Drop Evaporation Research Articles

Vapor concentration and temperature field measurement of an evaporating sessile drop by tomographic laser absorption spectroscopy

Drop evaporation is a ubiquitous phenomenon that has been studied for over a century. However, the surrounding gas-phase field including the temperature and vapor concentration distribution is not sufficiently studied experimentally. In this paper, a sensor based on tunable laser absorption spectroscopy is designed to study the vapor-phase temperature and concentration distribution of evaporating sessile drops, and data processing method involving data pre-processing and tomographic reconstruction is proposed to realize high-precision, spatially resolved measurement, which was realized by scanning the mechanical galvanometer in the horizontal direction. With free-knot splines smoothing and “denucleated” onion-peeling algorithm, temperature and H2O concentration distributions surrounding the evaporated drop at three different substrate plate temperatures are observed. The concentration and temperature in close vicinity to the gas–liquid interface are reconstructed accurately despite the high-gradient changes. A spatial resolution of under 100 μm with a temporal resolution of 10 s has been realized. Quantitative depiction of the temperature and concentration fields shows evidence of convection and indicates that while the concentration level sharply peaks at the interface, temperature in the close vicinity to the drop shows flattening or even dipping trends. The in situ laser measurement results are validated against contact measurement, theoretical prediction with saturated vapor pressure, and model simulation of COMSOL. Uncertainties have been evaluated based on both repeated measurements and model prediction of input uncertainty propagation. Temperature and concentration measurement uncertainties are estimated to be <1.5% and <3.5%, respectively, even though all experiments were performed in open air with non-negligible buoyancy-induced convection.

A double rigidity transition rules the fate of drying colloidal drops.

The evaporation of drops of colloidal suspensions plays an important role in numerous contexts, such as the production of powdered dairies, the synthesis of functional supraparticles, and virus and bacteria survival in aerosols or drops on surfaces. The presence of colloidal particles in the evaporating drop eventually leads to the formation of a dense shell that may undergo a shape instability. Previous works propose that, for drops evaporating very fast, the instability occurs when the particles form a rigid porous solid, constituted of permanently aggregated particles at random close packing. To date, however, no measurements could directly test this scenario and assess whether it also applies to drops drying at lower evaporation rates, severely limiting our understanding of this phenomenon and the possibility of harnessing it in applications. Here, we combine macroscopic imaging and space- and time-resolved measurements of the microscopic dynamics of colloidal nanoparticles in drying drops sitting on a hydrophobic surface, measuring the evolution of the thickness of the shell and the spatial distribution and mobility of the nanoparticles. We find that, above a threshold evaporation rate, the drop undergoes successively two distinct shape instabilities, invagination and cracking. While permanent aggregation of nanoparticles accompanies the second instability, as hypothesized in previous works on fast-evaporating drops, we show that the first one results from a reversible glass transition of the shell, unreported so far. We rationalize our findings and discuss their implications in the framework of a unified state diagram for the drying of colloidal drops sitting on a hydrophobic surface.

Experimental studies of evaporation of paraffin drops

Paraffin is a low-melting material that can be used as an environmentally friendly and high-energy fuel. It can also be considered as a promising material with phase transitions capable of accumulating heat. It is known that the period of induction of ignition of paraffin droplets consists of successive stages, the main ones of which are melting and evaporation. The paper examines the processes of evaporation of octadecane and docosane droplets in air at different temperatures. A method of studying the kinetics of evaporation of paraffin fuel droplets using digital photography of the research object and computer image processing has been developed.
 It is proved that the evaporation kinetics of the studied alkanes is described by the linear law of the change of the square of the diameter of the drop over time. Evaporation rate constants were found. There are no such data in the literature for these paraffins. The use of evaporation constants made it possible to find the time of complete evaporation of drops and to estimate the diffusion coefficient of paraffin vapor in air at different temperatures. As a result of the experiments, it was established that the rate of evaporation of octadecane is more than 4 times higher than the rate of evaporation of docosane. It was found that when the air temperature reaches the specified values, the rate of evaporation of the specified alkanes increases significantly.

Investigation of single drop evaporation characteristics of two-component fatty acid methyl esters at high temperature

To investigate the evaporation and micro-explosion characteristics of blended droplets at 773 K and 973 K ambient temperatures, methyl palmitate (MP) and methyl oleate (MO) are selected as typical fuels in this study. The MP concentration ranges from 10 % to 50 % to investigate the micro-explosion characteristics of the blended droplets. No micro-explosion that occurs during its whole lifetime when MP is pure, but the micro-explosion can be observed in all blended droplets. Interestingly, the micro-explosion intensity, average evaporation rate, micro-explosion delay time, and evaporation duration of droplet all reach the optimum value when the MP concentration is 20 % at 773 K and 973 K. The micro-explosion can be divided into three categories by intensity, namely strong micro-explosion, weak micro-explosion, and puffing. The strong micro-explosions occur at 973 K when the concentration is 20 %. It can be inferred that micro-explosion is the critical factor in shortening the evaporation duration. Cleavage reaction that occurs in the MO produced bubbles to increase the micro-exploration intensity. Meanwhile, the polymerization that carried out in the MO inhibits the evaporation with producing solid. It is observed experimentally that the residual volume caused by the polymerization decreases with increasing MP concentration. Furthermore, a new relationship is established in terms of the evaporation rate, micro-explosion intensity and ambient temperature by numerical fitting.

Effect of temperature and surfactants on evaporation and contact line dynamics of sessile drops

Researches on droplet evaporation has a vast range of applications in different fields. The mechanisms of droplets evaporation have been a topic of investigation for decades. The droplet after spreading on the substrate exhibits constant evaporation rate for a constant base diameter and decreasing contact angle; followed by another evaporation mode with constant contact angle and decreasing base diameter; and finally, evaporation with decreasing of both contact angle and base diameter. In the present study, the evaporation rate and mechanisms influenced by surfactant and temperature effects are analysed. The evaporation was demonstrated to be related to the dynamics of the sessile droplet spreading ability and the depinning event. It remains complex to predict the evaporation of the sessile droplet due to the continuous change in surfactant local concentration. Moreover, the temperature change modifies the surface tension and the induced inner flows. These two questions find answers in the triple line dynamic which controls the evaporation rate. The forces controlling the depinning with surfactant was quantified. A clear constant force was found following depinning. Such dynamic equilibrium demonstrates also the effect of the friction, surface roughness and moreover the limit of surfactant concentration in the triple line direct vicinity. The evaporation rate shows two particularities: one is the classical decrease of the evaporation rate after depinning; the second in case of surfactant, there is an enhancement appearing at later droplet lifetime, which could be a consequence of film evaporation contributing to the evaporation near the triple line. The local evaporation does not change significantly despite the change in the contact angle with respect to surfactant concentration. The substrate temperature enhances evaporation rate. The coupled temperature and surfactant effect show a local evaporation rate decreasing with surfactant concentration increase at ambient temperature (around 20 °C) and enhancement with surfactant concentration at higher temperature.

Ultrasensitive Surface‐Enhanced Raman Spectroscopy Detection by Porous Silver Supraparticles from Self–Lubricating Drop Evaporation

AbstractThis work demonstrates an original and ultrasensitive approach for surface‐enhanced Raman spectroscopy (SERS) detection based on evaporation of self‐lubricating drops containing silver supraparticles. The developed method detects an extremely low concentration of analyte that is enriched and concentrated on sensitive SERS sites of the compact supraparticles formed from drop evaporation. A low limit of detection of 10−16 m is achieved for a model hydrophobic compound rhodamine 6G (R6G). The quantitative analysis of R6G concentration is obtained from 10−5 to 10−11 m. In addition, for a model micro‐pollutant in water triclosan, the detection limit of 10−6 m is achieved by using microliter sample solutions. The intensity of SERS detection in this approach is robust to the dispersity of the nanoparticles in the drop but became stronger after a longer drying time. The ultrasensitive detection mechanism is the sequential process of concentration, extraction, and absorption of the analyte during evaporation of self‐lubrication drop and hot spot generation for intensification of SERS signals. This novel approach for sample preparation in ultrasensitive SERS detection can be applied to the detection of chemical and biological signatures in areas such as environment monitoring, food safety, and biomedical diagnostics.

A New Two-Phase Multiplier for Flow Pressure Drop in Multiport Minichannel Condensers and Evaporators

Due to flow maldistribution, the condensation and evaporation flow patterns in multiport minichannels are considerably different from single minichannels or macrochannels. This article compiled the friction pressure drop data from experimental phase-change investigations in multiport minichannel condensers and evaporators over the past two decades. The data was reduced to friction pressure gradients, and a new two-phase multiplier was proposed for estimating the phase-change pressure drop in multiport condensers and evaporators. Thirteen hundred and forty-four condensation pressure drop and six hundred and twenty-three evaporation pressure drop data from twenty-nine studies were correlated to yield four predictive two-phase multiplier equations in the laminar and turbulent flow regimes. The predictive correlations fit 67% of the laminar condensation and 80% of the turbulent pressure drop data within ±50%. Further, 57% of the laminar evaporation and 100% of the turbulent evaporation pressure drop data were fit within ±50%. The correlations were compared with widely published correlations and were a significant improvement. Meta-analysis revealed that multiport minichannels are most effective for reducing turbulent flow condensation pressure drop and laminar flow evaporation pressure drop. The compiled data and presented correlations and analysis should be helpful to the process, electronics packaging, aviation, and aerospace industries designing compact, lightweight, and high-efficiency condensers, and evaporators.

Analytical solutions for modelling the evaporation of sessile drops

An analytical model for drop evaporation in gaseous environments that accounts for the effect of Stefan flow and temperature dependence of the thermo-physical properties is extended to the case of sessile drops. The exact solution to the conservation equations in toroidal coordinates based on the Mehler-Fock transform is reformulated in terms of simple functions by using the method of images for specific values of the contact angle. The method is then extended in approximate form to general values of the contact angles. The accuracy of this approximation is quantified for hydrophobic and hydrophilic cases, thus showing the applicability of the model to a wide range of wettability conditions.

Numerical model for sessile drop evaporation on heated substrate under microgravity

• One-sided numerical model is developed for sessile drop evaporation on heated substrate under microgravity. • Flow motion is analyzed by 3D resolved computations of evaporating sessile drop. • Transition from primary to secondary instability occurs at critical aspect ratio. • First time fine effects of secondary instabilities on evaporation rate are captured. Although sessile drops have simple geometries, the physics involved in their evaporation process is complex owing to their numerous intricate interactions and their fluid nature. An accurate quantitative model of the evaporation process will enable increased understanding and control over the process. In this study, a numerical model is developed for sessile drop evaporation on a heated substrate under microgravity based on the results of rocket and parabolic experiments to understand the ’internal dynamics of a sessile drop. The model is quantitatively validated through experiments. Subsequently, a correlation between substrate temperature and evaporation rate is suggested for an ethanol sessile drop. The flow motion is analyzed by conducting three-dimensional resolved computations of an evaporating sessile drop. This provides insights into the Marangoni effect in the dynamics of the evaporation process and the occurrence of secondary instabilities. For the first time, the fine effects of secondary instabilities on the evaporation rate are captured. Our numerical model is valid in the absence of convection in the vapor phase, producing an interface of an evaporating drop in a fully saturated vapor, which typically exists under microgravity conditions.

Evaporation and liquid-phase separation of ethanol–cyclohexane binary drops under acoustic levitation

Evaporation of cyclohexane and ethanol binary drops under acoustic levitation was investigated. The aim was to understand the effect of acoustic levitation on the evaporation dynamics and involved physical processes of the binary drops. We report the occurrence of liquid-phase separation of the binary drop during evaporation under acoustic levitation. Through systematic experiments, it was found that the enhanced evaporation rate of the drop under acoustic levitation led to a significant temperature decrease. In addition, driven by external acoustic streaming, water vapor tended to be enriched and condensed on the drop surface. Because ethanol is extremely soluble in water, tiny cyclohexane droplets were extracted from ethanol owing to water condensation. In addition, driven by the internal flow of the acoustically levitated drop, the extracted cyclohexane droplets coalesced, eventually resulting in macro-segregation in the drop. These findings provide new insights into the evaporation dynamics of acoustically levitated drops, thus, shedding light on industrial purification and separation of volatile liquids with opposite water solubilities, such as cyclohexane and ethanol.

Effect of Contact Angle Hysteresis on Evaporation Dynamics of a Sessile Drop on a Heated Surface

Contact angle hysteresis (CAH) is a significant factor affecting the drop motion on solid substrates. A model of CAH is introduced to explore the influence of CAH on the dynamics of a sessile drop on a uniformly heated surface, and a two-dimensional evolution equation of the drop thickness is established using the lubrication approximation and Navier slip boundary conditions. A numerical simulation is performed to examine the dynamic behaviors of an evaporating drop, and the drop profile, contact angle, contact line, and moving speed are investigated. Simulated results indicate that the drop evolution process involves drop spreading, pinning, and depinning of the contact line. In the drop spreading stage, when the hysteresis angle increases, the spreading period is shortened, and the spreading radius and spreading speed are reduced; in contrast, the pinning period is raised, and the mass of the drop is apparently reduced with increasing hysteresis angle. In the depinning stage, the CAH declines the contact angle, and a flatter pattern is evolved, thereby improving the heat transfer performance, promoting drop evaporation, and shortening the depinning time. The presence of CAH can speed up the drying of the drop, and the large hysteresis angle leads to faster evaporation. Regulating the CAH is an effective way to manipulate the motion of the contact line for an evaporating drop.

Evaporation of liquid coating a fiber

We investigate theoretically and numerically the diffusion-limited evaporation of a liquid deposited on a fiber in two configurations: a sleeve and an axisymmetric barrel-shaped droplet. For a sleeve, the local flux depends on both the aspect ratio and the smallest length of the problem. By using analytical calculations and 3D finite elements simulations, we predict a divergence of this flux further localized at the edge as the aspect ratio increases. The evaporation of axisymmetric drops on a fiber is studied with numerical simulations. For sufficiently large volumes, we evidence that the evaporation rate is almost independent of the wetting properties of the liquid, even for small contact angles, and that the droplets evaporate as spheres of the same volume.

Sessile drop evaporation approach to detect starch adulteration in milk

Milk adulteration is a major challenge faced by the dairy sector. Starch is a common milk adulterant with negative impact on consumer health. The major shortfalls in the present methods for detection and quantification of starch in milk are the higher limit of detection (LOD) and the need for additional chemical reagents. Hence, developing a chemical-free method with lower LOD for starch in milk is urgently required, especially in low- and middle-income countries. We propose a sessile drop evaporation-based approach to detect starch adulteration in milk. The influence of starch adulteration on the evaporative deposition patterns is studied using light microscopy as a function of dilution level (10–50%, v/v), starch type (potato and tapioca starches) and starch concentration (0.005–0.2%, w/v). The dried deposit patterns formed after drying drops of pristine and adulterated milk at 60 °C are dictated by the competing effects of outward capillary flow and inward Marangoni flows. Confocal Raman spectroscopy indicated the presence of fatty acids and other lipophilic constituents at the center, a combination of proteins and lipophilic constituents at the edge, and starch in regions close to the coffee stain, i.e., at the edge of the dried deposits. Higher level of water addition and lower level of starch adulteration resulted in the formation of coffee ring deposits with larger width. Surface profilometry indicated a decrease in the height of coffee-ring with increase in the concentration of both water and starch. The width and maximum height of edge deposits thus measured are proposed as the parameters to quantify starch concentration in milk. The proposed approach is effective in detecting starch concentration as low as 0.005% (w/v), as well as identifying the starch type, thereby facilitating facile, rapid, chemical-free, and in situ detection of starch adulteration in milk.

Modeling the evaporation of multicomponent drops of general shape on the basis of an analytical solution to the Stefan–Maxwell equations

This paper proposes an analytical approach to model the evaporation of multicomponent drops of general shape, which is based on the solution to Stefan–Maxwell equations. The model predicts the quasi-steady molar fractions and temperature distributions in the gas phase as well as the heat rate and the species evaporation rates. The model unifies previous approaches to this problem, namely, for spherical and spheroidal drops, under a unique model and proposes solutions for other shapes and geometries, such as sessile drops and drop pairs. To assess the model, a comparison with a numerical solution to the conservation equations is also reported for both different drop configurations and different compositions.

Chromatographic framework for coffee ring effect-driven separation of small molecules in surface enhanced Raman spectroscopy analysis

The applications of coffee ring effect (CRE) in analytical chemistry have been increasingly expanded from particles and macromolecules to small molecules, in particular coupled to surface-enhanced Raman spectroscopy (SERS). Despite the theory behind the formation of CRE itself from a single drop evaporation onto the dry surface is well established, the theoretical aspects of CRE-driven separation, especially the analyte-surface interactions involving small molecules, have not been conceived. Herein, we have developed a theoretical framework to describe the CRE-driven separation process of small molecules, using SERS analysis of dimethylarsinic acid (DMAV), dimethylmonothioarsinic acid (DMMTAV), and dimethyldithioarsinic acid (DMDTAV) on gold nanofilm (AuNF) as an example. By combining the CRE theory for the radial flow and the Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory for mass transfer between solution and AuNF surface, we adapted the conventional chromatographic theory to derive a modified van Deemter equation for the CRE-driven separation. By using this model, we predicted the travel distances of arsenicals based on the different affinity of analytes to the AuNF and evaluated the possibility of separation of unknown analytes by CRE-based SERS, demonstrating the successful adaptation of classic chromatographic theory to CRE-driven nanochromatography.

Spontaneous Condensation of RNA into Nanoring and Globular Structures.

The effect of polyvalent cations, like spermine, on the condensation of DNA into very well-defined toroidal shapes has been well studied and understood. A great effort has been made to obtain similar condensed structures from RNA molecules, but so far, it has been elusive. In this work, we show that single-stranded RNA (ssRNA) molecules can easily be condensed into nanoring and globular structures on a mica surface, where each nanoring structure is formed mostly by a single RNA molecule. The condensation occurs in a concentration range of different cations, from monovalent to trivalent, but at a higher concentration, globular structures appear. RNA nanoring structures were observed on mica surfaces by atomic force microscopy (AFM). The samples were observed in tapping mode and were prepared by drop evaporation of a solution of RNA in the presence of one type of the different cations used. As far as we know, this is the first time that nanorings or any other well-defined condensed RNA structures have been reported in the presence of simple salts. The RNA nanoring formation can be understood by an energy competition between the hydrogen bonding forming hairpin stems—weakened by the salts—and the hairpin loops. This result may have an important biological relevance since it has been proposed that RNA is the oldest genome-coding molecule, and the formation of these structures could have given it stability against degradation in primeval times. Even more, the nanoring structures could have the potential to be used as biosensors and functionalized nanodevices.

Numerical simulation of the evaporation process of pinned urea-water droplets in cavities

Modeling the evaporation process of pinned urea-water solution (UWS) droplets sitting on a heated surface is of large interest for reducing the deposit formation in Selective Catalytic Reduction (SCR) systems, which can lead to a significant loss in efficiency. The kinetics of diffusion-governed evaporation of sessile droplets having the shape of a spherical cap in an infinitely extended geometry has been extensively studied in the past. The present work is focused on modelling of the heat and mass transport in the gas phase during evaporation of drops with a size comparable with a capillary length. The model is valid for the wall temperatures below the saturation temperature, and the Rayleigh number is of order of 10. The influence of gravity on both the droplet shape and on the gas flow is taken into account. The droplet shape is determined by solving the Laplace-Young equation in cylindrical coordinates. The evaporation of droplets within a constrained surrounding geometry is considered. The domain geometry is close to the geometry of an experimental cell, in which validation tests have been performed. The simulations show that the influence of the droplet shape deviation from a spherical cap on the evaporation rate is negligible. The evolution of the droplet height and contact angle however are affected by the gravity. Furthermore, the evaporation rate is strongly affected by the natural convection and by the constrained geometry of the domain.

Flow and heat measurement within self-rewetting mixtures of 1-pentanol and water drops

Recently interest has arisen in the use of so-called self-rewetting mixtures for micro-scale heat transfer systems. Such fluids, in which the surface tension can increase with increasing temperature, are expected to offer superior evaporative cooling performance by extending the region of operation before dry-out of the heated surface sets in. Whilst improved performance has been shown in some practical situations using these fluids, it is not entirely clear as to the mechanism of such improvements. We have studied the flow within evaporating sessile drops of 1-pentanol-water mixtures using micro-PIV and have observed three stages in the evaporation process. During the first stage there appears to be a single toroidal vortex with flow inwards along the base of the drop. The vortex only occupies the central region of the drop and appears to pulsate, reducing in size during evaporation. This is followed by a second transition stage to a third stage in which the flow is directed radially outward, as observed by us for pure water droplet evaporation and in the latter stages of ethanol-water drop evaporation.

液滴蒸発と熱流動現象：赤外線カメラによるアプローチ

数多くの産業プロセスにおける素過程である液滴蒸発は，物質移動，伝熱，対流および濡れといった様々な熱流体現象が連成しており，適切な方法を用いることでその複雑かつ美しい現象を見て学ぶことができる．赤外線（IR）サーモグラフィは温度場や流れ場を乱すことなく液滴の温度分布情報を得ることができるので，この10年で使用が広がっている．しかし，IRカメラから直接得られた温度画像にはあまり意味がない場合がある．有意な温度情報を得るには，計測対象の赤外放射特性を考慮したデータ処理が必要である．本稿では，IR画像から液滴の気液界面温度を算出する方法を紹介する．これによって湿度環境下で蒸発するエタノール液滴の表面では，周囲の水蒸気が凝縮することが明らかになった．また，水滴を局所加熱することで発生させた熱毛管対流現象をIRカメラで詳細に観察することにも成功した．この現象は，微小量液体の混合・撹拌技術への応用が期待される．

Solar-driven interfacial evaporation of a hanging liquid marble

A liquid marble is a droplet coated with hydrophobic particles. Due to the large capillary energy associated with particle adsorption, this particle-armed liquid normally sustains long-term stability rather than evaporation under ambient condition. Conversely, we show here experimentally that liquid marbles coating with photothermal nanoparticles can significantly accelerate the liquid evaporation under solar irradiation. A series of experiment in solar evaporation of water drops, nanofluid drops as well as liquid marbles are conducted under controlled condition to verify this concept. It is found that the liquid marble exhibits the highest evaporation rate among the three cases. This high performance is attributed to the plasmon-coupling effect between neighboring nanoparticles along the drop surface, creating a high-temperature thermal boundary for intense evaporation of the interfacial liquid. While solar evaporation of nanofluid induces multiple scattering events among the nanoparticles that increases photon absorption within the drop domain, resulting in a collective heating to enhance the evaporation. Numerical modelling is further introduced to disclose the temperature distribution in these photothermic systems. Despite its diffusion nature, the temperature distribution can be engineered highly non-uniform and concentrated at the nanoscale using the particle assemblies in close proximity. A significant degree of nanoscale control over the individual temperatures of neighboring particles is demonstrated, depending on the interparticle gaps and dielectric surrounding media. Our findings open new possibilities for flexible control of photothermal processes and precisely heat delivery for solar vapor generation or local welding of textured nanomaterial.