Evaporation Of Water Droplets Research Articles

Dynamics of inner bubble during water droplet evaporation on conical micro-pillars surfaces

The experiments are conducted to investigate the geometrical parameters' effects of the water droplet evaporation process on the conical micro-pillars surfaces. Four conical micro-pillars surfaces with different pitches of adjacent micro-pillars (200, 300, 400 and 500 μm) are fabricated by nanosecond laser micro-nano processing technology on the copper surface. The results indicate that micro-pillars structures significantly enhance the droplet evaporation rate for the modified surfaces comparing to the smooth surface due to the easier initiation and more rapid growth of the bubbles. Conical micro-pillars surface with pitch of 500 μm has the largest solid-liquid interface among the studied conical micro-pillars surfaces. Meanwhile, some satellite bubbles depart from heated surface at the surface temperature of 107.8 °C, leading to the best nucleate boiling of droplet. On the other hand, a coalesced vapor bubble is observed in the tests on the other micro-pillars surfaces, which increases thermal resistance at the solid-liquid interface and hinders heat transfer from the heated surface to the droplet. The phenomenon of bubble oscillation occurs due to the combined effects of bubble evaporation and condensation. And this process repeats for several time at the early stage of evaporation. In addition, a strong turbulent fluctuation is formed after the breakup of vapor bubble on conical micro-pillars surfaces at the surface temperature of 107.8 °C. The observed strong turbulent fluctuation is also observed at higher surface temperature.

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 levitating liquid microdroplets over a dry heated surface

The present work is devoted to the study of the evaporation process of micro-sized water droplets levitating over a heated dry substrate. The study of this process is relevant in connection with the development of spray cooling systems. Due to the extreme complexity of this phenomenon the mechanism of spray cooling is still not fully understood. In this work we studied the evaporation of micro-sized water droplets levitating over a dry substrate heated from below. The working area was open to the atmosphere. Evaporation was studied in the temperature substrate range from 23 to 95°C. During the experiment local values of the substrate temperature and geometric characteristics of the drop were determined. In the experiment a shadow method with high spatial resolution was used, shooting was performed with a high-speed camera.

Evaporation of colloidal droplets from aluminum–magnesium alloy surfaces after laser-texturing and mechanical processing

Evaporation-induced self-assembly on solid surfaces is widely used for the production of photonic devices, circuit boards of electronic devices, and cooling technologies. There are numerous factors affecting self-assembly. However, the state of a solid surface is one of the most important. In this work, we analyzed the effect of mechanical processing methods (widely used in mechanical engineering) and promising pulsed laser texturing of aluminum-magnesium alloy on the characteristics of self-assembly of polystyrene (PS) particles during the evaporation of colloidal solution droplets. The patterns were visualized under a scanning electron microscope. The profile of an evaporating droplet was obtained using the optical shadow technique. It was found that during the evaporation of water droplets loaded with PS particles on aluminum-magnesium alloy samples, the ring-like patterns were formed. Such a pattern type was explained by the evaporation of a droplet in the constant contact diameter mode during almost the entire evaporation time (more than 90% of the total evaporation time). The dried patterns formed on aluminum-magnesium alloy surfaces after satin-finishing and laser texturing were found to be stretched parallel to the vector of laser beam or polishing tool movement due to capillary force. In addition, we determined the conditions when the particles formed spot-like patterns on laser-textured surfaces. These results provide new knowledge in understanding the effect of different processing methods of a material on the formation of dried patterns and are important for many applications.

Marangoni convection in an evaporating water droplet

The existence of Marangoni flow in an evaporating pure water drop has remained an open question. In this study, we report the occurrence of Marangoni convection within a water drop resting on a copper rod as it evaporates into the air. Through careful preparation of the test liquid suspension, we have observed convective structures inside the water drop. We also developed a mathematical model representing the evaporation system and solved it numerically to further strengthen support for the experimental data. Our study is different from previous studies on droplet evaporation in the sense that we allowed the droplet to evaporate spontaneously in the normal laboratory condition without heating the substrate. The substrate was neither externally heated nor did radiation from the illumination light stimulate a Marangoni flow. The velocities in the midsection of the droplet were measured using particle tracking velocimetry in both the sessile and pendant configurations. The typical velocities in the center of the droplet were found to be 10 mm/s experimentally and even higher velocities (about double that) were predicted by the mathematical model. This high value stands out among the thermocapillary-driven flows observed in an autonomously evaporated drop of water under ambient conditions.

Risk Exposure during Showering and Water-Saving Showers

Eco-friendly showers aim to lower energy and water consumption by generating smaller water droplets than those produced by traditional systems. To evaluate the risk of users inhaling the contaminants associated with such water droplets—namely, chemical components or opportunistic bacterial pathogens such as Legionella—we modeled the behavior of water droplets aerosolized by water-atomization technology at a flow rate of 2.2 L/min and compared the results obtained using this model with those determined experimentally in a typical shower stall. Additionally, we monitored the number and mass of inhalable water droplets emitted by twelve showerheads—eight using water-atomization technology and four using continuous-flow technology—which have distinct characteristics in terms of water flow rate, water pressure, spray angle, and number of and diameter of nozzles. The water-atomizing showers tested not only had lower flow rates, but also larger spray angles, less nozzles, and larger nozzle diameters than those of the continuous-flow showerheads. We observed a difference in the behavior of inhalable water droplets between the two technologies, both unobstructed and with the presence of a mannequin. The evaporation of inhalable water droplets emitted by the water-atomization showers favored a homogenous distribution in the shower stall. In the presence of the mannequin, the number and mass of inhalable droplets increased for the continuous-flow showerheads and decreased for the water-atomization showerheads. The water-atomization showerheads emitted less inhalable water mass than the continuous-flow showerheads did per unit of time; however, they generally emitted a slightly higher number of inhalable droplets (1.6 times more), including those large enough to carry a bacterium each—only one model performed as well as the continuous-flow showerheads in this regard. Further experiments are needed to assess whether this slight increase in the number of inhalable water droplets increases the biological risk.

Ring droplet formation during evaporation of a sessile water droplet on a biphilic surface

Numerous studies have investigated droplet evaporation on surfaces with different roughness and wettability values. However, most studies have focused on droplet evaporation from homogeneous surfaces at low surface temperatures (below 100 °C). In this study, we conducted droplet evaporation experiments on biphilic surfaces with different ring pattern sizes fabricated by a hybrid method consisting of the sol-gel approach and a picosecond laser system. This study investigated the droplet evaporation boiling phenomenon on these biphilic surfaces under superheated conditions. Changes in droplet geometry and inner bubble dynamics during evaporation were analyzed using a high-speed camera. A ring-shaped droplet formed during the final evaporation period. The formation of the ring enhanced the droplet evaporation efficiency. Furthermore, the experimental results from this study work were compared with the theoretical model proposed in the literature.

Two-phase Interactions between Propagating Shock Waves and Evaporating Water Droplets

One-dimensional numerical simulations based on hybrid Eulerian-Lagrangian method are performed to study the interactions between propagating shocks and dispersed evaporating water droplets. Two-way coupling for exchanges of mass, momentum, energy and vapour species is adopted for the dilute two-phase gas-droplet flows. Interphase interactions and droplet breakup dynamics are investigated with initial droplet diameters of 30, 50, 70 and 90 {\mu}m under an incident shock wave Mach number of 1.3. Novel two-phase flow phenomena are observed when droplet breakup occurs. First, droplets near the two-phase contact surface show obvious dispersed distribution because of the reflected pressure wave that propagates in the reverse direction of the leading shock. The reflected pressure wave grows stronger for larger droplets. Second, spatial oscillations of the gas phase pressure, droplet quantities (e.g., diameter and net force) and two-phase interactions (e.g., mass, momentum, and energy exchange), are observed in the post-shock region when droplet breakup occurs, which are caused by shock / droplet interactions. Third, the spatial distribution of droplets (i.e., number density, volume fraction) also shows strong oscillation in the post-shock region when droplet breakup occurs, which is caused by the oscillating force exerted on the droplets.

Reaction front development from ignition spots in n-heptane/air mixtures: Low-temperature chemistry effects induced by ultrafine water droplet evaporation

Effects of low-temperature chemistry induced by ultrafine water droplet evaporation on reaction front development from an ignition spot with temperature gradient are studied in this work. The Eulerian–Eulerian method is used to simulate the gas–liquid two-phase reactive flows, and the physical model is one-dimensional spherical reactor with stoichiometric gaseous n-heptane/air mixture and ultrafine monodisperse water droplets (initial diameter 5 μm). Homogeneous ignitions of two-phase mixtures are first simulated. The water droplets can completely evaporate in the reactor prior to ignition, and hence pronouncedly reduce gas temperature, which may induce the low-chemistry reactions. It is found that the turnover temperature for negative temperature coefficient range increases with droplet volume fraction. Three-stage ignitions are present when the volume fraction is beyond a critical value, that is, low-temperature, intermediate-temperature, and high-temperature ignitions. The chemical explosive mode analysis also confirms the low-chemistry reactions induced by the evaporation of ultrafine water droplets. Then, reaction front development from an ignition spot with temperature gradient in two-phase mixtures is analyzed based on one-dimensional simulations. Different modes for reaction front origin in the spot are identified, based on the initial gas temperature and lower turnover temperature. Specifically, the reaction front can be initiated at the left and right ends of the ignition spot, and inside it. Detailed reaction front developments corresponding to the above three modes are discussed. In addition, the pressure wave from high-temperature ignition is important, compared to those from low and intermediate chemistries. The reaction front propagation speed and thermal states of fluid particles corresponding to different reaction front initiation modes are analyzed. Moreover, autoignition modes are summarized in the diagrams of normalized temperature gradient vs normal acoustic time and droplet volume fraction. The detonation limits of two-phase mixtures highly depend on the droplet volume fraction and are not regularly peninsular-shaped, like those for purely gaseous mixtures.

Revisiting Wetting, Freezing, and Evaporation Mechanisms of Water on Copper.

Wetting of metal surfaces plays an important role in fuel cells, corrosion science, and heat-transfer devices. It has been recently stipulated that Cu surface is hydrophobic. In order to address this issue we use high purity (1 1 1) Cu prepared without oxygen, and resistant to oxidation. Using the modern Fringe Projection Phase-Shifting method of surface roughness determination, together with a new cell allowing the vacuum and thermal desorption of samples, we define the relation between the copper surface roughness and water contact angle (WCA). Next by a simple extrapolation, we determine the WCA for the perfectly smooth copper surface (WCA = 34°). Additionally, the kinetics of airborne hydrocarbons adsorption on copper was measured. It is shown for the first time that the presence of surface hydrocarbons strongly affects not only WCA, but also water droplet evaporation and the temperature of water droplet freezing. The different behavior and features of the surfaces were observed once the atmosphere of the experiment was changed from argon to air. The evaporation results are well described by the theoretical framework proposed by Semenov, and the freezing process by the dynamic growth angle model.

A free-form patterning method enabling endothelialization under dynamic flow

Endothelialization strategies aim at protecting the surface of cardiovascular devices upon their interaction with blood by the generation and maintenance of a mature monolayer of endothelial cells. Rational engineering of the surface micro-topography at the luminal interface provides a powerful access point to support the survival of a living endothelium under the challenging hemodynamic conditions created by the implant deployment and function. Surface structuring protocols must however be adapted to the complex, non-planar architecture of the target device precluding the use of standard lithographic approaches. Here, a novel patterning method, harnessing the condensation and evaporation of water droplets on a curing liquid elastomer, is developed to introduce arrays of microscale wells on the surface of a biocompatible silicon layer. The resulting topographies support the in vitro generation of mature human endothelia and their maintenance under dynamic changes of flow direction or magnitude, greatly outperforming identical, but flat substrates. The structuring approach is additionally demonstrated on non-planar interfaces yielding comparable topographies. The intrinsically free-form patterning is therefore compatible with a complete and stable endothelialization of complex luminal interfaces in cardiovascular implants.

The effect of surface structures on evaporation and free convection in evaporating sessile drops

The effect of various surface structures of the AlMg6 aluminum-magnesium alloy on the evaporation rate and convection in a sessile droplet was experimentally studied. Structures on the surface were created using laser exposure. The evaporation of water droplets on heated structured surfaces was compared with a smooth wall. The wall temperature Tw = 68 °C. The data obtained for the evaporation rate without taking into account the diameter of the droplet base for a structured hydrophilic surface is 11-13% higher than for a smooth surface. The difference between structured hydrophilic and hydrophobic surfaces was 4-6%. The strongest effect of convection enhancement was observed for a structured hydrophobic surface.

Evaporation Kinetics of Sessile Water Droplets on a Smooth Stainless Steel Surface at Elevated Temperatures and Pressures.

The evaporation of water droplets on a solid surface at elevated temperatures under a pressurized condition has not yet been well understood, although this phenomenon is of both theoretical and practical significance. In this work, water droplet evaporation on smooth stainless steel surfaces in nitrogen gas atmosphere at elevated pressures and temperatures (up to 2 MPa and 202.4 °C, respectively) was investigated experimentally. It was observed that the increase in pressure diminishes the proportion of the constant contact radius stage over the entire evaporation time, whereas an opposite trend was found when raising the temperature, due to the change in the surface pinning ability with pressure (and temperature). The results also suggested that the evaporation mode transition is mainly affected by temperature rather than pressure. In addition, the evaporation rate was calculated under various degrees of subcooling, revealing that the evaporation rate increases almost linearly with pressure when keeping the degree of subcooling constant. However, when fixing the test temperature, a nonlinear decrease of the evaporation rate with pressure was observed. A power law growth of the evaporation rate with temperature was also found at a constant pressure. Last, it was uncovered by a theoretical analysis that the saturated vapor concentration is the dominant factor dictating the evaporation rate.

Evaporation characteristics and heat transfer enhancement of sessile droplets under non-uniform electric field

Electrohydrodynamic (EHD) enhancement of water droplet evaporation by non-uniform electric field and the subsequent dynamic characteristics were experimentally investigated in this paper. A pin-to-plate electric field was produced by a needle connected to a high-voltage power supply and the grounded heated surface coated by thin Teflon film with a contact angle of about 100°. The droplet was generated onto the hot surface by a micro-syringe with a constant initial volume of 4 μl, and a high-speed camera was used to capture the droplet morphology. Effects of electric field and surface temperature on the contact angle, droplet height, evaporation stage, volume variation, and evaporation rate were studied. The results showed that both the increase in external electric field strength and surface temperature enhance droplet evaporation with a maximum enhancement ratio of 6.8 times in the present work. In addition, a special extra constant contact angle (CCA) stage was found in droplet evaporation under electric field, while only constant contact radium (CCR) and Mixed stages existed in neutral (no voltage) evaporation. In particular, the evaporation enhancing mechanism of no-uniform electric field considering corona wind blowing, surface tension weakening and molecular orientational alignment was also presented.

A purely kinetic description of the evaporation of water droplets.

The process of water evaporation, although deeply studied, does not enjoy a kinetic description that captures known physics and can be integrated with other detailed processes such as drying of catalytic membranes embedded in vapor-fed devices and chemical reactions in aerosol whose volumes are changing dynamically. In this work, we present a simple, three-step kinetic model for water evaporation that is based on theory and validated by using well-established thermodynamic models of droplet size as a function of time, temperature, and relative humidity as well as data from time-resolved measurements of evaporating droplet size. The kinetic mechanism for evaporation is a combination of two limiting processes occurring in the highly dynamic liquid-vapor interfacial region: direct first order desorption of a single water molecule and desorption resulting from a local fluctuation, described using third order kinetics. The model reproduces data over a range of relative humidities and temperatures only if the interface that separates bulk water from gas phase water has a finite width, consistent with previous experimental and theoretical studies. The influence of droplet cooling during rapid evaporation on the kinetics is discussed; discrepancies between the various models point to the need for additional experimental data to identify their origin.

Analysis of Circulation Reversal and Particle Transport in Evaporating Drops

We report experimental evidence of circulation reversal in evaporating drops. The internal flow patterns and particle distribution inside the evaporating water droplet with suspended polystyrene particles was studied using the Particle Image Velocimetry (PIV) technique. Droplet placed on externally heated substrate showed two symmetric and counter rotating Marangoni convection cells at the beginning of evaporation process. Towards the final stages of evaporation, the two counter rotating Marangoni convection cells reversed the direction of circulation. Interestingly, it was observed that between these two stages four symmetric and counter rotating convection cells appeared inside the evaporating droplet. Our results are in support of recent theoretical investigations on circulation reversal in evaporating drops. The flow patterns were driven by the surface tension gradient caused by the surface temperature distribution along the droplet-air interface. The surface temperature distribution is not only dependent on the non-uniform evaporation flux but also on the conduction path from the substrate to the droplet surface. Towards the end of evaporation, the fluid flow becomes radially outward from the center to the pinned edge of the droplet. We have also investigated the evaporating droplets placed on a curved substrate. Unlike the case of flat substrate, the evaporation from curved surface resulted monotonic temperature profile. The particle distribution and resulting deposition patterns was strongly influenced by the internal fluid flow pattern and was observed to be different from that of a flat substrate.

Numerical Study on Spray Drying Process: Effect of Nonuniform Temperature Field and Interaction between Droplets on Evaporation Rates of Individual Droplets.

Spray drying process is widely used to produce particulate materials in the pharmaceutical industries, such as porous materials for direct compression, solid dispersion for improvement of drug dissolution properties, micro encapsulation to stabilize active compounds, taste masking, preparation of dry powder for inhalation. However, as many factors affect the physical properties of dried particles and the spray drying processes have complex behaviors in which heat and mass transfer occur simultaneously, the detailed mechanisms of dry particle generation have yet to be sufficiently elucidated. In this study, computational fluid dynamics was used to simulate water droplet evaporation in a spray dryer, and the evaporation kinetics of "individual droplets" in the droplet aggregate (group) were analyzed. The numerical simulation revealed that each droplet had different evaporation rates owing to the following two reasons. First, the driving force of evaporation, ΔT, changed every moment as the droplets traveled through different temperature fields in the drying tower. Second, it was calculated the driving force for droplet evaporation differed from the ideal system because the evaporation of other droplets changed the fluid characteristics around the droplets. The obtained results are important findings that lead to the understanding the spray drying process to design and manufacture the pharmaceutical products.

Breakup dynamics of capillary bridges on hydrophobic stripes

The breakup dynamics of a capillary bridge on a hydrophobic stripe between two hydrophilic stripes is studied experimentally and numerically using direct numerical simulations. The capillary bridge is formed from an evaporating water droplet wetting three neighboring stripes of a chemically patterned surface. By considering the breakup process in a phase space representation, the breakup dynamics can be evaluated without the uncertainty in determining the precise breakup time. The simulations are based on the Volume-of-Fluid (VOF) method implemented in Free Surface 3D (FS3D). In order to construct physically realistic initial data for the VOF simulation, Surface Evolver is employed to calculate an initial configuration consistent with experiments. Numerical instabilities at the contact line are reduced by a novel discretization of the Navier-slip boundary condition on staggered grids. The breakup of the capillary bridge cannot be characterized by a unique scaling relationship. Instead, at different stages of the breakup process different scaling exponents apply, and the structure of the bridge undergoes a qualitative change. In the final stage of breakup, the capillary bridge forms a liquid thread that breaks up consistently with the Rayleigh-Plateau instability.

EFFECT OF COATINGS WITH VERTICALLY AND HORIZONTALLY ORIENTED CARBON NANOTUBES ON EVAPORATION OF WATER DROPLETS

EFFECT OF COATINGS WITH VERTICALLY AND HORIZONTALLY ORIENTED CARBON NANOTUBES ON EVAPORATION OF WATER DROPLETS

Влияние концентрации капель воды в аэрозольном облаке на скорости их испарения

The results of experimental studies of the integral characteristics of the water droplets evaporation in aerosol cloud are presented. Variable parameters: initial radius of droplets 0.1–0.25 mm, temperature of combustion products 573–873 K, concentration of water droplets 0.03–0.1 l /m3. The ranges of variation of the water mass evaporation rate are determined depending on the concentration of droplets in the aerosol cloud and their initial sizes. Approximation expressions for the established dependencies are obtained. For the first time, an approach was proposed to determine the evaporation rate of aerosol droplets taking into account the known/calculated values of the evaporation rate of a single drop.