Evaporation Characteristics Of Water Droplets Research Articles

Prediction of water droplet behavior on aluminum alloy surfaces modified by nanosecond laser pulses

Metal processing by laser radiation is shown to be a promising way to obtain surfaces with controllable wetting and evaporation characteristics. Such surfaces can be used in designing the modern cooling systems based on phase transitions. However, large-scale applications of metal processing by laser radiation are hampered by the lack of a theory that allows predicting the wetting and evaporation of liquid droplets on modified surfaces. In this work, we study a water droplet behavior during its wetting and evaporation on hydrophilic and hydrophobic aluminum alloy surfaces modified by single nanosecond laser pulses. To predict the wettability of the laser-processed surfaces, we proposed to use the empirical correlation of the contact angle on the surface free energy obtained in terms of dynamic contact angles. We analyzed the applicability of two well-known theoretical models to determine the evaporation rates of droplets on hydrophilic and hydrophobic laser-processed surfaces. The Spalding and diffusive evaporation models can be used for water droplet evaporating from the hydrophilic aluminum alloy surface at a temperature difference between the surface and air in the chamber of 0–6.5 K. At higher temperature difference from 6.5 K to 9 K, only the Spalding model is applicable. In the case of the hydrophobic laser-processed aluminum alloy surface, the mass loss rate of evaporating droplet at the temperature difference range of 0–9 K can be predicted by the diffusive model. In addition, this model was found to be appropriate for predicting the geometric dimensions of a droplet evaporating in a pinned contact line mode on hydrophilic surfaces without heating at any stage of its evaporation. The results of this study can be used to predict the wetting and evaporation characteristics of water droplets on aluminum alloy surfaces modified by single nanosecond laser pulses.

Evaporation-Rate Control of Water Droplets on Flexible Transparent Heater for Sensor Application.

To develop high-performance de- or anti-frosting/icing devices based on transparent heaters, it is necessary to study the evaporation-rate control of droplets on heater surfaces. However, almost no research has been done on the evaporation-rate control of liquid droplets on transparent heaters. In this study, we investigate the evaporation characteristics of water droplets on transparent heater surfaces and determine that they depend upon the surface wettability, by modifying which, the complete evaporation time can be controlled. In addition, we study the defrosting and deicing performances through the surface wettability, by placing the flexible transparent heater on a webcam. The obtained results can be used as fundamental data for the transparent defrosting and deicing systems of closed-circuit television (CCTV) camera lenses, smart windows, vehicle backup cameras, aircraft windows, and sensor applications.

Evaporation characteristics of water droplets in Cassie, Wenzel, and mixed states on superhydrophobic pillared Si surface

Evaporation characteristics of various droplets on superhydrophobized Si micro-pillar (SiμP) substrate have been investigated experimentally. Especially, the unique characteristics of evaporating mixed drop have been elucidated for the first time, to the best of the authors’ knowledge. Processes for forming different droplets, i.e., Cassie, Wenzel, and mixed ones, are developed. Exposure to humidifier mist has enabled the formation of Cassie droplets, droplets sitting on pillars top with entrapped air underneath. Water vapor condensation has led to the formation of mixed droplets, that is, Cassie droplet with impaled plugs. The mixed droplets have been converted into Wenzel ones by simply adding additional water and applying pressure to them. These droplets have shown different evaporation characteristics. As expectedly, mixed droplets have shown evaporation time in-between of those for Cassie and Wenzel ones. Further, compared to the well-known constant contact angle (CCA) mode of evaporation for Cassie and constant contact radius (CCR) mode of evaporation for Wenzel droplets, respectively, the evaporation of the mixed droplet has been found to be pseudo-CCA mode, where the contact angle decreases/recovers with stepwise decrease in contact radius. This evaporation behavior is quite similar to the stick-slip motion of droplet evaporation on superhydrophobic surface. Finally, spontaneous droplet movement, depending on the droplet type, has been traced during the evaporation. While the Cassie drop moves rather randomly during evaporation, the mixed droplet showed predictable movement depending on the size and location of Wenzel plugs. The evaporation characteristics of different droplets can be a valuable help for a wide variety of applications such as self-cleaning, dewing, fog-basking, dropwise/filmwise condensation, etc.

Integral characteristics of water droplet evaporation in high-temperature combustion products of typical flammable liquids using SP and IPI methods

This paper examines the evaporation features of water droplets moving through high-temperature combustion products of typical flammable liquids (on an example of gasoline, kerosene, ethanol, and acetone). Using Shadow Photography and Interferometric Particle Imaging methods contributed to perform the given experimental research. Such approach enabled to establish the integral characteristics of droplet motion (sizes and velocities) and evaporation process (evaporation rate). The initial droplet sizes (radii) and the temperatures of combustion products varied in a wide range: 0.05–4 mm and 400–1900 K, respectively. The choice of these ranges allowed us to summarize the research results, which can be applied in a group of promising high-temperature (usually more than 1000 K) gas-vapor-droplet technologies (thermal or flame methods of liquid waste treatment; heat carriers based on flue gases, steam, and water droplets; polydisperse firefighting). Conditions of almost the complete and insignificant (with minimum change of liquid mass) droplet evaporation were determined. In addition, the integral characteristics of water droplet evaporation under non-stationary heat transfer (at a temperature of combustion products from 1900 K to 400 K) were compared with the same characteristics under nearly stationary heat transfer (at a temperature of 1100 ± 30 K). During short-time heating (less than 0.5 s), values of these parameters correlated well (deviations did not exceed 8–12%).

The Characteristics of Water Droplet Evaporation in the Flames of Various Flammable Liquids

This paper investigates the integral characteristics of the evaporation of water droplets (the initial diameter 4–6 mm) during their motion in flames obtained through the combustion of typical flammable liquids for various purposes, such as kerosene and denatured alcohol. We performed the experiments using high-speed video cameras and panoramic cross-correlation techniques PTV and SP. The difference between evaporation rates under appropriate conditions has been defined. The influence of a droplet initial size on evaporation rate has been demonstrated.

Investigation of Regularities of Heat and Mass Transfer and Phase Transitions during Water Droplets Motion through High-Temperature Gases

The macroscopic regularities of heat and mass transfer and phase transitions during water droplets motion through high-temperature (more than 1000 K) gases have been investigated numerically and experimentally. Water droplet evaporation rates have been established. Gas and water vapors concentrations and also temperature values of gas-vapor mixture in small neighborhood and water droplet trace have been singled out. Possible mechanisms of droplet coagulation in high-temperature gas area have been determined. Experiments have been carried out with the optical methods of two-phase gas-vapor-droplet mixtures diagnostics (“Particle Image Velocimetry” and “Interferometric Particle Imaging”) usage to assess the adequateness of developed heat and mass transfer models and the results of numerical investigations. The good agreement of numerical and experimental investigation results due to integral characteristics of water droplet evaporation has been received.

Prediction of droplet evaporation characteristics of nebuliser based humidification and drug delivery devices

This study reports on a model for the evaporation of mono-disperse water droplets into air that can be used to aid the design of nebuliser based humidification systems for respiratory treatments and also for predicting droplet size distributions for pulmonary drug delivery devices under either isothermal or adiabatic conditions. The governing equations are developed from the coupled mass and energy balances for the droplets and for the surrounding air. An adjustment to one of these equations allows for the modelling of either isothermal or adiabatic conditions. Additionally, the proposed model allows for the variation of fluid transport and thermodynamic properties during the solution of the governing equations. The model is compared to and validated against an overall mass and energy balance and the literature. Confirmation of the general evaporation characteristics of water droplets evaporating into air is gained from the model results and, its accuracy, usability and applicability are improved over existing models. The proposed model indicates that the droplet evaporation process in humidification devices is largely unaffected by the initial water temperature and also indicates advantages of adiabatic conditions when using cascade impactor technology for droplet sizing.

Water droplet evaporation on Cu-based hydrophobic surfaces with nano- and micro-structures

The characteristics of water droplet evaporation on three different hydrophobic surfaces, PCu (Plain Copper, θ=115°), MSCu (Micro-Structured Copper, θ=126°) and NSCuO (Nano-Structured Copper Oxide, θ=159°) with coating of the same SAM (Self-Assembled Monolayer) material, were experimentally investigated. For industrial heat transfer applications, copper material was used as the substrate, and the simple and cost-effective fabrication technique to prepare the superhydrophobic surface, NSCuO, was introduced. Based on the observations, the behavior of droplet evaporation was divided into three stages: Stage I (constant contact area stage), Stage II (constant contact angle stage) and Stage III (mixed stage). When studying the PCu surface, the Stages I, II, and III were observed, consistent with previous reports. For the MSCu surface, Stages I and III appeared without Stage II, and the pinning period of contact line was the longest among the test samples due to the formation of Wenzel state droplet. In the case of the superhydrophobic NSCuO surface, only Stage III occurred, and the contact line moved freely during the entire evaporation time because of the formation of Cassie state droplet. The total evaporation time of the NSCuO was the longest out of all the samples tested. At the last stage of evaporation, the edge of the droplet shrank at a much faster rate in all surfaces. On the other hand, the shrinking velocity of the droplet height drastically increased only on the NSCuO, which was considered as the unique behavior of superhydrophobic surface. In this experiment, it was found that the surface structure determines the motion of the contact line on the surface, which, in turn, strongly influences the characteristics of the droplet evaporation.

Evaporating characteristics of sessile droplet on hydrophobic and hydrophilic surfaces

This study investigates evaporation characteristics of water droplets on hydrophobic and hydrophilic surfaces experimentally. The time-dependent contact angle, center-height, contact radius, surface area, and volume of droplet were measured for slide glass, octadecyltricholorosilane (OTS), and alkylketene dimmer (AKD) surfaces using the digital image analysis technique. First, during the evaporation of droplet on hydrophilic surface, the measured contact angle, liquid volume, and height of droplet were in good agreements with numerical predictions. It was found that there are no three distinct stages and no pinning section during the evaporation for the super-hydrophobic surface, and the surface characteristics highly affect evaporation speed rather than the droplet surface area contact with air. Moreover, as the hydrophobicity of surfaces becomes strong, the pinning time becomes short and the total evaporation time lasts long. In particular, the edge shrinking velocity of the droplet shows nonlinear characteristics, and its magnitude becomes extremely high at the last period of total evaporating time.