Sessile Water Droplets Research Articles

Open‐Chip Droplet Splitting in Electrowetting

AbstractElectrowetting‐on‐dielectric (EWOD) has emerged as a powerful technique to perform on‐chip droplet operations like transportation, dispensing, splitting, and mixing in sandwiched droplets. In contrast, open‐chip droplet manipulation using electrowetting enables micro‐total‐analysis systems with multiple sensor integration and re‐routing capabilities. Droplet splitting has been the bottleneck in developing open‐chip platforms. Droplet splitting on an open‐chip platform using electrowetting‐on‐dielectric is presented. An energy‐based simulation model has been developed. It shows that splitting a sessile water droplet is impossible on an open‐chip configuration because of the low pad contact angle requirement. Low contact angles cannot be achieved due to contact angle saturation in electrowetting. It is experimentally shown that splitting is possible if the droplet is engulfed in an oil shell (i.e., in compound droplets). The planar electrode configurations and regime of electrowetting numbers for which splitting can be achieved are identified. It is observed that larger gaps and higher electrowetting numbers favor symmetrical splitting because the electrostatic force driving the actuation is significantly higher than the retarding interfacial forces. Conversely, asymmetrical splitting has been obtained when the actuation force is barely sufficient. Further the splitting of surfactant‐loaded single‐phase sessile droplets is demonstrated and a preferential surface charging phenomenon is explained.

Modelling study on freezing process of water droplet on inclined cold plate surface with droplet dynamic behavior considered

Droplet freezing on inclined surfaces exists widely in engineering fields. To accurately predict and control the freezing process of a sessile water droplet on inclined surface, a theoretical model based on the heat-enthalpy method is presented in this study, with two types of dynamic behavior considered, deformation and spreading. After the validation of model by droplet profiles and freezing duration from experiments, the freezing characteristics are analyzed, including contact area, frozen height and vertex offset, etc. As found, the effect of inclined angle on less than 10.34 µL water droplet is greater than that on larger than 10.34 µL droplet, due to the mutual relation between surface tension and gravity effect. When the inclined angle of surface changes from 0° to 40°, the contact area keeps at 11.61 mm2 for 10 µL water droplet, and increases by 5.64% from 23.04 to 24.34 mm2 for 25 µL water droplet. The initial heights of 10 and 25 µL water droplets decrease by 0.85% from 1.18 mm to 1.17 mm and by 1.91% from 1.51 mm to 1.49 mm, respectively. That means it is easier frozen for the same water droplet on bigger inclined angle surface. This study is beneficial for the optimization of anti-frosting and defrosting technologies.

Optofluidic Platform for the Manipulation of Water Droplets on Engineered LiNbO3 Surfaces

AbstractThe actuation and control of liquid droplets on a surface have important implications in many industrial applications and microfluidics. In recent years, various strategies have been used. Here, an optofluidic platform that performs the basic droplet handling operations required in a common microfluidic device is presented. It is based on z‐cut, iron‐doped lithium niobate crystals that, when illuminated, generate surface charges of opposite sign at the two crystal faces because of the photovoltaic effect. The face of the crystal in contact with the droplets is coated with a lubricant‐infused layer, which guarantees hydrophobicity and, more importantly, a very slippery and robust surface for prolonged use. In this way, sessile water droplets having volumes of microliters, corresponding to millimeters in size, can be easily actuated, guided, merged, and split by projection on the crystal of suitable static or dynamic light patterns. The actuated droplets can cover distances of centimeters on a timescale of a few seconds. The resulting platform is highly flexible and reconfigurable and does not require moving parts.

Zirconium diboride assisted superhydrophobic/superoleophilic surface modification of polyurethane sponge for continuous oil-water separation and emulsion purification

BackgroundWith rapid industrialization, water pollution by accidental oil spills and industrial wastes dumping into water bodies has become a severe threat to the marine ecosystem. In this regard, usage of superhydrophobic/superoleophilic sorbent is forecasted perhaps one of the best solution to recover oil and natural solvents from contaminated water owing to its high absorption capacity, and recyclability. MethodsHerein, we report zirconium diboride modified polyurethane sponge (ZrB2@PUS) prepared by facile dip coating technique at ambient condition for selective oil/water removal. The ZrB2@PUS was further functionalized using octadecyl trichlorosilane (OTS) in presence of toluene to impart superhydrophobicity. Significant findingsMicrostructural analysis revealed that ZrB2 platelets anchored onto smooth struts of PUS thereby increasing surface roughness, whereas, OTS helps in decreasing the surface tension and enhancing the non-wettability. The ZrB2@PUS exhibited a contact angle ∼166° and ∼0° for a sessile water and oil droplet, respectively. Moreover, ZrB2@PUS was found suitable for removal of various oil and organic solvents from oil-water mixture with absorption capacity reaching ∼83–129 g/g and capacity retention ∼92–97% after 20 cycles in static and dynamic conditions. OTS-functionalized ZrB2@PUS also successfully explored for separating SDS stabilized diesel-in-water and toluene-in-water emulsion, demonstrating feasibility for wastewater treatment containing oil and organic solvents.

Effects of surface topography at different scales on the dispersion of the wetting data for sessile water droplets on nitrided austenitic stainless steels

Plasma nitriding is used to increase the durability of steels. The wettability of nitrided surfaces is poorly studied. The ability of nitriding in modifying the wetting and evaporation of sessile droplets on austenitic stainless steel is studied. Transferred plasma for nitriding provides independent substrate biasing giving the opportunity to tailor the surface before and during the nitriding treatments. It was shown that a cleaning treatment (ArH2 plasma) produces topographical modifications of the surface by selective sputtering of the grains giving contact angles between 80 and 90°. Similar results are obtained when the nitriding treatments are carried out with a rather high bias voltage. It is explained using the skewness (Ssk) topographical roughness parameter which reveals the asymmetry of the profile. The more the Ssk is, the higher the contact angle is. During nitriding treatments, the very high internal stresses leads to the formation of spikes at the grain boundaries. Pinning of the three-phase contact line on peaks localized at grain boundaries are responsible for high contact angles. For nitriding treatments carried out with lower bias, surface contamination occurs and a discontinuous oxynitride layer is formed. The nanostructure formed by these oxides gives contact angles up to 120°.

Competition between thermal and surfactant-induced Marangoni flow in evaporating sessile droplets

HypothesisThermal Marangoni flow in evaporating sessile water droplets is much weaker in experiments than predicted theoretically. Often this is attributed to surfactant contamination, but there have not been any in-depth analyses that consider the full fluid and surfactant dynamics. It is expected that more insight into this problem can be gained by using numerical models to analyze the interplay between thermal Marangoni flow and surfactant dynamics in terms of dimensionless parameters. SimulationsTwo numerical models are implemented: one dynamic model based on lubrication theory and one quasi-stationary model, that allows for arbitrary contact angles. FindingsIt is found that insoluble surfactants can suppress the thermal Marangoni flow if their concentration is sufficiently large and evaporation and diffusion are sufficiently slow. Soluble surfactants, however, either reduce or increase the interfacial velocity, depending on their sorption kinetics. Furthermore, insoluble surfactant concentrations that cause an order 0.1% surface tension reduction are sufficient to reduce the spatially averaged tangential flow velocity at the interface by a factor 100. For larger contact angles and smaller droplets this required concentration is larger (typically <1% surface tension reduction). The numerical models are mutually validated by comparing their results in cases where both are valid.

Evaporation of a sessile droplet pinned by a groove on a heated substrate

The process of evaporation of sessile water droplets on a heated substrate was experimentally studied. A droplet evaporation is a fundamental phase change process which requires thorough consideration in various conditions. In this work we investigated the evaporation of water droplets on the smooth copper substrate with a circle groove made on its surface, which helped to keep the sessile droplet stable. Thus, the contact area of the evaporating droplet with the substrate remained constant, while the contact angle varied over a wide range. The experiment was conducted with a help of Krüss DSA100.

Corrosion resistance and near-infrared light induced self-healing behavior of polycaprolactone coating with MIL-53@TA on magnesium alloy

A novel self-healing composite coating was fabricated on magnesium alloy by using polycaprolactone (PCL) as resin matrix and iron-based metal-organic framework MIL-53 modified with tannic acid (TA) as function filler. The surface morphology, structure and wettability of composite coating were characterized by scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) and sessile water droplet static contact angle. The corrosion resistance and healing performance of PCL coatings was determined by electrochemical impedance spectroscopy (EIS) and photothermal tests. Results indicate that MIL-53@TA/PCL coating possesses a flatter surface, a higher corrosion resistance and better photothermal effect than that of blank PCL coating and MIL-53/PCL coating. Under the near-infrared (NIR) laser irradiation with the wave length of 808 nm, the scratched MIL-53@TA/PCL coating could be healed within 10 min because of the excellent photothermal conversion ability of MIL-53@TA component.

Water droplet behavior in between hydrophilic and hydrophobic surfaces and dust mitigation.

An innovative method is introduced for environmental dust mitigation from a hydrophobic surface by a sessile water droplet. The sessile water droplet is located between two parallel plates having hydrophilic (at the top) and hydrophobic (at the bottom) states. The water droplet is located at the top hydrophilic plate, and the effect of the plate spacing on dust mitigation rate is examined. The droplet behavior is analyzed for different plate spacings and various droplet sizes using a high-speed camera. The fluid and the particle motions are simulated inside the droplet while adopting the experimental conditions. The findings demonstrate that the sessile droplet can effectively mitigate dust. Reducing the plate spacing increases the droplet meniscus diameter and enhances the dust removal rate. The surface tension force on the hydrophilic surface remains greater than that of the pinning force on the dusty hydrophobic surface even though the Magdeburg and surface tension forces contribute to the droplet pinning force on the hydrophobic dusty surface. Flow current is developed in the droplet fluid during the squeezing period, which considerably enhances the dust removal rate from the hydrophobic surface. The cleaned area increases with the droplet volume and plate spacing. Stria patterns are observed on the circumference of the dust-removed area. The present study provides a detailed analysis of a new method of dust removal from surfaces for self-cleaning applications.

Droplet Evaporation on Hot Micro-Structured Superhydrophobic Surfaces: Analysis of Evaporation from Droplet Cap and Base Surfaces

In this study, evaporation of sessile water droplets on hot micro-structured superhydrophobic surfaces is experimentally and theoretically investigated. Water droplets of 4 µL are placed on micro-pillared silicon substrates with the substrate temperature heated up to 120°C. A comprehensive thermal circuit model is developed to analyze the effects of substrate roughness and substrate temperature on the sessile droplet evaporation. For the first time, two components of heat and mass transfer, i.e., one from the droplet cap surface and the other from the droplet base surface, during droplet evaporation are distinguished and systematically studied. As such, the evaporation heat transfer rates from both the droplet cap surface and the interstitial liquid-vapor interface between micropillars at the droplet base are calculated in various conditions. For droplet evaporation on the heated substrates in the range of 40°C – 80°C, the predicted droplet cap temperature matches well with the experimental results. During the constant contact radius mode of droplet evaporation, the decrease of evaporation rate from the droplet base contributes most to the continuously decreasing overall evaporation heat transfer rate, whereas the decrease of evaporation rate from the droplet cap surface is dominant in the constant contact angle mode. The influence of internal fluid flow is considered for droplet evaporation on substrates heated above 100°C, and an effective thermal conductivity is adopted as a correction factor to account for the effect of convection heat transfer inside the droplet. Temperature differences between the droplet base and the substrate base are estimated to be about 2°C, 5°C, 8°C, 13°C and 18°C for droplet evaporation on substrates heated at 40°C, 60°C, 80°C, 100°C, and 120°C, respectively, elucidating the delayed or depressed boiling of water droplets on a heated rough surface due to evaporative cooling.

Ice nucleation forced by transient electric fields.

Icing affects many technical systems, like aircraft or high-voltage power transmission and distribution in cold regions. Ice accretion is often initiated by ice nucleation in sessile supercooled water droplets and is influenced by several influencing factors, of which the impact of electric fields on ice nucleation is still not completely understood. Especially the influence of transient electric fields is rarely or not at all investigated, even though it is of great interest, e.g., for high-voltage transmission lines or for the food industry. In the present study the impact of transient electric fields on ice nucleation in supercooled sessile water droplets is experimentally investigated under well-defined conditions. A set of droplets is cooled down to a certain temperature and is subsequently exposed to electric fields generated from standard lightning or standard switching impulse voltages, which are commonly used for testing of high-voltage equipment. The nucleation behavior of individual droplets is captured using a high-speed camera and the effect of the transient electric field on ice nucleation is analyzed by considering both the singular and the stochastic nature of nucleation. While the singular nature of nucleation is referred to during analysis of the relative number of droplets remaining liquid long times after the impulse voltage, its stochastic nature is accounted for in the analysis of the temporal evolution of the relative number of frozen droplets. It is shown that low electric field strengths (E[over ̂]≤6.52kV/cm) only have a negligible impact on ice nucleation, independent of the supercooling. In contrast, high electric field strengths (E[over ̂]≥9.78kV/cm) promote significantly ice nucleation. It is also shown that depending on the supercooling, the freezing delay of the different droplets in the ensemble may vary over several magnitudes for the same conditions. It is demonstrated that the electric field appears to indirectly affect the nucleation rate by generating droplet oscillations, finally promoting ice nucleation. The experiments clearly demonstrate the possibility to actively force ice nucleation by applying transient electric fields. These results improve the understanding of ice accretion on high-voltage insulators and may also lend insight into freezing processes in food industry. We expect that these results will be a valuable contribution in formulating and/or validating new nucleation models.

Modeling study on sessile water droplet during freezing with the consideration of gravity, supercooling, and volume expansion effects

Droplet freezing phenomenon widely exists in many fields, including aerospace, power production, and cryopreservation, etc. Considering the effects of supercooling, gravity and volume expansion, a theoretical model of droplet freezing is developed. A good agreement is found between the predicted results of freezing front radius and height and experiments for both hydrophilic and hydrophobic surfaces. The developed model also shows good performance in predicting freezing times. The average prediction deviation is 7.63%, and more than 93% simulation results show the deviation within ±15%. Gravity has a more obvious influence on final freezing times with the increase of cold plate temperature, contact angle, and droplet volume. For the droplet freezing process under various contact angles and cold plate temperatures, the fastest average temperature change rates inside water droplet are -2.48 °C/s and -1.72 °C/s, respectively. This study is beneficial for the better understanding of the droplet solidification as well as the optimization of refrigeration and defrosting technologies.

Ecological drivers of eggshell wettability in birds.

Complex and at times extreme environments have pushed many bird species to develop unique eggshell surface properties to protect the embryo from external threats. Because microbes are usually transmitted into eggs by moisture, some species have evolved hydrophobic shell surfaces that resist water absorption, while also regulating heat loss and the exchange of gases. Here, we investigate the relationship between the wettability of eggshells from 441 bird species and their life-history traits. We measured the initial contact angle between sessile water droplets and the shell surface, and how far the droplet spread. Using phylogenetic comparative methods, we show that body mass, annual temperature and eggshell maculation primarily explained variance in water contact angle across eggshells. Species nesting in warm climates were more likely to exhibit highly hydrophobic eggshells than those nesting in cold climates, potentially to reduce microbial colonization. In non-passerines, immaculate eggs were found to have more hydrophobic surfaces than maculate eggshells. Droplets spread more quickly on eggshells incubated in open nests compared to domed nests, likely to decrease heat transfer from the egg. Here, we identify clear adaptations of eggshell wettability across a diverse range of nesting environments, driven by the need to retain heat and prevent microbial adhesion.

Ring droplet formation during evaporation of a sessile water droplet on a biphilic surface

Numerous studies have investigated droplet evaporation on surfaces with different roughness and wettability values. However, most studies have focused on droplet evaporation from homogeneous surfaces at low surface temperatures (below 100 °C). In this study, we conducted droplet evaporation experiments on biphilic surfaces with different ring pattern sizes fabricated by a hybrid method consisting of the sol-gel approach and a picosecond laser system. This study investigated the droplet evaporation boiling phenomenon on these biphilic surfaces under superheated conditions. Changes in droplet geometry and inner bubble dynamics during evaporation were analyzed using a high-speed camera. A ring-shaped droplet formed during the final evaporation period. The formation of the ring enhanced the droplet evaporation efficiency. Furthermore, the experimental results from this study work were compared with the theoretical model proposed in the literature.

Droplet evaporation characteristics on heated superhydrophobic surface subjected to airflow

In a novel frost suppression method for air source heat pumps, the frost-free operation of the system is realized by vaporizing the condensed droplets on superhydrophobic heat exchanger fins before freezing. Due to the control requirements of this method, a multiphase flow model based on VOF method for droplet evaporation is proposed. The evaporation characteristics and survival time of a sessile water droplet on heated superhydrophobic surface subjected to airflow are investigated through the model. The convection and evaporative cooling effect are considered in the simulation, experiments showed that the prediction error for droplet survival time is within 8%. The effects of boundary layer thickness, substrate temperature, surface wettability, air temperature and humidity on the heat and mass transfer process of droplet evaporation are studied with this model. The survival time of water droplet with diameter below 100 μm evaporating in different evaporation modes is also calculated. The results showed that under the conditions of common parameters in winter, the survival time of a droplet evaporating in fully developed flow is in the range of 4∼20 s. The survival time of droplets evaporating in constant contact radius mode is 15.4% shorter than that evaporating in constant contact angle mode.

Freezing characteristics of deposited water droplets on hydrophilic and hydrophobic cold surfaces

Freezing phenomena of water droplets on metal surfaces widely exist in nature and industry. While the freezing of a sessile water droplet has been extensively studied, here we report an experimental and thoeritical investigation of the freezing characteristics of a deposited water droplet on cold hydrophilic and hydrophobic aluminum surfaces and also compare them with those of the sessile droplet. The results show that the deposited droplet starts to freeze from the liquid-solid interface after the spreading contact line is arrested by the cold surface and the recalescence stage observed during the freezing of a (supercooled) sessile water droplet does not appear. Furthermore, under our experimental conditions of surface temperature and wettability, the frozen shapes of the deposited droplets on the hydrophilic and hydrophobic surfaces are only dependent on the surface temperature while those of the sessile droplets unnoticeably change with the surface temperature but significantly vary with the surface wettability. Additionally, we present a scaling analysis to describe the temporal evolution of freezing front and the relationship between freezing time and surface temperature for the deposited droplet.

A combined experimental and computational study of phase-change dynamics and flow inside a sessile water droplet freezing due to interfacial heat transfer

This study experimentally and numerically investigates the freezing characteristics and fluid dynamics of millimetre-sized sessile water droplets submerged in silicone oil at sub-zero temperatures under free convection. Individual water droplets were cooled to sub-zero temperatures (260-270 K) via interfacial heat transfer between the two liquid phases, in an approach different to studies in the literature where the cooling is done either from the solid substrate or from a low-temperature gas phase (such as air) surrounding the droplets. Laser-induced fluorescence was employed to perform spatiotemporally-resolved measurements of the phase distribution (from which interface distributions, freezing fronts, and rates were extracted). The particle image velocimetry was used to generate information on the velocity fields inside the liquid droplets. The experimental data are complemented by computational fluid dynamics (CFD) simulations, which showed acceptable qualitative and quantitative agreement with the experimental results. The experimental and simulation results indicated that prior to the initiation of freezing, two counteracting recirculation zones are generated in the central plane of the droplets, one on either side of the centreline, leading to a net upward flow at the edges and a downward flow in the centre due to the natural convection driven by internal temperature gradients. The nucleation sites appear on the external regions of the recirculation structures (which are locations with higher shear). Once freezing starts, the natural circulation patterns are suppressed, and instead, a sole downwards flow dominates, which is the result of the freezing layer suppressing the water phase. CFD results demonstrated a relatively wide temperature and pressure distribution in the water droplet at the beginning of the freezing stage, which gradually diminishes as the freezing process proceeds. The effect of droplet size and oil temperature on the freezing rates were investigated parametrically. The finding showed that when the droplet size doubles, the time required for the droplet to completely freeze extended by almost an order of magnitude. In addition, lower oil temperatures appeared to modify the normalised freezing rate, with sharper exponential growth rates and reduced initial and final stages at higher oil temperatures.

Interplay of transport mechanisms during the evaporation of a pinned sessile water droplet

Droplet evaporation has been intensively investigated in past decades owing to its emerging applications in diverse fields of science and technology. Yet the role transport mechanisms has been the subject of a heated debate, especially the presence of Marangoni flow in water droplets. This work aims to draw a clear picture of the switching transport mechanisms inside a drying pinned sessile water droplet in both the presence and absence of thermocapillarity by developing a comprehensive model that accounts for all pertinent physics in both phases as well as interfacial phenomena at the interface. The model reveals a hitherto unexplored mixed radial and buoyant flow by shedding light on the transition from buoyancy induced Rayleigh flow to the radial flow causing coffee ring effect. Predictions of the model excellently match previous experimental results across varying substrate temperatures only in the absence of Marangoni flow. When thermocapillarity is accounted for, strong surface flows shape the liquid velocity field during most of the droplet lifetime and the model starts to overestimate evaporation rates with increasing substrate temperature.

A modeling study of sessile water droplet on the cold plate surface during freezing under natural convection with gravity effect considered

Freezing of water droplet is widely seen and important in the fields of aerospace, cold energy storage, and power production. To investigate the freezing process of a sessile water droplet on a horizontal cold plate, a theoretical model was developed. Different from previously reported models, the effects of supercooling and gravity on the physical properties and the water droplet profile are both considered, respectively. This model is validated with the experimental data of two parameters, including the freezing time and the freezing front radius. The results indicate that the deviation of freezing time is decreased from 7.69% to 0.17%, while the accuracy improved by 7.52%. The average deviation of the freezing front radius is decreased from 142.90 μm to 57.94 μm, with the accuracy improved by 59.46%. At the freezing stage, the appearance of the dynamic growth angle contributes to the less deviation of the freezing front radius. The eccentricity of droplet shape decreases from 0.45 to 0.03, with the eccentricity decreased by 93.45%. The temperature change rates inside a droplet show a gradually decreasing tendency, and the temperatures at different droplet locations present different limiting values. The findings of this study are beneficial for understanding droplet solidification process as well as new technologies for refrigeration, deicing, and defrosting.

High-resolution neutron imaging reveals kinetics of water vapor uptake into a sessile water droplet

•High-resolution neutron imaging visualizes the evaporation of a sessile droplet •The neutron imaging traces a deuterated component in the evaporating droplet •FTIR spectroscopy and diffusion-based model support the neutron imaging observation The spatiotemporal distribution of multiple components and phases governs their evaporation and condensation at the liquid-vapor interface. However, in situ methods to characterize the distribution remain challenging, despite the significance of understanding the ubiquitous mass transport phenomena. Here, we introduce high-resolution neutron imaging as a versatile method to quantify the composition of a sessile droplet in situ, under evaporation and condensation. To prove the concept, we perform a neutron transmittance analysis of a sessile heavy water (D2O) droplet and measure the fraction change of H2O to D2O by the sorption of ambient H2O vapor during the evaporation. Our observations are consistent with ex situ Fourier transform infrared spectroscopy measurements and our diffusion-based numerical model. Our results demonstrate that, with deuterated components having a physicochemical similarity with their hydrogenated counterparts, high-resolution neutron imaging can trace composition changes in nonequilibrium phenomena, such as evaporation and condensation. The spatiotemporal distribution of multiple components and phases governs their evaporation and condensation at the liquid-vapor interface. However, in situ methods to characterize the distribution remain challenging, despite the significance of understanding the ubiquitous mass transport phenomena. Here, we introduce high-resolution neutron imaging as a versatile method to quantify the composition of a sessile droplet in situ, under evaporation and condensation. To prove the concept, we perform a neutron transmittance analysis of a sessile heavy water (D2O) droplet and measure the fraction change of H2O to D2O by the sorption of ambient H2O vapor during the evaporation. Our observations are consistent with ex situ Fourier transform infrared spectroscopy measurements and our diffusion-based numerical model. Our results demonstrate that, with deuterated components having a physicochemical similarity with their hydrogenated counterparts, high-resolution neutron imaging can trace composition changes in nonequilibrium phenomena, such as evaporation and condensation.