Evaporation Modes Research Articles

Precipitation dynamics of surrogate respiratory sessile droplets leading to possible fomites

HypothesisThe droplets ejected from an infected host during expiratory events can get deposited as fomites on everyday use surfaces. Recognizing that these fomites can be a secondary route for disease transmission, exploring the deposition pattern of such sessile respiratory droplets on daily-use substrates thus becomes crucial. ExperimentsThe used surrogate respiratory fluid is composed of a water-based salt-protein solution, and its precipitation dynamics is studied on four different substrates (glass, ceramic, steel, and PET). For tracking the final deposition of viruses in these droplets, 100 nm virus emulating particles (VEP) are used and their distribution in dried-out patterns is identified using fluorescence and SEM imaging techniques. FindingsThe final precipitation pattern and VEP deposition strongly depend on the interfacial transport processes, edge evaporation, and crystallization dynamics. A constant contact radius mode of evaporation with a mixture of capillary and Marangoni flows results in spatio-temporally varying edge deposits. Dendritic and cruciform-shaped crystals are majorly seen in all substrates except on steel, where regular cubical crystals are formed. The VEP deposition is higher near the three-phase contact line and crystal surfaces. The results showed the role of interfacial processes in determining the initiation of fomite-type infection pathways in the context of COVID-19.

Effect of heat treatment on corrosion of laser-textured aluminum alloy surfaces

The kinetics of electrochemical corrosion of aluminum alloy (AlMg6) surfaces with different wettability was analyzed. The surfaces were processed by three different methods, in particular, polishing, laser texturing, the combination of laser texturing and low-temperature heating. After laser processing, the dimple-like texture was formed on the surface, and the wettability significantly enhanced. Low-temperature heating of laser-textured AlMg6 alloy surfaces led to the wettability inversion from strongly hydrophilicity to superhydrophobicity. Microscopic and profilometric methods were used to estimate the surface degradation due to corrosion when aggressive solution droplets (a mixture of NaCl and hydrogen peroxide aqueous solutions) evaporated. The potentiodynamic polarization measurements of AlMg6 alloy surfaces were conducted. The typical modes of corrosion and evaporation of aggressive solution droplets were detected. The kinetics of corrosion was estimated by the behavior of the corrosion area evolution. In addition, when immersing laser-textured sample with strongly hydrophilic properties into aggressive solution, the higher corrosion rate was found in the liquid meniscus region (aggressive mixture / alloy / air interface) compared to the textured site immersed in the mixture. This was explained by convection increasing the rate of reaction products removal and promoting a stronger deviation from the equilibrium state in the aggressive mixture. Experimental results of the potentiodynamic polarization measurements revealed that laser-textured samples exhibit enhanced corrosion protective properties compared to polished samples.

Experimental investigation of a capillary-assisted high performance evaporator

A particular architecture for a miniaturized evaporator is proposed in which liquid is spread on the heat source using a thin capillary structure connected to two manifolds. The vapor produced by the heat source is extracted from the device using a separate outlet. A prototype has been designed, manufactured and integrated in a mechanically pumped loop in order to evaluate the evaporator efficiency in terms of heat transfer coefficient and induced pressure drop. In this paper, a capillary structure consisting in parallel rectangular micro-grooves is investigated with HFE7000 used as the working fluid. Results show that the evaporator is able to extract heat flux of the order of 10 W.cm-2 with heat transfer coefficients close to 104 W.m-2.K-1 while inducing pressure losses below a few hundred Pascal. Direct visualizations of the flow in the capillary structure allowed a precise characterization of the evaporator functioning mode, in particular the existence of two distinct two-phase flow regimes depending on the flooding of the capillary structure: nucleate boiling and liquid films evaporation. In order to predict the occurrence of these operating modes, a hydraulic model of the evaporator assembly has been developed and validated thanks to dedicated experimental campaigns. In particular, this model is able to predict the separation between the nucleate boiling and evaporation mode and highlights the fact that the capillary limit is not the main restricting factor in terms of the device maximum manageable heat flux.

Evaporation Kinetics of Sessile Water Droplets on a Smooth Stainless Steel Surface at Elevated Temperatures and Pressures.

The evaporation of water droplets on a solid surface at elevated temperatures under a pressurized condition has not yet been well understood, although this phenomenon is of both theoretical and practical significance. In this work, water droplet evaporation on smooth stainless steel surfaces in nitrogen gas atmosphere at elevated pressures and temperatures (up to 2 MPa and 202.4 °C, respectively) was investigated experimentally. It was observed that the increase in pressure diminishes the proportion of the constant contact radius stage over the entire evaporation time, whereas an opposite trend was found when raising the temperature, due to the change in the surface pinning ability with pressure (and temperature). The results also suggested that the evaporation mode transition is mainly affected by temperature rather than pressure. In addition, the evaporation rate was calculated under various degrees of subcooling, revealing that the evaporation rate increases almost linearly with pressure when keeping the degree of subcooling constant. However, when fixing the test temperature, a nonlinear decrease of the evaporation rate with pressure was observed. A power law growth of the evaporation rate with temperature was also found at a constant pressure. Last, it was uncovered by a theoretical analysis that the saturated vapor concentration is the dominant factor dictating the evaporation rate.

Retarding frosting of an air source heat pump by using vapor-bypassed evaporation technique

• Vapor-bypassed evaporation is firstly used to retard frosting for ASHP applications. • A tested ASHP system that contains two operation modes is established. • Frosting characteristics of the two operation modes are investigated in detail. • Vapor-bypassed evaporation is more suitable for high heating capacity conditions. This paper presents a comparative experimental study on the frosting characteristics of an air source heat pump system that contains two operation modes, i.e., the vapor-bypassed evaporation (VB) mode and the conventional operation (CO) mode. For VB mode, a phase separator is installed inside the evaporator refrigerant circuit to bypass vapor refrigerant, which can increase the average evaporation temperature for retarding frosting by reducing refrigerant pressure drop and decreasing the area of refrigerant inferior heat transfer coefficient region (mist flow region). According to the experimental results, under H2 condition (indoor: 20/15 °C; outdoor: 2/1 °C), as the initial refrigerant mass flow rate ranges 35–45 g / s , the VB mode yields 4.30–7.06% lower frost mass, 3.9–7.3% higher average heating capacity and 6.4–10.8% higher coefficient of performance (COP) over CO mode with the frosting duration of 60 min. Moreover, under H3 condition (indoor: 20/15 °C; outdoor: -7/-8 °C), the frosting duration is set as 100 min, and the VB mode obtains 9.67% lower frost mass, 7.0% higher average heating capacity and 13.3% higher COP over CO mode. Generally, the vapor-bypassed evaporation technique has a great application potential in retarding frosting and improving performance of an air source heat pump system especially under high heating capacity or low ambient temperature conditions.

Direct writing of colloidal suspensions onto inclined surfaces: Optimizing dispense volume for homogeneous structures

HypothesisA process to fabricate structures on inclined substrates has the potential to yield novel applications for colloidal-based structures. However, for conventional techniques, besides the coffee ring effect (CRE), anisotropic particle deposition along the inclination direction (IE) is expected to occur. We hypothesize that both effects can be inhibited by reducing the dispense volume during printing by direct writing. ExperimentsWe combined an additive manufacturing technique, namely direct writing, with colloidal assembly (AMCA) for an automated and localized drop-cast of polystyrene and silica suspensions onto inclined surfaces. Herein, we investigated the influence of the substrate tilting angle and the dispense volume on the printing of colloids and the resulting structures’ morphology. FindingsThe results demonstrate that a reduction in the dispense volume hinders the CRE and IE for both particles’ systems, even though the evaporation mode is different. For polystyrene, the droplets evaporated solely in stick-mode, enabling a “surface capturing effect”, while for silica, droplets evaporated in mixed stick–slip mode and a “confinement effect” was observed, which improved uniformity of the deposition. These findings were used to generate a model of the critical droplet radius needed to print homogeneous colloidal-based structures onto inclined substrates.

Calculation of electric energy savings and simulation of tank operation with variable extraction of steam and liquid phases of propane-butane mixtures

Currently, when consumers use various categories of propane-butane mixtures of liquefied petroleum gas as the main or backup energy carrier, natural and artificial regasification systems based on electric or direct fire evaporators with intermediate heat transfer agent or solid-state intermediate heat transfer agent of various evaporation capacity, which operate depending on the design in various evaporation modes, are widely used. An important element for reducing energy consumption in the artificial liquefied petroleum gas regasification systems is the switching of tank to the combined regasification mode. The combined cycle provides two main periods of tank operation: the initial period with natural regasification of liquefied gas in the day tank, ground tube evaporator, and the subsequent period with artificial regasification of the propane-butane mixture in the evaporator with an intermediate heat transfer agent. For the purpose of theoretical justification of the proposed cycle, a mathematical model of combined regasification of liquefied petroleum gases was developed, which comprehensively takes into account the main periods of tank operation: with natural regasification mode in the day tank, ground heat exchanger and artificial regasification mode in the evaporator. The results of the conducted research show that the implementation of the combined regasification cycle in the liquefied gas supply can significantly reduce the energy costs of its evaporation, increases the energy efficiency of liquefied petroleum gas supply systems and provides an average annual savings of 33.7% in electric energy for regasification.

Evaporating droplets on inclined plant leaves and synthetic surfaces: Experiments and mathematical models

HypothesisEvaporation of surfactant droplets on leaves is complicated due to the complex physical and chemical properties of the leaf surfaces. However, for certain leaf surfaces for which the evaporation process appears to follow the standard constant-contact-radius or constant-contact-angle modes, it should be possible to mimic the droplet evaporation with both a well-chosen synthetic surface and a relatively simple mathematical model. ExperimentsSurfactant droplet evaporation experiments were performed on two commercial crop species, wheat and capsicum, along with two synthetic surfaces, up to a 90° incline. The time-dependence of the droplets’ contact angles, height, volume and contact radius was measured throughout the evaporation experiments. Mathematical models were developed to simulate the experiments. FindingsWith one clear exception, for all combinations of surfaces, surfactant concentrations and angles, the experiments appear to follow the standard evaporation modes and are well described by the mathematical models (modified Popov and Young–Laplace-Popov). The exception is wheat with a high surfactant concentration, for which droplet evaporation appears nonstandard and deviates from the diffusion limited models, perhaps due to additional mechanisms such as the adsorption of surfactant, stomatal density or an elongated shape in the direction of the grooves in the wheat surface.

Experimental study on droplets evaporation on hydrophobic surface at different tilt angles

Droplets evaporation exists widely in industrial manufacturing and engineering applications. Optical visualization system was used to investigate the evaporation process of deionized water droplets on fresh taro leaves with different tilt angles. The evaporation time and mode of droplets were experimental studied at different tilt angles. The results show that the time for the droplet to completely evaporate decreased as the surface tilt angle increases. During the first half of droplet evaporation process on the slant hydrophobic surface, the evaporation modes were different between the two sides of the droplet. When the droplet on an inclined hydrophobic surface evaporated to a certain volume or a certain contact angle, its subsequent evaporation behavior was similar to that of a droplet on a non-tilted hydrophobic surface.

Evaporation-induced alterations in oscillation and flow characteristics of a sessile droplet on a rose-mimetic surface.

Strategic control of evaporation dynamics can help control oscillation modes and internal flow field in an oscillating sessile droplet. This article presents the study of an oscillating droplet on a bio-inspired "sticky" surface to better understand the nexus between the modes of evaporation and oscillation. Oscillation in droplets can be characterized by the number of nodes forming on the surface and is referred to as the mode of oscillation. An evaporating sessile droplet under constant periodic perturbation naturally self-tunes between different oscillation modes depending on its geometry. The droplet geometry evolves according to the mode of evaporation controlled by substrate topography. We use a bio-inspired, rose patterned, "sticky" hydrophobic substrate to perpetually pin the contact line of the droplet in order to hence achieve a single mode of evaporation for most of the droplet's lifetime. This allows the prediction of experimentally observed oscillation mode transitions at different excitation frequencies. We present simple scaling arguments to predict the velocity of the internal flow induced by the oscillation. The findings are beneficial to applications which seek to tailor energy and mass transfer rates across liquid droplets by using bio-inspired surfaces.

Morphology driven spatial dependence of wetting, evaporation, and unidirectional spreading of water on hexagonally patterned gold microstructure arrays

We report here our investigations on the wettability, evaporation, and unidirectional spreading behavior of water on a patterned substrate. Using colloidal lithography, hexagonally ordered gold microstructure arrays with varying morphologies were fabricated along the length of the substrate. Three different regions of the substrate exhibiting different morphologies (R1: microshells, R2: microshells with interstice, and R3: microholes) and their transition positions were identified using a field emission scanning electron microscope. Spatial dependence of wetting and the influence of surface functionalization were investigated in these regions. In R1 and R2, wetting studies show hydrophobic behavior for both the as-prepared (θ=116°−120°) and octadecanethiol (ODT, θ=128°–132°) functionalized substrates. In contrast, R3 exhibits hydrophilicity (θ=84°), which transforms to hydrophobicity (θ=102°) post ODT-functionalization. Interestingly, underwater wettability investigations of the same substrate using oil (air bubble) show superoleophobicity (superaerophobicity) that after functionalization transforms to oleophilicity (aerophobicity) for all regions. Furthermore, the evaporation of the water droplet has been studied for different regions. It shows clear distinction in the modes of evaporation for the as-prepared and functionalized substrates. Taking into account geometrical parameters, we calculated theoretical contact angles for different wetting states and compared them with our experimental results. We have found evidence for the occurrence of the penetrating Cassie–Baxter state for both as-prepared (water penetration angle, δ=50° for all regions) and ODT-functionalized substrates (δ=56° for R1 and R2 and δ=0° for R3). Unidirectional spreading of water against gravity is observed for different inclinations of the substrate at the transition positions (e.g., at 10 mm, droplet velocity = 0.83 mm/s). Our study has potential implications in microfluidic devices, biosensors, and water transportation.

Suppression of coffee ring effect in high molecular weight polyacrylamide droplets evaporating on hydrophobic surfaces

In this work, we experimentally report the concentration and molecular weight dependence of the deposit patterns of polyacrylamide (PAM) droplets on hydrophobic surfaces. We find that with an increase in non-dimensional concentrations c/c* ranging from 0.16 (dilute) to 66.66 (semi-dilute entangled) there is a gradual transition from ring to non-ring patterns. However, with a decrease in the molecular weight of the polymer by one order, the coffee ring formation was not suppressed for the reported range of concentration. We attribute these results to the role played by the critical overlap concentration (c*) and diffusion coefficient of polymer along with the evaporation modes. The present study focuses on the drying of pure polymer droplets instead of the widely studied coffee ring effect in drying of droplets with rigid solid phase colloidal particles. We believe that these experimental findings would be of immense insight in applications such as inkjet printing, surface coatings, paints and detection assays in biotechnology for tuning polymer deposit patterns as a function of polymer molecular weight and concentration.

Chemically striped surface accelerate the droplet evaporation

The chemically striped surface contains both hydrophilic and super-hydrophobic stripes. During the droplet spreading on the striped surface, the inconsistent retraction of liquids on hydrophilic stripes and super-hydrophobic stripes causes the droplet to split out satellite droplets. The generation of the satellite droplet can significantly increase the liquid-solid contact area and the length of the three-phase contact line. The increase of the contact line length broadens the range of higher heat flux, and the rise of the contact area enlarges the heat transfer area. Therefore, the chemically striped surface can promote droplet evaporation and increase the heat transfer 41.51 % higher than the hydrophilic surface. Due to different sizes, the satellite droplets enter the CCA evaporation mode earlier than the main droplet. Therefore, the droplet will experience a CCR−CCA mixed mode. During this mode, the contact line length and contact area will decrease rapidly. The satellite droplets on the chemical surface with thinner stripes are smaller than those on the chemical surface with wider stripes, leading the droplet on the former surface to enter the CCR−CCA mixed mode earlier than the droplet on the latter surface. Therefore, the lifetime of droplets on the chemical surface with wider stripes is shorter than that of droplets on the chemical surface with thinner stripes. Besides, the lower impact velocity is not conducive to droplet splitting and, therefore, evaporation.

Investigation on the Mechanism of Heat Load Reduction for the Thermal Anti-Icing System

The aircraft ice protection system that can guarantee flight safety consumes a part of the energy of the aircraft, which is necessary to be optimized. A study for the mechanism of the heat load reduction in the thermal anti-icing system under the evaporative mode was presented. Based on the relationship between the anti-icing heat load and the heating power distribution, an optimization method involved in the genetic algorithm was adopted to optimize the anti-icing heat load and obtain the optimal heating power distribution. An experiment carried out in an icing wind tunnel was conducted to validate the optimized results. The mechanism of the anti-icing heat load reduction was revealed by analyzing the influences of the key factors, such as the heating range, the surface temperature and the convective heat transfer coefficient. The results show that the reduction in the anti-icing heat load is actually the decrease in the convective heat load. In the evaporative mode, decreasing the heating range outside the water droplet impinging limit can reduce the convective heat load. Evaporating the runback water in the high-temperature region can lead to the less convective heat load. For the airfoil, the heating power distribution that has an opposite trend with the convective heat transfer coefficient can reduce the convective heat load. Thus, the optimal heating power distribution has such a trend that is low at the leading edge, high at the water droplet impinging limit and zero at the end of the protected area.

Different modes of heat transfer and crystallization in a drop of NaCl solution: The influence of key factors on the crystallization rate and the heat transfer coefficient

Evaporation, crystallization and heat transfer in a droplet of NaCl/H2O solution have been studied experimentally. Crystallization began at the edges of the droplet at solution supersaturation S = 1.94 (S = Csup/Csat, where Csup is the salt concentration in a metastable region (supersaturation), and Csat corresponds to saturation), which is noticeably higher than the previously obtained supersaturation for the droplet, microcapillary, and when using surfactant. When the droplet evaporated on the heated wall, the supersaturation of the salt solution was noticeably higher than when evaporating without heating. Four characteristic modes of evaporation and crystallization are distinguished. The ratio of the crystallization rate to the evaporation rate (jcr/jev) may be both substantially greater and less than one. It is shown that the heat transfer and the crystallization rate are regulated by Marangoni convection. The data obtained are of interest for the development of modern concepts of high-temperature crystallization in the presence of intense Marangoni flow.

Evaporation and Electrowetting of Sessile Droplets on Slippery Liquid-Like Surfaces and Slippery Liquid-Infused Porous Surfaces (SLIPS).

Sessile droplet evaporation underpins a wide range of applications from inkjet printing to coating. However, drying times can be variable and contact-line pinning often leads to undesirable effects, such as ring stain formation. Here, we show voltage programmable control of contact angles during evaporation on two pinning-free surfaces. We use an electrowetting-on-dielectric approach and Slippery Liquid-Infused Porous (SLIP) and Slippery Omniphobic Covalently Attached Liquid-Like (SOCAL) surfaces to achieve a constant contact angle mode of evaporation. We report evaporation sequences and droplet lifetimes across a broad range of contact angles from 105°–67°. The values of the contact angles during evaporation are consistent with expectations from electrowetting and the Young-Lippman equation. The droplet contact areas reduce linearly in time, and this provides estimates of diffusion coefficients close to the expected literature value. We further find that the total time of evaporation over the broad contact angle range studied is only weakly dependent on the value of the contact angle. We conclude that on these types of slippery surfaces, droplet lifetimes can be predicted and controlled by the droplet’s volume and physical properties (density, diffusion coefficient, and vapor concentration difference to the vapor phase) largely independent of the precise value of contact angle. These results are relevant to applications, such as printing, spraying, coating, and other processes, where controlling droplet evaporation and drying is important.

Effect of alternative solvent evaporation techniques on mechanical properties of primer–adhesive mixtures.

This study evaluated the influence of the mode and time of solvent evaporation on the tensile strength (TS), flexural strength (FS) and elastic modulus (EM) of two adhesive systems: Scotchbond Multipurpose (SBMP) and Clearfil SE (CSEB). For this purpose, rectangular samples (2x1x7 mm) were prepared with 10 μL of primer and the solvents were evaporated with air spray at (23±1) ºC, (40±1) ºC and negative control (without spray). For each temperature, the times of 5, 20, 30, and 60 seconds were investigated. The statistical results showed that evaporation at 40±1ºC resulted in better EM for the two adhesives tested and all the evaporation times evaluated. However, there were no significant differences between the times and modes of evaporation for TS. The results of this study indicate that evaporation at a temperature of (40±1) °C could improve the elastic modulus of both adhesives tested, regardless of the evaporating time.

Numerical study of droplet evaporation on heated flat and micro-pillared hydrophobic surfaces by using the lattice Boltzmann method

The evaporation processes of sessile droplets on both heated flat and structured hydrophobic surfaces are numerically studied by using a thermal multi-phase lattice Boltzmann model. The contact-line dynamics and heat transfer regarding the evaporation of droplets are simulated and the effects of the heat conductivity, wettability, and roughness of the substrate on the droplet evaporation are investigated. The results indicate that the different evaporation modes of the droplets on the rough surfaces are attributed to the microscopic “stick–slip” motion of the contact line. The constant contact angle (CCA) mode of droplet evaporation on the rough hydrophobic surface can be regarded as a macroscopic average of the microscopic “stick–slip” processes with a short stick period and small change in apparent contact angle. The weaker stick effect of the surface with a higher hydrophobicity and smaller solid fraction makes the droplet evaporation more likely to occur in the CCA manner.

Influence of Acetylene on Ti Target Poisoning During Pulse-Enhanced Vacuum Arc Evaporation

Vacuum arc evaporation is a prospective technique to fabricate hard and wear material due to its high ion energy and plasma density. Nevertheless, target tends to be poisoned by the compound layer in common reactive gases such as acetylene. Then the coating deposition rate and the properties of the coatings would be strongly influenced. Thus, the formation of the compound layer has to be inhibited. Previous studies pointed out the potential of pulse-enhanced vacuum arc evaporation mode for inhibiting the formation of the compound layer. Within the presented study, motion of group spots (GSs), optical emission spectra, and substrate current based on PEVAE mode were examined entirely. The result shows that the area ratio of the compound layer in the target was decreased from about 75% to 42%. The ionization degree of titanium and mean substrate current could be substantially increased by up to 40% and 100%, respectively, in a mixture gas of nitrogen and acetylene due to larger plasma density indicated by higher spectra intensity of plasmas. All of the above made PEVAE prospective candidate to inhibit the formation of the compound layer. Beyond this PEVAE would lead to higher coating rate and denser coating in nitrogen plus acetylene atmosphere.

Evaporation dynamics of water droplets on superhydrophobic nanograss surfaces

Evaporation of a sessile droplet on superhydrophobic surfaces has various applications due to their utmost water-repellent and self-cleaning wetting properties. We experimentally investigate the evaporation dynamics of water droplets and the time evolution of the contact angle and drop dimensions on superhydrophobic, nanograss substrates of an extremely-low solid packing fraction (ϕ ≈ 0.005). The experimental data shows that all the droplets deposited initially form a gas-trapping, Cassie-Baxter state. Small droplets subsequently evaporate with a constant contact angle mode, followed by a mixed mode at the end of the droplet lifetime. On the contrary, for relatively large droplets, two distinct evaporation modes are found. Some of the larger evaporating droplets were initially in a constant contact angle mode and underwent a mixed mode, while others began with a mixed mode with slowly decreasing base diameter and contact angle. Intriguingly, stick-slip motions of the contact line for large droplets are observed using superhydrophobic nanograss surfaces. Such slip or jumping motion could be related to the excess free energy available in the system when the receding contact angle is reached, resulting in the contact line movement. Finally, the experimental data of contact angle dependent evaporative mass flux are found to nearly collapse onto one universal curve for different droplet sizes and initial contact angles, in agreement with an evaporative cooling model.