Evaporation Of Sessile Droplets Research Articles

Evaporation of Sessile Droplets Laden with Particles and Insoluble Surfactants.

We consider the flow dynamics of a thin evaporating droplet in the presence of an insoluble surfactant and noninteracting particles in the bulk. On the basis of lubrication theory, we derive a set of evolution equations for the film height, the interfacial surfactant, and bulk particle concentrations, taking into account the dependence of liquid viscosity on the local particle concentration. An important ingredient of our model is that it takes into account the fact that the surfactant adsorbed at the interface hinders evaporation. We perform a parametric study to investigate how the presence of surfactants affects the evaporation process as well as the flow dynamics with and without the presence of particles in the bulk. Our numerical calculations show that the droplet lifetime is affected significantly by the balance between the ability of the surfactant to enhance spreading, suppressing the effect of thermal Marangoni stresses-induced motion, and to hinder the evaporation flux through the reduction of the effective interfacial area of evaporation, which tend to accelerate and decelerate the evaporation process, respectively. For particle-laden droplets and in the case of dilute solutions, the droplet lifetime is found to be weakly dependent on the initial particle concentration. We also show that the particle deposition patterns are influenced strongly by the direct effect of the surfactant on the evaporative flux; in certain cases, the "coffee-stain" effect is enhanced significantly. A discussion of the delicate interplay between the effects of capillary pressure and solutal and thermal Marangoni stresses, which drive the liquid flow inside of the evaporating droplet giving rise to the observed results, is provided herein.

Evaporation dynamics and sedimentation pattern of a sessile particle laden water droplet

The dynamics of the flow inside an evaporating sessile droplet of water with polystyrene micro-spheres of 1.0 μm in diameter in suspension is described. The initial volume of the droplets is in the range from 0.6 to 1.0 μl, and observations were made in the last stages before total evaporation. The flow was recorded in a sequence of images that were analyzed with a micro-PIV system to extract quantitative information. Also, using image analysis techniques we determined the dynamics of the retreating liquid film once unpinned from the original contact line. Additionally, we have explored its correlation to the formation of the sediment pattern which is organized in elongated mounds roughly deposited in azimuthal and radial orientations. It is found that the aggregation dynamics of micro-spheres in the segments of the two orientations is different. This might have a substantial influence on the final arrangement of micro-spheres in the sediments.

Velocity field measurements in an evaporating sessile droplet by means of micro-PIV technique

Velocity fields are measured in evaporating sessile droplets on two substrates with different contact angles and contact angle hysteresis using micro resolution particle image velocimetry technique. Different flow patterns are observed in different stages of droplet evaporation: a flow with vortices and a radial flow. Flow structure is found to be similar for droplets on different substrates.

Towards universal buckling dynamics in nanocolloidal sessile droplets: the effect of hydrophilic to superhydrophobic substrates and evaporation modes.

The evaporation of a nanocolloidal sessile droplet exhibits preferential particle assembly, nanoporous shell formation and buckling to form cavities with unique morphological features. Here, we have established many universal trends that explain the buckling dynamics under one umbrella irrespective of hydrophobicity, evaporation mode and particle loading. We provide a regime map explaining the droplet morphology and buckling characteristics for droplet evaporation on various substrates. Specifically, we find that the final droplet volume and the radius of curvature at the buckling onset are universal functions of particle concentration. Furthermore, we establish that post-buckling cavity growth is evaporation driven regardless of the substrate.

Effect of Marangoni Flows on the Shape of Thin Sessile Droplets Evaporating into Air.

Freely receding evaporating sessile droplets of perfectly wetting liquids, for which the observed finite contact angles are attributed to evaporation, are studied with a Mach-Zehnder interferometer. The experimentally obtained droplet shapes are found to depart, under some conditions, from the classical macroscopic static profile of a sessile droplet. The observed deviations (or the absence thereof) are explained in terms of a Marangoni flow due to evaporation-induced thermal gradients along the liquid-air interface. When such a Marangoni effect is strong, the experimental profiles exhibit a maximum of the slope at a certain distance from the contact line. In this case, the axisymmetric flow is directed from the contact line to the apex (along the liquid-air interface), hence delivering more liquid to the center of the droplet and making it appear inflated. These findings are quantitatively confirmed by predictions of a lubrication model accounting for the impact of the Marangoni effect on the droplet shape.

Contribution of convective transport to evaporation of sessile droplets: Empirical model

Despite the fact that sessile droplets evaporation dynamics has been studied for more than half a century, the scientific community struggles with the creation of an accurate quantitative description of the rate of evaporation. The classically used description considers evaporation as a quasi-steady process controlled by the diffusion of vapour into the air and the whole system is assumed to be isothermal at the ambient temperature. However, when two types of fluids (alcohols and alkanes) are let to evaporate on heated substrates, their evaporation rates tend to be underestimated by this model, mostly due to convection. This experimental study aims to understand how atmospheric convective transport in the vapour phase influences evaporation in order to develop an empirical model that accurately describes the evaporation rate. The Rayleigh number is used to analyse the contribution of natural convection and an empirical model is developed combining diffusive and convective transport for each type of fluid. The influence of the molecular chain length (and the increasing number of carbon atoms) is also being discussed.

Internal fluid motion and particle transport in externally heated sessile droplets

Experiments and numerical simulations were carried out for an evaporating sessile droplet. The droplet was confined in the narrow gap between two glass plates, making it a “Hele‐Shaw” droplet and particle image velocimetry technique was used. In case of the evaporating droplet with pinned contact line and exposed to ambient condition, two symmetric but counterrotating convection cells were observed. After complete evaporation, the particles deposited on the substrate near the contact line. The direction of the flow was reversed for a droplet placed on uniformly heated substrate, and the final deposition pattern was a large spot at the center with a thin line at the periphery. For asymmetrically heated substrate a single convection cell appeared, and the final deposition was also asymmetric. When the liquid was subjected to localized heating, the contact line no longer remains pinned and a relatively uniform deposition was obtained after complete drying. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1308–1321, 2016

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

Over the past decade, the most common approach to creating liquid shedding surfaces has been to amplify the effects of nonwetting surface chemistry, using micro/nanotexturing to create superhydrophobic and superoleophobic surfaces. Recently, an alternative approach using impregnation of micro/nanotextured surfaces with immiscible lubricating liquids to create slippery liquid-infused porous surfaces (SLIPS) has been developed. These types of surfaces open up new opportunities to study the mechanism of evaporation of sessile droplets in zero contact angle hysteresis situations where the contact line is completely mobile. In this study, we fabricated surfaces consisting of square pillars (10-90 μm) of SU-8 photoresist arranged in square lattice patterns with the center-to-center separation between pillars of 100 μm, on which a hydrophobic coating was deposited and the textures impregnated by a lubricating silicone oil. These surfaces showed generally low sliding angles of 1° or less for small droplets of water. Droplet profiles were more complicated than on nonimpregnated surfaces and displayed a spherical cap shape modified by a wetting ridge close to the contact line due to balancing the interfacial forces at the line of contact between the droplet, the lubricant liquid and air (represented by a Neumann triangle). The wetting ridge leads to the concept of a wetting "skirt" of lubricant around the base of the droplet. For the SLIP surfaces, we found that the evaporation of small sessile droplets (∼2 mm in diameter) followed an ideal constant contact angle mode where the apparent contact angle was defined from the intersection of the substrate profile with the droplet spherical cap profile. A theoretical model based on diffusion controlled evaporation was able to predict a linear dependence in time for the square of the apparent contact radius. The experimental data was in excellent quantitative agreement with the theory and enabled estimates of the diffusion constant to be obtained.

Effect of Surface Reactivity on Watermark Formation Studied By Sessile Droplet Evaporation Approach

: Wet nanoscale etching of silicon is very dependent to surface reactive sites like step edges, defect densities, and crystallographic planes [1]. Ultra-pure water (UPW) used during rinsing and drying steps is etching silicon in the sub-nanometer range furthermore this silica residues will cause ring shape drying marks on the surface [2-3]. In this paper drying marks are characterized by physical and chemical characterization techniques to evaluate the silicon etching process. The conversion of the silicon hydrogen bond to silicon hydroxide is investigated on different crystal orientations to monitor the reactivity of the surface. Experiment & Measurement: The p-type (100) and (111) 200 mm silicon wafers (SunEdison) used had a resistivity of 1-5 Ω.cm. The samples were cleaned with an HF/HCl mixture to obtain a hydrophobic surface with a contact angle of around 70◦. Atomically smooth silicon (111) was prepared by a NH4F (Sigma-Aldrich 99%) etching process in a closed oxygen free etching cell [4]. Double side polished silicon (100) and (111) wafer were used as a crystal in attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) to monitor the hydrogen termination. The 3D profile of the WM formed after evaporation of a UPW sessile droplet was measured using a KLA-Tencor High Resolution Profilometer [3]. Results & Discussion:The impact of the crystal orientation on the WM residue is studied by comparing the results of droplet drying on p-type Si (100) versus (111) in the dark (40% relative humidity). Although AFM analysis confirms a smooth surface finish (RMS = 0.2 nm) for the HF treated (100) and (111) surface, no clear atomic planes are observed. Fig. 1 shows the etch rate of silicon in the dark under clean room conditions. The etch rate is calculated based on residue volume (measured by profiliometry) divided by the droplets initial wetting area, and evaporation time. Fig. 2 shows the decrease in the amplitude of the silicon hydride peak, measured by ATR-FTIR spectroscopy, as a function of time (the rate of decrease is faster in the Si (100), case as might be expected). The fact that hydride termination of the surface disappears only slowly gives information about the etching mechanism, that will be discussed in the proceeding. The higher stability of the (111) surface is attributed to the higher number of back-bonds for Si-surface atoms in case of (111) (with 3 back-bonds) versus the (100) case (with 2 back-bonds). The experimental result confirms the chemical dissolution process but a difference in surface stability is observed. WM formation was studied in the dark since previous work [3] showed that light increases the etch rate markedly, thus complicating the interpretation. In order to investigate the impact of surface morphology on WM formation, atomically smooth surfaces were prepared by immersing a p-type Si (111) substrates (miscut angle <0.07º) in oxygen free NH4F. The results are shown in Figure 3. By comparing the result of HF-pretreated and NH4F-pretreated surfaces it is clear that the atomically smooth surface shows reduced reactivity compared to the rough HF last surface. Due to lower step edge density the chemical reactivity of the surface decreases and WM residue formation is suppressed. Conclusion: This study showed the effect of surface imperfections on dissolution process by WM formation. It is shown that the silicon (111) dissolution process decreased in case of atomically smooth surface. The dissolution can be further suppressed by performing rinsing and drying process in dark ambient.

Effect of Poly(ethylene oxide) Molecular Weight on the Pinning and Pillar Formation of Evaporating Sessile Droplets: The Role of the Interface.

We report on the drying process of sessile droplets of aqueous poly(ethylene oxide) (PEO) solutions studied by contact angle analysis. Liquid samples were prepared with the same initial concentration of four different molecular weights, Mw, of PEO. Droplets with initial volumes of between 1 and 5 μL were left to evaporate while temperature, pressure, and relative humidity were kept constant. Residues were formed with either a disklike puddle or a distinctive tall conical pillar shape. The latter occurred following a four-stage deposition process: pinned drying, during which the contact line is stationary; pseudodewetting, where the receding contact line is induced by precipitation; bootstrap building, during which the liquid droplet is lifted on freshly precipitated solid; and late drying. Contact angle analysis allowed us to monitor all stages during drying and consider transitions between stages for different molecular weights. We illustrate the mechanisms taking place during the crucial stages of pinning and depinning, revealing the effect of adhesion and contact line friction for high molecular weights and its influence on the final morphology of the dried PEO solute. To this end, we performed PEO solution droplet evaporation on PEO and PTFE films demonstrating the importance of interfacial interaction phenomena. We show that the formation of disklike puddles for high molecular weights on glass is associated with continuous droplet contact line pinning. This results from the strong adhesion due to the interdigitation of the loops and tails of a polymer layer (adsorbed on glass during evaporation) with the polymer gel network inside the droplet that forms as water evaporates.

Thermodynamic Behaviors of Macroscopic Liquid Droplets Evaporation from Heated Substrates

Evaporation of a macroscopic-scale sessile droplet on different hot isothermal substrates has been experimentally investigated, for the framework of planning space experiments onboard Chinese recoverable satellite to explore the interface effect, heat and mass transfer during the phase transition process. Undoubtedly, the evaporation phenomenon of a sessile drop on heated substrates is a complex problem which involves the behavior of triple line, heat transfer with thermal conduction and convection, mass transfer into the vapor phase. Therefore, preparations from scientific view have been carried out to validate setup of the space experiment modes. Based on the experiments performed in the terrestrial gravity, we found that the evolution of a water droplet could be separated into three stages, began with the constant contact area, then switched to the depin stage and ended up with the flushing stage. The average evaporation rate was measured and the thermal effects of different substrates were studied. Results revealed a linear variation of contact diameter with its average evaporation rate, which has the similar tendency with small drops. The varieties of the heat flux density during evaporating showed that droplet absorbed energy from the heated substrate, then with the help of the internal flow of thermocaplliry and buoyant convection, heat was transported to the liquid-vapor interface providing the energy for evaporation.

Evaporation of nanofluid droplet on heated surface

In this study, an experiment on the evaporation of nanofluid sessile droplet on a heated surface was conducted. A nanofluid of 0.5% volumetric concentration mixed with 80-nm-sized CuO powder and pure water were used for experiment. Droplet was applied to the heated surface, and images of the evaporation process were obtained. The recorded images were analyzed to find the volume, diameter, and contact angle of the droplet. In addition, the evaporative heat transfer coefficient was calculated from experimental result. The results of this study are summarized as follows: the base diameter of the droplet was maintained stably during the evaporation. The measured temperature of the droplet was increased rapidly for a very short time, then maintained constantly. The nanofluid droplet was evaporated faster than the pure water droplet under the experimental conditions of the same initial volume and temperature, and the average evaporative heat transfer coefficient of the nanofluid droplet was higher than that of pure water. We can consider the effects of the initial contact angle and thermal conductivity of nanofluid as the reason for this experimental result. However, the effect of surface roughness on the evaporative heat transfer of nanofluid droplet appeared unclear.

Transient effects on sessile droplet evaporation of volatile liquids

For a very volatile liquid, a generally used quasi-steady diffusion model often fails in representing accurately an evaporation phenomenon. The substrate thermal diffusivity and the liquid volatility rule this assumption. The present study uses an unsteady diffusion model to analyze the effect of these two parameters. Heat diffusion in solid, liquid and gas phases and mass diffusion in the surrounding air are solved by a finite volume method associated with an ADI scheme. In order to investigate the effects of volatility, water, ethanol and methanol are considered on very thin substrates of aluminum or PTFE. Thicker substrates are then used to analyze the effect of their thermal properties under non-heating and heating conditions. For an unheated substrate, the volatility reduces the droplet lifetime, but has little influence on the unsteady effects. But, the thermal diffusivity of a heated substrate strongly affects the evaporation rate. Unsteady effects are important for a small substrate thermal diffusivity. Their duration can be more than 80% of the drop lifetime for an ethanol droplet on PTFE. Consequently, the quasi-steadiness assumption does not hold in this case, while it remains valid on a heated aluminum substrate.

Effect of substrate temperature on pattern formation of nanoparticles from volatile drops.

This study investigates pattern formation during evaporation of water-based nanofluid sessile droplets placed on a smooth silicon surface at various temperatures. An infrared thermography technique was employed to observe the temperature distribution along the air-liquid interface of evaporating droplets. In addition, an optical interferometry technique is used to quantify and characterize the deposited patterns. Depending on the substrate temperature, three distinctive deposition patterns are observed: a nearly uniform coverage pattern, a "dual-ring" pattern, and multiple rings corresponding to "stick-slip" pattern. At all substrate temperatures, the internal flow within the drop builds a ringlike cluster of the solute on the top region of drying droplets, which is found essential for the formation of the secondary ring deposition onto the substrate for the deposits with the "dual-ring" pattern. The size of the secondary ring is found to be dependent on the substrate temperature. For the deposits with the rather uniform coverage pattern, the ringlike cluster of the solute does not deposit as a distinct secondary ring; instead, it is deformed by the contact line depinning. In the case of the "stick-slip" pattern, the internal flow behavior is complex and found to be vigorous with rapid circulating flow which appears near the edge of the drop.

Precision stacking of nanoparticle laden sessile droplets to control solute deposit morphology

Stacking pure solvent droplets on a solid substrate is apparently impossible in the absence of an external force as the second droplet will invariably spill over the first leading to a large wetted area. However, the unique feature that emerges during the drying of a nanoparticle laden droplet is the progressively enlarging thin solid film along the evaporating sessile droplet liquid periphery. This solid interface: the edge of which we shall refer to as the agglomeration front comprises of a thin layer of nanoparticle assembly and can support a carefully dispensed second droplet thereby allowing droplet stacking. It will be shown that the growth of this agglomeration front can also be effectively controlled by the dispensing time difference and the nanoparticle concentration in the two droplets. So far, we are commonly aware of material stacking in solid phase. This letter demonstrates stacking in the liquid phase and control over the thin solid interface growth.

Dependence of fluid flows in an evaporating sessile droplet on the characteristics of the substrate

Temperature distributions and the corresponding vortex structures in an evaporating sessile droplet are obtained by performing detailed numerical calculations. A Marangoni convection induced by thermal conduction in the drop and the substrate is demonstrated to be able to result not only in a single vortex, but also in two or three vortices, depending on the ratio of substrate to fluid thermal conductivities, on the substrate thickness and the contact angle. The “phase diagrams” containing information on the number, orientation and spatial location of the vortices for quasistationary fluid flows are presented and analysed. The results obtained demonstrate that the fluid flow structure in evaporating droplets can be influenced in a controlled manner by selecting substrates with appropriate properties.

Heat Flux at the Surface of Metal Foil Heater under Evaporating Sessile Droplets

Evaporating water drops on a horizontal heated substrate were investigated experimentally. The heater was made of a constantan foil with the thickness of 25 μm and size of 42 × 35 mm2. The temperature of the bottom foil surface was measured by the infrared (IR) camera. To determine the heat flux density during evaporation of liquid near the contact line, the Cauchy problem for the heat equation was solved using the temperature data. The maximum heat flux density is obtained in the contact line region and exceeds the average heat flux density from the entire foil surface by the factor of 5–7. The average heat flux density in the region wetted by the drop exceeds the average heat flux density from the entire foil surface by the factor of 3–5. This fact is explained by the heat influx from the foil periphery to the drop due to the relatively high heat conductivity coefficient of the foil material and high evaporation rate in the contact line region. Heat flux density profiles for pairs of sessile droplets are also investigated.

Buoyancy-induced on-the-spot mixing in droplets evaporating on nonwetting surfaces.

We investigate hitherto-unexplored flow characteristics inside a sessile droplet evaporating on heated hydrophobic and superhydrophobic surfaces and propose the use of evaporation-induced flow as a means to promote efficient "on-the-spot" mixing in microliter-sized droplets. Evaporative cooling at the droplet interface establishes a temperature gradient that induces buoyancy-driven convection inside the droplet. An asymmetric single-roll flow pattern is observed on the superhydrophobic substrate, in stark contrast with the axisymmetric toroidal flow pattern that develops on the hydrophobic substrate. The difference in flow patterns is attributed to the larger height-to-diameter aspect ratio of the droplet (of the same volume) on the superhydrophobic substrate, which dictates a single asymmetric vortex as the stable buoyancy-induced convection mode. A scaling analysis relates the observed velocities inside the droplet to the Rayleigh number. On account of the difference in flow patterns, Rayleigh numbers, and the reduced solid-liquid contact area, the flow velocity is an order of magnitude higher in droplets evaporating on a superhydrophobic substrate as compared to hydrophobic substrates. Flow velocities in all cases are shown to increase with substrate temperature and droplet size: The characteristic time required for mixing of a dye in an evaporating sessile droplet is reduced by ∼8 times on a superhydrophobic surface when the substrate temperature is increased from 40 to 60 °C. The mixing rate is ∼15 times faster on the superhydrophobic substrate compared to the hydrophobic surface maintained at the same temperature of 60 °C.

Evaporation of sessile droplets affected by graphite nanoparticles and binary base fluids.

The effects of ethanol component and nanoparticle concentration on evaporation dynamics of graphite-water nanofluid droplets have been studied experimentally. The results show that the formed deposition patterns vary greatly with an increase in ethanol concentration from 0 to 50 vol %. Nanoparticles have been observed to be carried to the droplet surface and form a large piece of aggregate. The volume evaporation rate on average increases as the ethanol concentration increases from 0 to 50 vol % in the binary mixture nanofluid droplets. The evaporation rate at the initial stage is more rapid than that at the late stage to dry, revealing a deviation from a linear fitting line, standing for a constant evaporation rate. The deviation is more intense with a higher ethanol concentration. The ethanol-induced smaller liquid-vapor surface tension leads to higher wettability of the nanofluid droplets. The graphite nanoparticles in ethanol-water droplets reinforce the pinning effect in the drying process, and the droplets with more ethanol demonstrate the depinning behavior only at the late stage. The addition of graphite nanoparticles in water enhances a droplet baseline spreading at the beginning of evaporation, a pinning effect during evaporation, and the evaporation rate. However, with a relatively high nanoparticle concentration, the enhancement is attenuated.

Self-assembly of colloidal sulfur particles on a glass surface from evaporating sessile drops: influence of different salts

Evaporation of sessile droplets containing colloidal particles generates different self-assembled fractal patterns apart from the well-known “coffee-ring” effect, which have many applications. In this work, the formation of a self-assembled fractal pattern using colloidal sulfur particles (synthesized in situ by the reaction of sodium thiosulfate and different inorganic or organic acids) on the glass surface was investigated with the help of optical microscopy. The results indicate that the self-assembled structure is highly dependent on the type of acid (mono-, di-, or tri-) used for the reaction, which in turn forms different salts after completion of the reaction. Particles alone form the “coffee-ring” structure without forming any self-assembled fractal pattern, but the presence of salt in the reaction media helps to form different fractal patterns. Fractal patterns are, in fact, influenced by the crystal morphology of salts.