2)-laser radiation are presented. The theoretical model for this process is based on the solution of the value of the thermal-boundary problem in an inhomogeneously heated droplet, including the kinetic equation describingvapor generation in a superheated liquid. The main characteristics of droplet explosions (e.g., degree of explosive evaporation and time of explosion) are calculated. It is established that these characteristics depend on the heating rate of the droplet and on its radius. The results point to the fact that two droplet-heating regimes can be distinguished-slow heating and rapid heating-based on the behavior of the explosive boiling process. This division represents the competition of real physical processes in an irradiated droplet and makes it possible to separate the basic, specific features of the explosion process.