Droplet Evaporation Process Research Articles

Model for transient n-heptane droplet ignition at elevated pressure

The present work deals with transient droplet evaporation and ignition process of an isolated n-heptane droplet at high pressure. The vapor–liquid equilibrium has been described by Peng–Robinson equation of state. In present numerical model, the effects due to elevated pressure, inert species mixing in the liquid phase and thermo-physical properties are considered as variable. The evaporation model has been comprehensively validated with experimental results available in the literature. For the present investigation, the ambient pressure has been varied from $$5$$ to $$80\;{\text{bar}}$$ and at constant ambient temperature of $$1000\;{\text{K}}$$ . The range of diameter was varied from $$0.5$$ to $$2.0\;{\text{mm}}$$ . The ignition delay was found to be strong functions of temperature, pressure and diameter. There is monotonic decrease in the ignition time due to increasing ambient temperature and pressure. At each pressure, there existed a minimum diameter below which ignition does not takes place and this minimum diameter decreases with increase in pressure.

Droplet Deposition Pattern Affected by Different Heating Directions

The coffee ring effect commonly exists in droplet deposition patterns, which fundamentally affects scientific research and industrial applications, like pharmaceutical purification, salt manufacturing, etc. Some researchers have tried different solutions to control the distributions of droplet deposition patterns, but most control deposits by adjusting droplet characteristics. In this work, droplet deposition patterns with different wettability are investigated by both localized and substrate heating. A whole process of droplet evaporation is recorded. The droplet generally evaporates from constant contact radius (CCR) mode to constant contact angle (CCA) mode, and CCR stage occupies the most of time. Experimental results show that, without any chemicals, laser induced local heating transitions particle deposition patterns from ring-like structure to dot-like patterns on a hydrophilic surface, driving most saline solvent to the center. Meanwhile, a hydrophobic surface is also investigated showing that the particles tend to assemble at the central area, but the pattern is slightly different compared to that on hydrophilic surface. In addition, physical mechanisms of local heating and heating from substrate are also explored in the present study.

Investigation of Droplet Evaporation on Copper Substrate with Different Roughness

In the present study, we investigated the evaporation process and deposition pattern of saline droplet on a copper substrate with different roughness under 40 °C ambient temperature. These four substrates are classified as smooth surface and rough surface based on their droplet contact angles. It has been found in this study that the evaporation pattern of droplets has a strong relationship to substrate roughness. The thickness boundary of the evaporation pattern on a smooth surface is larger than that on a rough surface and the particles are closer to boundary and the tendency is more obvious on a smooth surface. The below factors contribute to the result. On the smooth surface, the contact angle of droplet increases as the roughness decreases. On the rough surface, the contact angle increases as the roughness increases. With contact angle decreasing, the evaporation rate at the boundary increases leading to the particles at the boundary more easily sedimentate. Moreover, the capillary flow is hindered by increasing the substrate roughness, while the Marangoni flow remains constant, resulting in more particles remain in the center of the droplet on the rough surface. To sum up, the coffee-ring formation is suppressed by increasing the substrate roughness on a copper substrate under 40 °C temperature.

Self-pinning of silica suspension droplets on hydrophobic surfaces

HypothesisSelf-pinning induced by the aggregation of particles at the edge of a pinned drop is a pre-requisite for the coffee ring formation. The edge (three-phase contact line) of a suspension drop on a hydrophobic surface would depin and shrink in the early stage of evaporation process. It is plausible to conjecture that the self-pinning of silica suspension drops depends on the particle size and surface property. ExperimentsTwo substrate materials, the alkylsilane coated surfaces and the polydimethylsiloxane surfaces, and three different sizes of silica particles are used to explore the criterion of self-pinning of silica suspension drops on these hydrophobic surfaces. The evaporation process of droplets is recorded and further analyzed. FindingsThe pinning concentration of silica suspensions of a fixed size linearly depends on the receding contact angle of the surface, irrelevant to the substrate material and initial particle concentration. The pinning concentration decreases along with an increase in particle size. In addition, the pinning concentrations of bi-dispersed silica (e.g. 400 + 1000 nm) suspensions have an excellent agreement with that of larger size (1000 nm) particle system. That implies that the larger particle dominates the system of bi-dispersed silica suspensions to initiate the self-pinning, further verified by SEM images.

Engineering Micropatterned Surfaces for Controlling the Evaporation Process of Sessile Droplets

Controlling the evaporation process of a droplet is of the utmost importance for a number of technologies. Also, along with the advances of microfabrication, micropatterned surfaces have emerged as an important technology platform to tune the wettability and other surface properties of various fundamental and applied applications. Among the geometrical parameters of these micropatterns, it is of great interest to investigate whether the arrangement of the patterns would affect the evaporation process of a sessile liquid droplet. To address this question, we fabricated four microhole arrays with different arrangements, quantified by the parameter of “eccentricity”. The results suggested that, compared to smooth substrates, the evaporation mode was not only affected by engineering the microhole arrays, but also by the eccentricity of these micropatterns. The values of contact angle hysteresis (CAH) were used to quantify and test this hypothesis. The CAH could partially explain the different evaporation modes observed on the microhole arrays with zero and non-zero values of eccentricity. That is, on microhole arrays with zero eccentricity, CAH of water droplets was comparatively low (less than 20 ° ). Consistently, during the evaporation, around 60% of the life span of the droplet was in the mixed evaporation mode. Increasing the eccentricity of the microhole arrays increases the values of CAH to above 20 ° . Unlike the increasing trend of CAH, the evaporation modes of sessile droplets on the microhole array with non-zero values of eccentricity were almost similar. Over 75% of the life span of droplets on these surfaces was in constant contact line (CCL) mode. Our findings play a significant role in any technology platform containing micropatterned surfaces, where controlling the evaporation mode is desirable.

A Phenomenon Study on Spreading and Evaporating Process of Droplet on DBD Actuator for Wind Turbine Anti-icing

This paper reports on the phenomenon of the unique spreading and evaporation process of water droplets on the surface of a dielectric barrier discharge (DBD) actuator. High-speed photography was used to capture the droplet spreading dynamics, and the temperature field evolution was characterized by infrared imaging. On the DBD actuator, the maximum spreading diameters of the droplets are shown to be increased by ∼95%, and the contact angles decrease from 58º to 13º, indicating that the DBD plasma significantly increases the hydrophilicity of the actuator surfaces. It is also revealed that the evaporation time induced by the DBD plasma is 6.54 times faster than that of an electric heater. It is argued that coupled effects of the hydrophilicity caused by plasma and the heat flux in the streamers bridging the electrodes and the droplets should be the crucial points of plasma evaporation physics of impinging water droplets, which are especially meaningful in anti-icing applications.

Ultrafast laser micro-nano structured superhydrophobic teflon surfaces for enhanced SERS detection via evaporation concentration

Abstract Evaporation concentration of target analytes dissolved in a water droplet based on superhydrophobic surfaces could be able to break the limits for sensitive trace substance detection techniques (e.g. SERS) and it is promising in the fields such as food safety, eco-pollution, and bioscience. In the present study, polytetrafluoroethylene (PTFE) surfaces were processed by femtosecond laser and the corresponding processing parameter combinations were optimised to obtain surfaces with excellent superhydrophobicity. The optimal parameter combination is: laser power: 6.4 W; scanning spacing: 40 μm; scanning number: 1; and scanning path: 90 degree. For trapping and localising droplets, a tiny square area in the middle of the surface remained unprocessed for each sample. The evaporation and concentration processes of droplets on the optimised surfaces were performed and analyzed, respectively. It is shown that the droplets with targeted solute can successfully collect all solute into the designed trapping areas during evaporation process on our laser fabricated superhydrophobic surface, resulting in detection domains with high solute concentration for SERS characterisation. It is shown that the detected peak intensity of rhodamine 6G with a concentration of 10−6 m in SERS characterisation can be obviously enhanced by one or two orders of magnitude on the laser fabricated surfaces compared with that of the unprocessed blank samples.

Charge Retention/Charge Depletion in ESI-MS: Experimental Evidence.

The effects of liquid and gas phase additives (chemical modifiers) on the ion signal distribution for Substance P (SP), recorded with a nanoelectrospray setup, are evaluated. Depletion of the higher charge state of Substance P ([SP+3H]3+) is observed with polar protic gas phase modifiers. This is attributed to their ability to form larger hydrogen-bonded clusters, whose proton affinity increases with cluster size. These clusters are able to deprotonate the higher charge state. "Supercharging agents" (SCAs) as well as aprotic polar gas phase modifiers, which promote the retention of the higher charge state of Substance P, do not form such large clusters under the given conditions and are therefore not able to deprotonate Substance P. Both SCAs and aprotic modifiers form clusters with the higher charge state, leading to stabilization of the charge. Whereas supercharging agents have low vapor pressures and are therefore enriched in late-stage electrospray droplets, the gas phase modifiers are volatile organic solvents. Collision induced dissociation experiments revealed that the addition of a modifier significantly delays the droplet evaporation and ion release process. This indicates that the droplet takes up the gas phase modifier to a certain extent (accommodation). Depending on the modifier's properties either charge depletion or retention may eventually be promoted.

Evaporation Modeling of Water Droplets in a Transonic Compressor Cascade under Fogging Conditions

High-fogging is widely used to rapidly increase the power outputs of stationary gas turbines. Therefore, water droplets are injected into the inflow air, and a considerable number enter the compressor. Within this paper, the primary process of droplet evaporation is investigated closely. A short discussion about the influential parameters ascribes a major significance to the slip velocity between ambient gas flow and droplets. Hence, experimental results from a transonic compressor cascade are shown to evaluate the conditions in real high-fogging applications. The measured parameter range is used for direct numerical simulations to extract evaporation rates depending on inflow conditions and relative humidity of the air flow. Finally, an applicable correlation for the Sherwood number in the form of S h ( R e 1 / 2 S c 1 / 3 ) is suggested.

A multiphase SPH framework for solving the evaporation and combustion process of droplets

PurposeThis paper aims to introduce a two-dimensional smoothed particle hydrodynamics (SPH) framework for simulating the evaporation and combustion process of fuel droplets.Design/methodology/approachTo solve the gas–liquid two-phase flow problem, a multiphase SPH method capable of handling high density-ratio problems is established. Based on the Fourier heat conduction equation and Fick’s law of diffusion, the SPH discrete equations are derived. To effectively characterize the phase transition problem, inspired by volume of fluid method, the concept of liquid phase mass fraction of the SPH particles is proposed. The one-step global reaction model of n-hexane is used for the vapor combustion.FindingsThe evaporation and combustion process of single droplet conforms to the law. The framework works out well when the evaporation of multiple droplets involves coalescence process. Three different kinds of flames are observed in succession in the combustion process of a single droplet at different inflow velocity, which agree well with the results of the experiment.Originality/valueTo the best of the authors’ knowledge, this is the first computational framework that has the capability to simulate evaporation and combustion with SPH method. Based on the particle nature of SPH method, the framework has natural advantages in interface tracking.

Device for experimental characterization of the 4D flow inside an evaporating sessile water droplet.

We describe an experimental system based on optical microscopy, permitting the analysis of the four dimensional structure of the flow inside evaporating sessile droplets by monitoring the motion of tracers in horizontal planes localized at different heights. Inter-plane particle identification is accomplished via 3D tracking algorithms. The multiple plane observation is achieved using a piezoelectric device to make the microscope objective oscillate vertically, while a high-speed camera captures images. The droplet evaporation process lasts several minutes and greatly accelerates as the fluid advances toward complete evaporation. In order to capture the dynamics of the whole process, two cameras with the same optical output but different temporal resolution are used sequentially. Using image processing algorithms, we obtain the full trajectories of multiple tracers, velocities of particles on the free surface of the droplets, and velocity fields. The information available may be used to understand the geometry of the sedimentation pattern.

Evaporation and drying kinetics of water-NaCl droplets via acoustic levitation.

The acoustic levitation method (ALM) is expected to be applied as a container-less processing technology in the material science, analytical chemistry, biomedical technology, and food science domains because this method can be used to levitate any sample in mid-air and prevent nucleation and contamination due to the container wall. However, this approach can lead to nonlinear behavior, such as acoustic streaming, which promotes the evaporation of a levitated droplet. This study aims to understand the evaporation and precipitation kinetics of an acoustically levitated multicomponent droplet. An experimental investigation of the evaporation process of a salt solution droplet was performed, and the experimental results were compared with those of the d2-law. The droplet was noted to evaporate in two stages owing to the precipitation of the salt. Because of the vapor pressure depression, the experimental data did not agree with the classical prediction obtained using the d2-law. However, the experimental results were in partial agreement with those of the d2-law when the vapor pressure depression was considered by using the concentration estimate at each time, as obtained from the experimental results. In addition, it was observed that the time when the salt completely precipitated could be estimated by using the extended theory. These findings provide physical and practical insights into the droplet evaporation mid-air for potential lab-in-a-drop applications.

Coupled effect of concentration, particle size and substrate morphology on the formation of coffee rings

Our study reports the coupled effects of particle size, concentration and the substrate roughness on the evaporation process of an aqueous sessile droplet containing silica nanoparticles. The patterns formed after complete drying of the sessile drops have also been studied. On smooth substrates the droplets evaporate in constant contact angle (CCA) and mixed modes (MM) with CCA mode as the predominant mode of evaporation. For substrates with nanoscale roughness all the three, namely CCA, constant contact radius (CCR) and MM modes exist with CCR mode as the predominant mode of evaporation. On nanorough substrates, droplets leave circular ring-shaped stains due to strong pinning along the droplet perimeter whereas irregular shape stains are formed on smooth substrates. We found that the rim width of the ring-like particulate deposits scales with the particle concentration following a power law behavior. Furthermore, we compared crack density and uniformity of cracks in the deposits formed on both substrates obtained by drying aqueous drops containing particles of different sizes and concentrations. For a given roughness, delamination is found to occur in the deposits formed by drying drops with higher particle concentration and smaller particle size. Finally, we summarize our study by presenting a phase diagram to classify the particulate deposits and the crack patterns as a function of particle concentration and substrate roughness.

A model for puffing/microexplosions in water/fuel emulsion droplets

A new analytical solution to the one-dimensional transient heat conduction equation in a composite spherically symmetric water-fuel emulsion droplet, suspended in a hot gas, is obtained. The Robin boundary condition at the surface of a droplet and conditions at the fuel-water interface are used. A water sub-droplet is assumed to be located at the centre of a fuel droplet, the radius of which was fixed at each time step; it could change at the next time step. The Abramzon and Sirignano model is applied for the approximation of the droplet evaporation process. This solution and the evaporation model are incorporated into a numerical code in which droplet heating/evaporation and the variable thermophysical properties are accounted for. The time instant at which the temperature at the fuel-water interface became equal to the boiling temperature of water is identified with the initiation of puffing, giving rise to microexplosion. This allowed us to compute the minimal microexplosion delay time. The new solution is applied to a typical case of puffing/microexplosion of water/diesel emulsion droplets in high temperature gas. It is shown that the new model allows us to understand better the underlying physics of the processes leading to puffing/microexplosion. The experimental observations of the microexplosion delay time for various initial droplet sizes are shown to be compatible with the predicted values of this time. It is shown that puffing/microexplosions are expected well before the droplet surface temperature reaches the boiling temperature of n-dodecane. The numerical code can be potentially implemented into Computational Fluid Dynamics codes, which can be applied to the modelling of other fuel and water/fuel blends.

Experimental study of evaporation and crystallization of brine droplets under different temperatures and humidity levels

The evaporation of brine droplets has critical impacts on the zero-liquid-discharge thermal desalination process. A good understanding of the evaporation of brine droplets provides guidelines for the design of thermal desalination systems, and offers insights for applications when solvent droplet evaporation processes are involved. In this study, brine droplets with different salt mass concentrations were placed in a chamber with controlled temperature and relative humidity. The evaporation and crystallization processes were then visualized through a high speed camera by employing the pendant droplet method. The results showed that the evaporation rate increases with the decrease of relative humidity, the increase of temperature, and the decrease of salt mass concentration. After the evaporation was finished, we can observe that the crystal grew along the filament during the evaporation and remained on it. A salt shell was formed at the outside, while the droplet still contained some amount of brine inside, when the evaporation rate was low. Consequently, the evaporation mechanism was changed once the salt shell was formed, the water molecules needed to overcome the pressure difference inside and outside the salt shell, and diffused through the shell for further evaporation. A multi-variable fitted quadratic regression model was developed with R2 = 0.974 to describe the relationship among the evaporation rate, mass concentration, relative humidity and temperature. Because the regression model was based on experimental data with temperature varying from 30 °C to 60 °C, a predicted result of brine droplet's evaporation rate with various mass concentrations under 75 °C and 0% RH showed a good agreement with the experiment data. Therefore the developed regression model can be extended for high temperature (<100 °C, no boiling) applications.

MORPHOLOGICAL FEATURES AND WETTABILITY OF POLYTETRAFLUOROETHYLENE SURFACE TEXTURED BY LASER ABLATION

In this paper, the morphological features of textures with non-uniform wettability created using femtosecond laser ablation of polytetrafluoroethylene substrates have been studied. Covering the surface of polytetrafluoroethylene with microcraters in accordance with a proper design a texture in the form of periodically located microcollets could be created. The period of the location of the columns is the same over the entire surface and is selected in the range from 15 to 100 microns. In the case when the period lies within 30–100 µm, the diameter of the bars is ~ 20 µm. If in the range of 15–20 µm, then this diameter decreases accordingly to ~ 10 µm. Depending on the pulse energy, the height of the pillars could be smoothly changed from 0 to 60 μm. However, to create a superhydrophobic concentrator, textures with the greatest depth were used so that the height of the columns does not limit the stability of the superhydrophobic state by the sagging mechanism. It was established that on the surface of each pillar during the process of laser ablation, a relief with a two-modal roughness in the form of short drop-shaped projections of the material covered with spherical globules is additionally formed. Thus, in one stage of laser micromachining, it is possible to create a surface with a three-modal roughness – microcolumns, drop-shaped projections and spherical globules. The process of droplet evaporation is represented by two main modes of constant contact angle and constant contact diameter, when the latter ceases to decrease and remains constant until the complete evaporation of the drop. As a result, a precipitate of the substance dissolved in a drop is formed on the substrate. It has been established that in the interval 0 &lt;τ &lt;0.9, evaporation occurs in the constant contact angle mode.

Evaporation of non-ideal binary solution drops

The study of the evaporation process of multicomponent liquid droplets was carried out in the work. In the simulation and experiments, an aqueous solution of ethanol of various concentrations, which is a binary non-ideal mixture with known physicochemical properties, was used as a mixture. The deviation of the solution properties from ideal was taken into account using the generalized dependence of the pressure of vapor components saturated above the solution on their concentration in the solution. Using an example of an aqueous ethanol solution, a deviation of the change in the diameter of the droplets from the “law d2” was shown, which is associated with a two-speed evaporation mode. During evaporation at a low concentration of ethanol, the temperature of the droplet was close to the temperature of adiabatic evaporation of water; at a high concentration of ethanol, it was close to the temperature of adiabatic evaporation of ethanol. Comparison of the results of the simulation with experiment showed satisfactory agreement.

Unravelling the Acoustic and Thermal Responses of Perfluorocarbon Liquid Droplets Stabilized with Cellulose Nanofibers.

The attractive colloidal and physicochemical properties of cellulose nanofibers (CNFs) at interfaces have recently been exploited in the facile production of a number of environmentally benign materials, e.g. foams, emulsions, and capsules. Herein, these unique properties are exploited in a new type of CNF-stabilized perfluoropentane droplets produced via a straightforward and simple mixing protocol. Droplets with a comparatively narrow size distribution (ca. 1-5 μm in diameter) were fabricated, and their potential in the acoustic droplet vaporization process was evaluated. For this, the particle-stabilized droplets were assessed in three independent experimental examinations, namely temperature, acoustic, and ultrasonic standing wave tests. During the acoustic droplet vaporization (ADV) process, droplets were converted to gas-filled microbubbles, offering enhanced visualization by ultrasound. The acoustic pressure threshold of about 0.62 MPa was identified for the cellulose-stabilized droplets. A phase transition temperature of about 22 °C was observed, at which a significant fraction of larger droplets (above ca. 3 μm in diameter) were converted into bubbles, whereas a large part of the population of smaller droplets were stable up to higher temperatures (temperatures up to 45 °C tested). Moreover, under ultrasound standing wave conditions, droplets were relocated to antinodes demonstrating the behavior associated with the negative contrast particles. The combined results make the CNF-stabilized droplets interesting in cell-droplet interaction experiments and ultrasound imaging.

Experimental investigation on the evaporation and micro-explosion mechanism of jatropha vegetable oil (JVO) droplets

The present study mainly focused on the evaporation and micro-explosion mechanism of jatropha vegetable oil (JVO) droplets. Six major components of JVO and their blends were investigated at ambient temperature of 873 K and 1073 K under normal gravity. The properties of JVO were analyzed by TG-DSC (thermogravimetric-differential scanning calorimetry) analysis and LC–MS (liquid chromatograph mass spectrometer). The results show that the evaporation process of JVO droplets are different from that of single component fatty acids and their blends at temperatures of 873 K and 1073 K. No micro-explosions occurred during the whole evaporation progress of single fatty acids and their blends at 873 K, while the intense and repeating micro-explosions occurred in JVO droplet evaporation process at 873 K and 1073 K. In addition, steam “Clouds” were observed during the JVO evaporation process of 873 K and 1073 K. Importantly, cracking reaction will occur in the JVO evaporation process, these gas products come from heavy component (triglyceride molecule) cracking and boiling of liquid products lead to repeating micro-explosions and totally different evaporation characteristics.

Role of Surfactant in Evaporation and Deposition of Bisolvent Biopolymer Droplets.

Inkjet printing of biopolymer droplets is gaining popularity because of its potential applications in regenerative medicine, particularly the fabrication of tissue-regenerative scaffolds. The quality of bioprinting, which affects cellular behaviors and the subsequent tissue formation, is determined by the solvent evaporation and deposition processes of biopolymer droplets, during which instantaneous local viscosity and surface tension changes occur because of the redistribution of the biopolymer inside the drop. Such dynamics is complex and not well understood. Most biopolymer inks also contain multiple solvents of distinct evaporation rates, further complicating the system dynamics. Using high-speed interferometry, we directly observe in real time the instantaneous drop shape of inkjet-printed picoliter gelatin drops containing glycerol and water. It is observed that, for bisolvent gelatin drops with surfactants, highly viscous gelatin and glycerol accumulated near the pinned contact line at an early stage suppress the evaporation-driven outward flow and create a stagnation zone near the contact line region. Lower surface tension at the contact line, because of its high local surfactant concentration, as compared to the drop apex induces a strong Marangoni recirculation, which in conjunction with a stagnation zone in the contact line region causes the instantaneous drop shape to transition from a spherical cap to a volcano shape during evaporation and resulting in a volcano-like deposition profile. In contrast, the suppressed evaporation outward flow together with a weak Marangoni flow leads to a domelike deposition for the case without surfactant. The role of surfactant in polymer drop deposition with water-only solvent is also investigated and compared against that of bisolvent drops. For the single-solvent case, the deposition profile is found to shift from a coffee-eye shape in the presence of surfactant to a uniform deposition without surfactant. The results reveal new insight into the complex role surfactant plays during polymer drop evaporation and deposition processes.