Sessile Drop Evaporation Research Articles

Fluid flow structures in an evaporating sessile droplet depending on the droplet size and properties of liquid and substrate

We investigate numerically quasi-steady internal flows in an axially symmetrical evaporating sessile droplet depending on the ratio of substrate to fluid thermal conductivities, fluid volatility, contact angle and droplet size. Temperature distributions and vortex structures are obtained for droplets of 1-hexanol, 1-butanol and ethanol. To this purpose, the hydrodynamics of an evaporating sessile drop, effects of the thermal conduction in the droplet and substrate and diffusion of vapor in air have been jointly taken into account. The equations have been solved by finite element method using ANSYS Fluent. The phase diagrams demonstrating the number and orientation of the vortices as functions of the contact angle and the ratio of substrate to fluid thermal conductivities, are obtained and analyzed for various values of parameters. In particular, influence of gravity on the droplet shape and the effect of droplet size have been considered. We have found that the phase diagrams of highly volatile droplets do not contain a subregion corresponding to a reversed single vortex, and their single-vortex subregion becomes more complex. The phase diagrams for droplets of larger size do not contian subregions corresponding to a regular single vortex and to three vortices. We demonstrate how the single-vortex subregion disappears with a gradual increase of the droplet size.

Evaporative self-assembly of soft colloidal monolayers: the role of particle softness.

We investigate the sessile drop evaporation aided self-assembly of microgel particles by varying their softness. Evaporation of sessile drops containing amphiphilic microgel particles at suitable concentrations results in uniform monolayer deposits that span the entire drop area. At lower concentrations, the deposits are in the form of monolayer coffee rings whose width scales with particle concentration. Using softer microgels synthesised with a lower quantity of crosslinker, we show that the monolayer coffee rings do not form at low particle concentrations. The microgels adsorbed at the interface deform, and the extent of deformation depends on the softness of the microgels as well as their concentration at the interface. Upon complete evaporation of the solvent, the microgel-laden interface is transferred to the substrate. The final deposit shows hexagonal particle arrays where the interparticle separation increases with increasing microgel softness and decreases with particle concentration in the drop. Further insight into the role of microgel softness in the microstructure of the particulate deposits is obtained by measuring the viscoelasticity of the particle-laden interface. Interestingly, the interface loaded with lesser crosslinked microgels exhibits viscoelastic nature even at lower particle concentrations, whereas the higher crosslinked microgels show viscous behaviour.

Sessile volatile drop evaporation under microgravity

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

Insights into the evolution of the thermal field in evaporating sessile pure water drops

We investigate the evolution of the thermal field during evaporation, a fundamental aspect of evaporating sessile drops. With numerous reports in the literature investigating the contact line dynamics, we aspire to identify generalized features in the evolution of the thermal field and ultimately correlate these with the contact line dynamics. Considering a broad range of experimental parameters such as substrate wettability, substrate temperature, initial volume of the drop, and ambient relative humidity results in a wide range of evaporation rates, in turn affecting the strength of internal convective flows. Infrared thermography is utilized to extract the thermal field at the liquid–vapor interface, and optical imaging is used to record the evolution of drop shape during evaporation. We observe that the onset and presence of a convective cell as a cold spot at the interface highlights a non-axisymmetry in the thermal field. In consequence, a hitherto unreported asymmetry in the internal flow field is observed, as evidenced by the particle image velocimetry. Among the multitude of experiments conducted, we report four distinct trends in the evolution of interfacial temperature difference depending on the presence and duration of the presence of the convective cell, which are elucidated by discussing the evolution of maximum and minimum temperatures at the interface. The interplay between heat conducted into the drop and heat released due to evaporation can result in a momentary decrease in temperature of the drop, which is not reported previously. Lastly, a theoretical estimate for the temperature difference within the drop is extracted using vapor diffusion model and energy balance during evaporation. Comparison of this theoretical temperature difference with experimental observations highlights the influence of internal convective flows in homogenizing the thermal field within the drop.

Transient flow patterns in an evaporating sessile drop: A numerical study on the effect of volatility and contact angle

Abstract In this paper, we numerically investigated the evolution of flow pattern inside an evaporating sessile drop using a transient numerical model based on the Arbitrary Lagrangian–Eulerian (ALE) frame that fully couples fluid flow and heat and mass transfer. Effects of both liquid volatility and initial contact angle on the transient flow pattern were explored. The results show that stronger volatile drops have more vortices whereas multi-vortices circulation lasts for shorter time. With the increasing of contact angle, the maximum number of vortices at first increases gently and then rapidly, while the duration of the multi-vortices circulation at first decreases rapidly and then increases a little bit. Marangoni flow is found to dominate the formation of vortices in most of the simulated cases, whereas natural convection plays a role in drops of large contact angles. These conclusions agree with the judgement criteria based on Marangoni number and Grashof number. Besides the classical symmetrical vortices, encompassing vortices are observed during the transient development of internal flow. These encompassing vortices normally form either through the merging of a big vortex with its adjacent small one or through the splitting of a big vortex into two smaller ones.

The influence of surface structuring on evaporation of sessile drops of water

The paper presents the results of experimental studies of the evaporation of sessile water droplets on smooth and structured surfaces of Teflon fluoropolymer. Using high-speed microphotographs, data were obtained on the change in the contact angle and the diameter of the contact spot of water droplets during evaporation on smooth and structured surfaces. Using the method of infrared thermography, the dynamics of changes in the average temperature of water droplets during evaporation was investigated. The results show that the structure of the surface has a decisive influence on the change in the geometric parameters and temperature of the sessile water droplets.

Deposition of Colloidal Particles during the Evaporation of Sessile Drops: Dilute Colloidal Dispersions

The deposition of colloidal silica particles during the evaporation of sessile drops on a smooth substrate has been modeled by the simultaneous solution of the Navier–Stokes equations, the convective-diffusive equation for particles, and the diffusion equation for evaporated vapor in the gas phase. Isothermal conditions were assumed. A mapping was created to show the conditions for various deposition patterns for very dilute suspensions. Based on values of the Peclet (Pe) number and Damkholer numbers (Da and Da−1), the effects of adsorption and desorption were discussed according to the map. Simulations were also done for suspensions with a high particle concentration to form a solid phase during the evaporation by using a packing criterion. The simulations predicted the height and width of the ring deposit near the contact line, and the results compared favorably to experimental particle deposition patterns.

A new method to study evaporation of sessile drop from permeable surfaces

Real time monitoring of the mass of an evaporating liquid drop from a real surface by optical methods cannot account for the volume of the drop that gets trapped between asperities. Mass measurement methods do not have high resolution, necessary to account for the small volume of the drop trapped in the pores. This work uses a non resonant mass sensor with a goniometer to simultaneously measure the mass and contact angle of the evaporating drop. Evaporative flux is used as the quantity of comparison across all surfaces. For a pinned sessile drop, the evaporative flux remains almost a constant for any kind of surface. For inkjet printing paper, the negative slope of the flux rate means evaporation is dominant. The drop breaches the hydrophobic coating on the paper to indicate a positive slope of this flux rate. In short, evaporation is strictly controlled by relative humidity and not the nature of the substrate.

Modeling of Sessile Droplet Evaporation on Engineered Surfaces

The evaporation of sessile drop has a wide range of application that includes printing, washing, cooling, and coating. Due to the complex nature of drop evaporation process, this phenomenon is reliant on several parameters such as ambiance and physiochemical properties of liquid and surface. In the present study, a mathematical model of water droplet evaporation on an engineered aluminum surface is developed. Experimental study is carried out for the validation of code. The data obtained from the simulation is validated against the data obtained from an experimental study as well as the data available in the literature and good agreement was found among them. Post-validation, the effect of surface wettability and environment conditions on a droplet evaporation rate is estimated. It is inferred from the outcomes that the temperature at the apex of the drop varies linearly with the increasing relative humidity. Droplet volume has a significant impact on the evaporation rate and comparatively higher evaporative flux for a smaller volume of the drop with large contact angles. This unveils the possibility of achieving the required evaporation rate by controlling surface wettability and relative humidity conditions near the drop.

Bénard-Marangoni Dendrites upon Evaporation of a Reactive ZnO Nanofluid Droplet: Effect of Substrate Chemistry.

Evaporation of a particle laden sessile drop can lead to complex surface patterns with structural hierarchy. Most commonly, the dispersed particles are inert. We have recently reported that when the sessile drop contains reactive ZnO nanoparticles, solidified Bénard-Marangoni (BM) cells with dendritic micromorphology were formed in the residual surface pattern from in situ-generated nanoclusters. Here, we report the effect of substrate chemistry on the residual pattern from the evaporation of nanofluids containing ZnO particles dispersed in a mixture of cyclohexane and isobutylamine, by comparing three different substrates: glass, silicon, and hydrophilized silicon. In particular, we performed a quantitative analysis of the BM cell size, distribution, and the cell morphological characteristics via the fractal dimension analysis. We find that the size dimension λBM of the dendritic Bénard-Marangoni cells varied on the different substrates, attributed to their different hydrophilicity and affinity for water molecules, evident from the different polar components γP in their surface free energy from the Owens-Wendt analysis. The average BM cell size was the smallest for the glass substrate (λBM = 289 μm) and comparable for the unmodified and UV/ozone-treated silicon wafers (with λBM = 466 and 423 μm, respectively). The fractal dimension analysis provided a quantitative description of the BM cells with complex structural hierarchy, highlighting the differences in the geometric features of the surface patterns resulting from different substrate chemistry. We also found that the fractal dimensions depended on the BM cell size, attributing it to two different regimes: the growing fractals and the maturing fractals.

Estimation of Size and Contact Angle of Evaporating Sessile Liquid Drops Using Texture Analysis

Size and contact angle of liquid drops are fundamental parameters in interfacial science. Accurate estimation of these parameters can provide objective information regarding several properties of the contacting surface. We leveraged principles of texture analysis to estimate the contact angle and the drop diameter from videos of evaporating sessile liquid drops deposited on solid surfaces. Specifically, we used a Harris corner detector to locate the corners and dynamically estimate the changing size and a Gabor wavelet-based approach to estimate the varying contact angle of the evaporating sessile drop. We demonstrated the ability of our approach to accurately estimate size and contact angles of drops deposited on a hydrophilic glass slide and on a paraffin film representing a hydrophobic surface. We also estimated the contact angle and size of drops deposited on horizontal and tilted surfaces to generate symmetric and asymmetric drop shapes, respectively. A software application that has the ability to analyze videos of sessile liquid drops as inputs is provided, and this tool can generate plots of the estimated contact angle and the drop diameter as a function of frame number.

Quantitative measurements of nanoparticle layer thicknesses near the contact line region after droplet drying-out

Confocal fringe patterns of evaporating sessile drops have provided initial evidence of the presence of a sub-micron thin liquid film emanating from an evaporating sessile droplet. The droplets studied were seeded with 50 nm aluminum oxide particles. Natural evaporation occurs on the substrates that are varied in wettability with static apparent contact angles as θ = 20o, 33o, 85o and 105o for a DI-water. Subsequent SEM, EDX and AFM characterization demonstrates the existence of aluminum oxide nanoparticle deposition in the submicron region. The profile of stuck nanoparticles shows nanometer-scaled height that is comparative in thickness to a transition film. Moreover, the radial extent of the deposition is consistent around the periphery of the original droplet and is found to be dependent upon the surface wettability. This study shows experimentally that the relative length of nanoparticle deposition increases with the surface wettability.

Sessile drop evaporation under an electric field

In this study, for the first time, the natural (diffusion-limited) evaporation of a sessile drop under an electric field was experimentally examined. A sessile drop natural evaporation is affected by the geometry of the drop, e.g. baseline, contact angle, and surface area, which all can be changed in the presence of an electric field. As such, first, the effect of electric field on sessile water droplet geometry was studied, together with how it differs for surfaces with various contact angle and contact angle hysteresis (for both hydrophilic and hydrophobic surfaces). Then, the dynamics (evaporation time and mode) of natural evaporation of sessile droplet under an electric field was studied by measuring the evaporation rate, surface area, contact angle, and baseline of the droplet. It is found that compare to when there is no electric field, the evaporation time of a sessile droplet increases when there is an electric field. This was statistically shown to be significant for drops placed on surfaces with contact angle hysteresis lower than 32°. For droplets that evaporate in constant baseline mode initially and then by evaporation in constant contact angle mode, the presence of the electric field resulted in a decrease in the duration of constant baseline mode when the surface had a low contact angle hysteresis.

A new model to predict the influence of surface temperature on contact angle

The measurement of the equilibrium contact angle (ECA) of a weakly evaporating sessile drop becomes very challenging when the temperatures are higher than ambient temperature. Since the ECA is a critical input parameter for numerical simulations of diabatic processes, it is relevant to know the variation of the ECA with the fluid and wall temperatures. Several research groups have studied the effect of temperature on ECA either experimentally, with direct measures, or numerically, using molecular dynamic simulations. However, there is some disagreement between the authors. In this paper two possible theoretical models are presented, describing how the ECA varies with the surface temperature. These two models (called Decreasing Trend Model and Unsymmetrical Trend Model, respectively) are compared with experimental measurements. Within the experimental errors, the equilibrium contact angle shows a decrease with increasing surface temperatures on the hydrophilic surface. Conversely the ECA appears approximately constant on hydrophobic surfaces for increasing wall temperatures. The two conclusions for practical applications for weakly evaporating conditions are that (i) the higher the ECA, the smaller is the effect of the surface temperature, (ii) a good evaluation of the decrease of the ECA with the surface temperature can be obtained by the proposed DTM approach.

Morphogenesis of polycrystalline dendritic patterns from evaporation of a reactive nanofluid sessile drop

We report polycrystalline residual patterns with dendritic micromorphologies upon fast evaporation of a mixed-solvent sessile drop containing reactive ZnO nanoparticles. The molecular and particulate species generated in situ upon evaporative drying collude with and modify the Marangoni solvent flows and B\'enard-Marangoni instabilities, as they undergo self-assembly and self-organization under conditions far from equilibrium, leading to the ultimate hierarchical central cellular patterns surrounded by a peripheral coffee ring upon drying.

Dynamics of evaporating drop on heated surfaces with different wettabilities

The dynamics of evaporating sessile drop on a uniformly heated, horizontal, solid substrate is considered. On the basis of lubrication theory and Navier slip condition, an evolution equation for the height of the two-dimensional drop is established. The numerical results show that the drop evolution is governed by capillary force, gravity, thermal capillary force and evaporation. Gravity exerts a promoting effect on drop spreading, while capillary force and thermal capillary force inhibit drop spreading. The typical dynamic features including contact line pinning or partial pinning modes during the drop evaporation are illustrated by changing the temperature-sensitive coefficient in the present model, and the drop lifetime of contact pinning mode is found to be shorter than that of contact line partial pinning mode. Under the same temperature-sensitive coefficient of three interfaces, the drop evolution is indicated with three typical stages: 1) spreading stage, 2) contact line pinning stage, and 3) both contact line and contact angle decreasing stage. As interface tension of liquid-gas or liquid-solid is more sensitive to temperature, the drop evolution is divided into two typical stages: 1) spreading stage and 2) contact line pinning stage. The equilibrium contact angle tends to be smaller and the substrate wettability is improved, leading to the increased spreading speed, the prolonged time of the contact line to reach pinning: the faster the evaporation rate, the shorter the lifetime of drop is. Additionally, the same effect of sensitivity of liquid-gas and liquid-solid interface tension to temperature on the wettability of substrate is observed. When the interface tension of solid-gas is more sensitive to temperature, the drop evolution is characterized in four typical stages: 1) spreading stage, 2) contact line pinning stage, 3) contact line de-pinning and constant contact angle stage, and 4) both contact line and contact angle decreasing stage. The equilibrium contact angle tends to be greater and the substrate wettability is deteriorated, leading the spreading speed to decrease. Hence, it is more effective to manipulate the drop movement in the presence of evaporation by regulating the temperature-sensitive coefficient of the solid-gas interface.

Correlation for sessile drop evaporation over a wide range of drop volatilities, ambient gases and pressures

A correlation for the evaporation of sessile drops over a very broad range of conditions was developed based on measured evaporation rate data obtained for drops of acetone, methanol, and six hydrocarbons ranging from hexane to isooctane, evaporating in air, helium, argon, and krypton, over a range of ambient pressures from 96 kPa to 615 kPa. The experiments were designed to produce a large variation in the rates of diffusion and buoyancy-induced (natural) convection of the vapor phase amongst the experimental conditions. The correlation, which fits the measurements with an RMS relative error of 5.2%, is a simple equation involving conventional parameters for diffusive and convective transport and is applicable to conditions for which vapor transport limits the rate of evaporation. Application of the correlation requires knowledge of eight basic properties: the ambient pressure and temperature, the equilibrium vapor pressure of the evaporating component, the diffusion coefficient for the evaporating component in the ambient gas, the viscosity of the ambient gas, the radius of the sessile drop, and the molecular weights of the evaporating component and the ambient gas. The correlation is much easier to implement than a computational model based on the coupled conservation equations of mass, energy, and momentum for the two phases, and it offers a single mathematical expression that provides valuable insight into how the roles of diffusive and convective transport change with physical and geometrical parameters. The correlation can be a valuable tool to aid in the analyses of applications involving sessile drop evaporation and to support the validation of complex computational models.The range of experimental conditions resulted in a large variation in the rates of diffusive and naturally convective transport of the vapor. Over the range of experimental conditions, the liquid volatility, as indicated by the equilibrium vapor pressure, was varied by a factor of 16.7, the mass diffusivity by a factor of 52.2, the density difference ratio (the impetus for natural convection) by a factor of 3557, and the drop radius by a factor of 22. In terms of the Rayleigh number, the experimental data covers a range from 5 to 361,000. Consequently, the correlation is applicable to a very broad range of conditions. To our knowledge these evaporation rate measurements of sessile drops in gases other than air and at pressures above one atmosphere are the first to be reported in the literature.

Fluid flow inside and outside an evaporating sessile drop

The sessile drop evaporation is a phenomena which is extensively studied in the literature, but the governing effects are far from being well understood especially those involving movements taking place in both liquid and gas phases. The present work numerically studies the flow within and around an evaporating sessile drop. The flow is induced by the strong mass loss at contact line, the thermo-capillary effect and the buoyancy effect in the surrounding air. The results showed that buoyancy-induced flow in gas phase weakly influences thermo-capillarity-induced flow in the liquid phase. Buoyancy effect can strongly modify the temperature distribution at liquid-gas interface and thus the overall evaporation rate of the drop when the substrate is heated.

Structure, morphology and wettability studies on Langmuir-Schaefer multilayer of poly(vinylidene fluoride)/poly(methyl methacrylate) blends

The intermolecular interaction between the polymer chains of different functionality is known to influence the aggregation and molecular chain packing. This way, it is possible to tune the structure, stability and surface quality through the blending of polymers. With this in focus, a systematic studies was carried out to understand structural, morphological and wettability variation in Langmuir-Schaefer multilayer of poly(vinylidene fluoride)/poly(methyl methacrylate) blends. Grazing incidence X-ray diffraction studies on blends showed a Bragg peak around 20.5° indicating the predominant presence of polar β phase up to 40% of poly(methyl methacrylate). Its fraction and dielectric environment are estimated using Fourier transform infrared spectroscopy and fluorescence studies, respectively. Morphological features of the blends studied using field emission scanning electron microscope showed a drastic change with increasing fraction of poly(methyl methacrylate). The surface quality and wettability of blends were assessed using contact angle studies. Contact angle and the volume of the sessile drop linearly decrease with time. A relatively faster evaporation of sessile drop was observed for higher fraction of poly(methyl methacrylate) attributed to its hydrophilicity. The evaporation kinetics was found to be largely dependent on the surface quality and chemical nature of poly(vinylidene fluoride)/poly(methyl methacrylate) multilayer.

Infra-Red measurements of an evaporating meniscus with imposed contact angle

In this paper temperature profiles in a meniscus formed by ethanol at the mouth of a capillary tube with the meniscus' shape dependent on the liquid supply rate are studied as evaporation occurs. An Infra-Red camera is used to measure the meniscus interfacial temperature and the results are compared for four tube sizes and three tube materials. This work follows on from work on evaporating sessile drops which has attracted much scientific attention in recent years. It is found that when the meniscus is still inside the tube, the interface temperature profile is similar to that of previous work (Buffone and Sefiane [19]). The interesting finding is that as the meniscus becomes flat at the tube mouth, the temperature profile of the interface evens out to the temperature of the contact line region. When the meniscus is eventually pushed out of the tube, the temperature of the meniscus interface becomes non-uniform again (depending on the contact angle). We demonstrate that this phenomenon is consistent for different tubes sizes and tube materials (with different wettabilities). We develop a simplified model for the meniscus inside the tube and the meniscus outside the tube which agrees qualitatively with the experimental findings.