Evaporation Of Sessile Water Droplets Research Articles

Construction of an active humidity regulation setup for NMR/MRI-Observation and simulation of the controlled evaporation of sessile water droplets

Controlling and improving processes like for example the production of organic semiconductors via printing depends on understanding the interplay of wetting and evaporation of complex fluids. Therefore, examination of the time dependent composition of complex fluid droplets during wetting or evaporation is of interest. The evaporation rate of sessile droplets containing largely water depends on the vapor pressures of the individual components and on the humidity (or partial pressure) of the surrounding gas phase. Hence, for a complete picture of an evaporation process and the comparability of the results of different measurements, it is essential to measure and control the humidity and temperature in the measurement compartment. Accordingly, climate chambers are available in different scales to fit a variety of techniques like contact angle goniometry to obtain results in a controlled atmosphere. We recently reported the application of MRI (Magnetic Resonance Imaging) and spatially resolved NMR (Nuclear Magnetic Resonance) spectroscopy for the examination of the evaporation of sessile droplets on surfaces in 10 mm NMR tubes. These are considered to be closed compartments. Here, we present an apparatus to a) measure and b) control the relative humidity within the sample compartment of the NMR setup by introducing preconditioned gas into the NMR tube. We monitored the evaporation of water droplets using RARE images and compared the volume decay with a) a simple diffusive evaporation model and b) with detailed FEM (finite element numerical model) simulations using COMSOL for validation. We find three evaporation regimes depending on the flow rate as well as on the distance of the gas outlet and the evaporating droplet. In one of the sample configurations tested the evaporation takes place in such a way that it can be described with the help of the simple diffusive model without convection. Thus, the presented approach opens comparative measurements with other methods as well as the observation of droplet evaporation in very dry or very humid environments with and without the influence of convection. Finally, using PRESS spectra, it is shown that the evaporation rate of water from a water/DMSO droplet can be controlled. This shows how the setup presented here can be used to study the evaporation of droplets of more complex mixtures.

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.

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.

Evaporation of Sessile Water Droplets on Horizontal and Vertical Biphobic Patterned Surfaces.

This paper presents an experimental study on thermal transport to single water droplets evaporating on heated biphobic surfaces consisting of a superhydrophobic matrix with a circular hydrophobic pattern with strong contact line pinning. A single water droplet of 8 μL volume is placed on a preheated surface and allowed to evaporate in an open laboratory environment. We investigate the influence of substrate orientation (horizontal and vertical) on evaporation dynamics. Using optical and infrared imaging, we report droplet fluid dynamics and heat transfer characteristics of the evaporating droplet. Overall, evaporation is more efficient on the vertical surface, exhibiting higher total heat transfer rates and up to 10% shorter evaporation times. Counterintuitively, on the vertical surface, the substrate-droplet interfacial heat flux was higher near the lower contact line than in the upper region, despite a high contact angle and an expected wedge effect at the bottom. At the same time, the temperature is colder in the lower part of the droplet. We attribute this apparent anomaly to the competition between sensible heating and evaporation, and a modified convective flow signature (both within the droplet and the gas phase) compared to a horizontal surface. We also show that the thermal signature becomes uniform once the contact angles at the upper and lower contact lines become equal toward the end of the evaporation process. Insights from this work can guide the design of spray cooling devices or be used to alter particle deposition patterns during evaporation-based fabrication techniques and ink-jet printing.

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.

Evaporation of sessile water droplet on heated surface with needle-shaped nanostructures by pre-boiling oxidation process

The effects of surface morphology by oxidation and surface temperature on the behavior of an evaporating water droplet on heated surface were investigated. As the test specimen, the pre-boiled (oxidized) aluminum surface was used with non-boiled (non-treated) surface. The surface temperature conditions were 40, 57, and 73 °C, respectively. By the pre-boiling process, a lot of unique needle-shaped nanostructures were spontaneously formed on the surface, which affected significantly the behaviors of an evaporating water droplet. The variations of the important parameters (i.e., contact diameter, contact angle, height, volume, surface area, and evaporation mass flux) were measured and discussed in detail. In addition, the evaporation time, evaporation rate, and heat transfer rate were evaluated and examined. Based on this work, it was found that potentially, using the pre-boiling process which is easier, simpler, and more cost-effective as well as applicable to the large-area, the droplet evaporation on the heated surface can be controlled and the heat transfer performance also can be enhanced through the change in surface morphology.

Evaporation of Nanosuspensions on Substrates with Different Hydrophobicity.

Liquid drop evaporation on surfaces is present in many industrial and medical applications, e.g., printed electronics, spraying of pesticides, DNA mapping, etc. Despite this strong interest, a theoretical description of the dynamic of the evaporation of complex liquid mixtures and nanosuspensions is still lacking. Indeed, one of the aspects that have not been included in the current theoretical descriptions is the competition between the kinetics of evaporation and the adsorption of surfactants and/or particles at the liquid/vapor and liquid/solid interfaces. Materials formed by an electrically isolating solid on which a patterned conducting layer was formed by the deposits left after drop evaporation have been considered as very promising for building electrical circuits on flexible plastic substrates. In this work, we have done an exhaustive study of the evaporation of nanosuspensions of latex and hydrophobized silver nanoparticles on four substrates of different hydrophobicity. The advancing and receding contact angles as well as the time dependence of the volume of the droplets have been measured over a broad range of particle concentrations. Also, mixtures of silver particles and a surfactant, commonly used in industrial printing, have been examined. Furthermore, the adsorption kinetics at both the air/liquid and solid/liquid interfaces have been measured. Whereas the latex particles do not adsorb at the solid/liquid and only slightly reduce the surface tension, the silver particles strongly adsorb at both interfaces. The experimental results of the evaporation process were compared with the predictions of the theory of Semenov et al. (Evaporation of Sessile Water Droplets: Universal Behavior in the Presence of Contact Angle Hysteresis. Colloids Surf. Physicochem. Eng. Asp. 2011, 391 (1-3), 135-144) and showed surprisingly good agreement despite that the theory was developed for pure liquids. The morphology of the deposits left by the droplets after total evaporation was studied by scanning electronic microscopy, and the effects of the substrate, the particle nature, and their concentrations on these patterns are discussed.

Thermal management of metallic surfaces: evaporation of sessile water droplets on polished and patterned stainless steel

This communication focus on the evaporation of sessile water droplets on different states of austenitic stainless steel surfaces: mirror polished, mirror polished and aged and patterned by sputtering. The evolution of the contact angle and of the droplet diameter is presented as a function of time at room temperature. For all the surface states, a constant diameter regime (CCR) is observed. An important aging effect on the contact angle is measured on polished surfaces due to atmospheric contamination. The experimental observations are compared to a quasi-static evaporation model assuming spherical caps. The evolution of the droplet volume as a function of time is almost linear with the evaporation time for all the observed surfaces. This is in accordance with the model prediction for the CCR mode for small initial contact angles. In our experiments, the evaporation time is found to be linearly dependent on the initial contact angle. This dependence is not correctly described by the evaporation model

Quasi-static motion of microparticles at the depinning contact line of an evaporating droplet on PDMS surface

In this paper, evaporation of sessile water droplets containing fluorescent polystyrene (PS) microparticles on polydimethylsiloxane (PDMS) surfaces with different curing ratios was studied experimentally using laser confocal microscopy. At the beginning, there were some microparticles located at the contact line and some microparticles moved towards the line. Due to contact angle hysteresis, at first both the contact line and the microparticles were pinned. With the depinning contact line, the microparticles moved together spontaneously. Using the software ImageJ, the location of contact lines at different time were acquired and the circle centers and radii of the contact lines were obtained via the least square method. Then the average distance of two neighbor contact lines at a certain time interval was obtained to characterize the motion of the contact line. Fitting the distance-time curve at the depinning contact line stage with polynomials and differentiating the polynomials with time, we obtained the velocity and acceleration of both the contact line and the microparticles located at the line. The velocity and the maximum acceleration were, respectively, of the orders of 1 µm/s and 20-200 nm/s2, indicating that the motion of the microparticles located at the depinning contact line was quasi-static. Finally, we presented a theoretical model to describe the quasi-static process, which may help in understanding both self-pinning and depinning of microparticles.

Analysis and feasibility of an evaporative cooling system with diffusion-based sessile droplet evaporation for cooling microprocessors

In this study, a feasibility analysis is undertaken to assess the capability of an evaporative cooling system using diffusion-based evaporation of sessile water droplets to provide sufficient cooling of microprocessors within the space requirements of the current heat sinks. The study investigates the cooling requirements for the Intel Xenon Processor and the Intel Core i7-900 Processor. An analytical model is developed to determine the capacity of a single layer of water droplets to provide sufficient cooling and calculates the size of the droplets required to meet the cooling needs. It is found that a single layer can provide sufficient cooling for the Xenon Processor with 21,316 droplets having a radius of 0.25mm and the Core i7-900 Processor with 27,556 droplets having a radius of 0.25mm. A numerical model is developed to analyze a tiered system that fits within the space restrictions corresponding to the current heat sinks, but can provide the required cooling needs with larger droplets and fewer of them. To decrease the complexity of manufacturing the evaporative cooling system, the numerical model simulated cases to find both (i) the minimum number of posts required to connect each of the tiers and (ii) the minimum number of tiers required to provide sufficient cooling for the microprocessors. The results of the numerical modelling work found that a minimum of 60 posts connecting each of the tiers were required to cool the Xenon Processor and 52 posts for the Core i7-900 Processor. It was also found that a minimum of 3 tiers was required for the Xenon Processor, with a total of 867 droplets having a radius of 2mm, and 5 tiers required for the Core i7-900 Processor, with a total of 1620 droplets having a radius of 2mm. The results of the work demonstrate that evaporative cooling systems with diffusion-based evaporation of sessile droplets can provide sufficient cooling for the selected microprocessors, with a number of feasible configurations.

PDMS表面上的液滴蒸发实验研究

Evaporation of sessile water droplets on polydimethylsiloxane (PDMS) surface was experimentally studied with the particle tracking velocimetry (PTV) technique. The fluorescent microspheres at the solid-liquid interface first moved towards the center and then back to the contact line. Because the evaporative flux near the contact line is less than that far from the line, there will be a capillary flow towards the center when the contact line is pinned. Such a flow will carry microspheres towards the center. Moving characteristics of the contact line were also investigated. It is found that the theoretical values of the moving velocity at different moments in the CCA stage is of the same order with the experimental values. However, the experimental moving accelerations were much larger than the theoretical ones because the microspheres at the contact line weaken the interaction between the PDMS surface and the contact line.

Study of the Contact and the Evaporation Kinetics of a Thin Water Liquid Bridge between Two Hydrophobic Plates

The evaporation of sessile water droplets on hydrophobic surfaces is a topic which led to numerous investigations. However, how does the liquid behave when the evaporation occurs between two of these particular substrates? The drying stage is governed by capillary phenomena which takes place in a confined space. In the field of material shaping, it is also possible that some regions of a green body exhibit hydrophobic properties. As part of a better understanding of the local mechanisms during drying, liquid bridges have been reproduced in an ideal case. Drying kinetics and parameters measurements from 303 to 343 K (relative humidity of 55%) of deionized water liquid bridges between two plates of hydrophobic substrates are presented. Experimental work was carried out using a specific device to create liquid bridges, coupled with image analysis within an adapted instrumented climatic chamber. While the volume and the exchange surface of liquid bridges decrease regularly throughout the process, contact angles constantly diminish and more significantly at the end. This is different from the evaporation between two hydrophilic plates. From these measurements, the change of curvature of the liquid bridges during evaporation is highlighted.

Theoretical study on evaporation of sessile water droplets on a glass panel with infrared radiation

An evaporation model of a water droplet on a glass substrate is developed to estimate the process time needed to manufacture a liquid crystal display panel (LCD). By using an infrared (IR) lamp as a radiation source to evaporate the droplet, the changes in droplet size and evaporation time are calculated. The peak wavelength of the lamp and initial droplet size are used to estimate the process time. A change in the distribution of droplets is calculated to practically apply the model used in the drying process. By analyzing the contributions of radiation and conduction, we found that the evaporation process is driven by different heat transfer mechanisms according to the peak wavelength of the lamp.

Experimental study of evaporation of sessile water droplet on PDMS surfaces

Evaporation of sessile water droplet on polydimethylsiloxane (PDMS) surfaces with three different curing ratios (5: 1, 10: 1, and 20: 1) was experimentally investigated in this paper. We show that the constant contact radius (CCR) evaporation on surface with high curing ratio lasts longer than that with low curing ratio. We also measured Young’s moduli of PDMS films by using atomic force microscopy (AFM) and simulated surface deformation of PDMS films induced by sessile water droplet. With increasing curing ratio of PDMS film, Young’s modulus of PDMS film is getting lower, and then there will be larger surface deformation and more elastic stored energy. Since such energy acts as a barrier to keep the three-phase contact line pinned, thus it will result in longer CCR evaporation on PDMS surface with higher curing ratio.

Evaporation of Nanoparticle Droplets on Smooth Hydrophobic Surfaces: The Inner Coffee Ring Deposits

The solid surfaces used in evaporation studies of nanoparticle sessile droplets usually exhibit significant surface roughness, causing significant pinning of the three-phase contact lines and producing different types of nanoparticle deposits, from single and multiple coffee rings (formed at the initial pining of triple contact lines) to central bumps. Here we used nanometer-scale smooth hydrophobic surfaces to investigate the evaporation of sessile water droplets containing silica nanoparticles and organic pigment nanoparticles. We observed a new type of coffee ring deposits which were not formed at the initial pinning but at the later pinning. We referred them to as the inner coffee ring deposits (ICRDs). The radius of ICRDs was smaller than the radius of the initially pinned contact area and increased with increasing concentration of added salts and nanoparticles and with increasing contact angle hysteresis of hydrophobic surfaces. We also observed different dendrite deposit patterns inside ICRDs. We a...

Directional motion of evaporating droplets on gradient surfaces

Evaporation of sessile water droplets on surfaces with wettability gradients was studied. The wettability gradient was generated by fabricating non-uniformly distributed cylindrical micropillars on silicon surfaces. During the evaporation, it was found that the center of mass of the droplet moved either in or against the direction of the wettability gradient, depending on the configuration of the micropillars. An energy-based theoretical criterion was derived to predict the moving direction. The theoretical predications agreed well with the experimental observations. The results provide a parametric design basis to control the contact line dynamics and directional transport of evaporating droplets.

Evaporation of Sessile Water Droplets in Presence of Contact Angle Hysteresis

In this paper we present a theory describing the diffusion limited evaporation of sessile water droplets in presence of contact angle hysteresis. Theory describes two stages of evaporation process: (I) evaporation with a constant radius of the droplet base; and (II) evaporation with constant contact angle. During stage (I) the contact angle decreases from static advancing contact angle to static receding contact angle, during stage (II) the contact angle remains equal to the static receding contact angle. Universal dependences are deduced for both evaporation stages. Obtained universal curves are validated against available in the literature experimental data.

Experimental and theoretical investigations of evaporation of sessile water droplet on hydrophobic surfaces

Experiments of sessile water droplet evaporation on both polydimethylsiloxane (PDMS) and Teflon surfaces were conducted. All experiments begin with constant contact area mode (the initial contact angle is greater than 90d), switch to constant contact angle mode and end with mixed mode. Based on the assumptions of spherical droplet and uniform concentration gradient, theoretical analyses for both constant contact area and constant contact angle modes are made and theoretical solutions are derived accordingly, especially a theoretical solution of contact angle is presented first for CCR stage with any value of the initial contact angle. Moreover, comparisons between the theoretical solutions and experimental data of contact angle in CCR stage demonstrate the validity of the theoretical solution and it would help for a better understanding and application of water droplet on solid surfaces, which is quite often encountered in lab-on-a-chip, polymerase chain reaction (PCR) and other micro-fluidics devices.

Experimental and theoretical investigations of evaporation of sessile water droplet on hydrophobic surfaces

Experiments of sessile water droplet evaporation on both polydimethylsiloxane (PDMS) and Teflon surfaces were conducted. All experiments begin with constant contact area mode (the initial contact angle is greater than 90°), switch to constant contact angle mode and end with mixed mode. Based on the assumptions of spherical droplet and uniform concentration gradient, theoretical analyses for both constant contact area and constant contact angle modes are made and theoretical solutions are derived accordingly, especially a theoretical solution of contact angle is presented first for CCR stage with any value of the initial contact angle. Moreover, comparisons between the theoretical solutions and experimental data of contact angle in CCR stage demonstrate the validity of the theoretical solution and it would help for a better understanding and application of water droplet on solid surfaces, which is quite often encountered in lab-on-a-chip, polymerase chain reaction (PCR) and other micro-fluidics devices.

Evaporation of sessile water droplets: Universal behaviour in presence of contact angle hysteresis

A theory is presented, which describes the diffusion limited evaporation of sessile water droplets in presence of contact angle hysteresis. Theory describes two stages of evaporation process: (I) evaporation with a constant radius of the droplet base; and (II) evaporation with a constant contact angle. During stage (I) the contact angles decreases from static advancing contact angle to static receding contact angle. During stage (II) the contact angle remains equal to the static receding contact angle. Universal dependences are deduced for both evaporation stages. Obtained universal curves are validated against available in the literature experimental data.