Evaporation Of Water Droplets Research Articles

Evaporation from thin porous Coatings: Pore size effects and predictive equation for homogeneous coatings

Evaporation of small water droplets on solids is hindered because surface tension pulls the droplet into a spherical cap that has a small perimeter. Our solution is to coat a solid with a very thin, porous layer into which the droplet flows to create a large-area disk with concomitant high rate of evaporation. We investigate evaporation by varying factors that have not been previously considered: pore size and distribution, contact angle, temperature, and relative humidity (RH).A larger pore size resulted in faster evaporation, which we explain through faster transport within the coating. Even faster evaporation occurred for a bilayer structure with small particles on the air side and larger particles on the solid side. The water advancing contact angle had an insignificant effect in the range from < 10° through to 60°.Our results for different pore sizes, temperature, humidity, and contact angle all collapse onto a single curve when appropriately normalized. This validates an equation that can be used for the evaporation from a homogeneous coating that depends only one empirical factor and the droplet volume. Since the volume is often user-controlled, we envisage that this equation can be used to predict evaporation and guide design of fast-drying coatings.

Fast and accurate phase processing in off-axis digital holography combining adaptive spatial filtering and an embedded GPU platform

Abstract Parallel-phase processing enables rapid phase extraction from off-axis digital holograms. To achieve fast and accurate results, the phase reconstruction processes were parallelized using improved filter algorithms and optimized programming strategies. First, an adaptive filtering method based on the Chan-Vese (CV) model which better suits parallelism was designed to extract the +1 term spectrum. We selected suitable computer unified device architecture (CUDA) libraries according to the characteristics of the key phase reconstruction steps. Acceleration technologies, such as virtual memory and shared memory, were used to improve the computational efficiency. Furthermore, we combined an improved 4f optical imaging system with an embedded graphic processing unit (GPU) platform to design a low-cost phase reconstruction system for off-axis digital holography. To verify the feasibility of our method, the reconstructed quality of the CV filtering method was estimated, and the run times of phase retrieval on the central processing unit (CPU) and embedded GPU were compared for off-axis holograms with different pixel sizes. Additionally, the dynamic fluctuation phase maps of water droplet evaporation were retrieved to demonstrate the real-time capability of the method.

Unveiling photon-driven nonlinear evaporation via liquid drop interferometry.

We investigated photomolecular-induced evaporation, wherein photons cleave off water clusters near water-vapor interfaces, bypassing the typical thermal evaporation process. However, thermal-induced evaporation is the main bottleneck to precisely identify photon-induced evaporation. Liquid drop interferometry (LDI) resolved this bottleneck, utilizing evaporating water drops as an active element. Interestingly, we first observed near-total internal reflection, a nonlinear increase in evaporation attributed to photomolecular-induced evaporation, which had never been studied before, to the best of our knowledge. Furthermore, by generating a standing wave on a partially metallic polished prism, we uncovered an unexpected enhancement in evaporation coinciding with the wave reaching its maxima at the air-water (AW) interface, validating that photomolecular-induced evaporation is a surface phenomenon. Significantly, our noninvasive measurements have identified transient deformation height as a key indicator of photon-induced cluster breaking and increased evaporation, thus significantly advancing our understanding of photomolecular effects on water droplet evaporation.

Evaporation Dynamics on a Lithium Niobate Surface.

Manipulating the water evaporation dynamics is a prerequisite in various modern-day applications like DNA stretching, rapid disease diagnostics, and inkjet printing. One method to affect the evaporation dynamics of droplets is to externally apply electric fields. However, surfaces that bear an intrinsic surface charge have not yet been investigated with respect to their evaporation behavior. In this study, we investigate water droplet evaporation on lithium niobate (LN), a ferroelectric material with a very high spontaneous polarization of 0.7 . Our results show that a droplet deposited on an LN surface evaporates in three stages: (i) constant contact radius (ii) mixed phase (iii) stick-slip, which is likely originating from the intrinsic surface charge. The influence of the polarization direction of the LN surface as well as the relative humidity of the environment on various evaporation characteristics were studied. The results suggest that the specific adsorption layers forming on charged surfaces, e. g. from the humidity of the surrounding air, play a key role in the evaporation process. Furthermore, compared to other materials with similar contact angles, LN demonstrated a significantly large evaporation rate. This property might also be attributed to the intrinsic surface charge and could be exploited in heat transfer applications.

Evaporation of Water Droplets and Corrosion on Various Graphene Coatings

Evaporation of Water Droplets and Corrosion on Various Graphene Coatings

THEORETICAL PERFORMANCE ASSESSMENT OF A PARABOLIC TROUGH HUMIDIFYING SOLAR COLLECTOR-BASED SOLAR STILL

In this paper, a parabolic trough humidifying solar collector-based solar still (PHSC-SS) is proposed. Its purpose is to apply some important performance improvement techniques to the flat plate humidifying solar collector-based solar still (flat plate HSC-SS), to significantly improve overall system performance. These included the use of parabolic trough solar concentrators and the design of humidifying solar collectors from evacuated tube collectors. The results reveal that, unlike flat plate HSC-SS, which must operate with a turbulent airflow regime to achieve optimum overall performance, PHSC-SS must operate with a laminar airflow regime and high inlet and outlet temperatures of air (at least 55 °C and less than 100 °C, at atmospheric pressure) in the heat collector element. For 900 W/m2 of incident solar irradiance, 2 m2 of solar collector area, and 0,00042 kg/s of air flow rate, the maximum energy efficiency, exergy efficiency and daily freshwater productivity of PHSC-SS were found to be 68,12%, 14,87% and 1,697 kg/h, respectively. Whereas for the same incident solar irradiance and solar collector area, and 0,1 kg/s of air flow rate, those of the flat plat HSC-SS were 72,9%, 1,12%, and between 1,07 – 2,923 kg/h (for inlet and outlet temperatures of air less than 30 °C, at atmospheric pressure), respectively. Although in some extreme cases freshwater productivity of flat plate HSC-SS can be higher than that of PHSC-SS, it should be noted that laminar airflow regime confers great advantages to PHSC-SS. These are higher air temperatures at condenser inlet (which ease water condensation process), no need of an auxiliary cooling device (needed in the flat plate HSC-SS), less mechanical vibrations of system, reduced condenser size, and less energy consumed by air blowers. Furthermore, the upper limit of the PHSC-SS is a PHSC-SS that operates without air flow, but rather by vaporization of water droplets at boiling point from absorber, followed by their suction to condenser, similarly to a flash evaporation.

Water droplet evaporation in varied gravity and electric fields

Sessile water droplet evaporation in varied gravity and electric fields has been experimentally studied. Specifically, the influences of gravity and electric fields are investigated in the context of the heat flux distribution beneath the droplets, as well as the droplet mechanics and resulting shapes. Experimental testing was carried out during a European Space Agency (ESA) Parabolic Flight Campaign (PFC 66). The droplets tested evaporated with a pinned contact line, a single wettability condition, and varied droplet volume and substrate heat flux. The peak heat transfer was located at the contact line for all cases. The peak heat flux, average heat flux, and droplet evaporation rate were shown to vary strongly with gravity, with higher values noted for hypergravity conditions and lower values in microgravity conditions. The droplet thermal inertia was shown to play a significant role, with larger droplets taking more time to reach thermal equilibrium during the parabolic testing period. No significant impact of the electric field on the droplet evaporation was noted for these test conditions.

Integrating hydrologic modeling and satellite remote sensing to assess the performance of sprinkler irrigation

ABSTRACT Improving irrigation water management is a key concern for the agricultural sector, and it requires extensive and comprehensive tools that provide a complete knowledge of crop water use and requirements. This study presents a novel methodology to explicitly estimate daily gross and net crop water requirements, actual crop water use, and irrigation efficiency of center pivot irrigation systems, by mainly utilizing the Sentinel-2 MultiSpectral Instrument (MSI) imagery at the farm scale. ETMonitor model is adapted to estimate actual water use (as the sum of canopy transpiration and evaporation of water intercepted by canopy and evaporation from soil) at daily/10-m resolution, benefiting from the high-resolution Sentinel-2 data and thus to assess the irrigation efficiency at the farm scale. The gross irrigation water requirement is estimated from the net crop water requirement and the water loss, including the water droplet evaporation directly into the air during application before droplets fall on the canopy and canopy interception loss. The method was applied to a pilot farmland with two major crops (wheat and potato) in the Inner Mongolia Autonomous Region of China, where modern equipment and appropriate irrigation methods are deployed for efficient water use. The estimated actual crop water use showed good agreement with the ground observations, e.g. the determination coefficients range from 0.67 to 0.81 and root mean square errors range from 0.56 mm/day to 1.24 mm/day for wheat and potato when comparing the estimated evapotranspiration with the measurement by the eddy covariance system. It also showed that the losses of total irrigated volume were 25.4% for wheat and 23.7% for potato, respectively, and found that the water allocation was insufficient to meet the water requirement in this irrigated area. This suggests that the amount of water applied was insufficient to meet the crop water requirement and the inherent water losses in the center pivot irrigation system, which imply the necessity to improve the irrigation practice to use the water more efficiently.

Construction of an Au/g-C3N4/GO/Au substrate by a layer-by-layer assembly approach for surface-enhanced Raman spectroscopy

In this work, an Au/g-C3N4/GO/Au hybrid nanofilm for surface-enhanced Raman scattering (SERS) substrate application was prepared by a facile layer-by-layer assembly technique using nanosheet-structured graphitic carbon nitride (g-C3N4), graphene oxide (GO) and 40 nm-gold nanoparticles (Au NPs). The effects of composition, assembly sequence and assembly cycles on the SERS performance were optimized to achieve the SERS-active substrate. The detection limit of Au/g-C3N4/GO/Au substrate for crystal violet and rhodamine 6 G probe molecules reached as high as 10−8 M, with good stability and uniformity. Its further application in food colorants detection (erythrosine, carmine and temptation red) had successfully met the national food safety standards and the quantitative detection for erythrosine and temptation red had also been established. Based on finite-difference time-domain element simulation and density functional theory quantum chemical calculation, the mechanism of the SERS effect of the substrate was unveiled, which is attributed to the combination of electromagnetic enhancement effect of Au nanoparticle aggregates and chemical enhancement effect of GO and g-C3N4. Notably, water droplet evaporation induced Raman enhancement was observed on the Au/g-C3N4/GO/Au substrate. This can be ascribed to the inhomogeneous evaporation of the droplet that promotes the enrichment of the probe molecules into the center of the droplet. This novel enhancement strategy provides a new idea for further improving detection sensitivity of SERS substrate materials.

Effects of liquid water injection on flame surface topology and propagation characteristics in spray flames: A direct numerical simulation analysis

The effects of water injection on flame surface topology and local flame propagation characteristics have been analyzed for statistically planar turbulent n-heptane spray flames with an overall (i.e., liquid + gaseous) equivalence ratio of unity using carrier-phase direct numerical simulations. Most fuel droplets have been found to evaporate as they approach the flame even though some droplets can survive until the burnt gas side is reached, whereas water droplets do not significantly evaporate ahead of the flame and the evaporation of water droplets starts to take place in the reaction zone and is completed within the burnt gas. However, the gaseous-phase combustion occurs predominantly in fuel–lean mode although the overall equivalence ratio remains equal to unity. The water injection has been found to suppress the fuel droplet-induced flame wrinkling of the progress variable isosurface under the laminar condition, and this effect is particularly strong for small water droplets. However, turbulence-induced flame wrinkling masks these effects, and thus, water injection does not have any significant impact on flame wrinkling for the turbulent cases considered here. The higher rate of evaporation and the associated high latent heat extraction for smaller water droplets induce stronger cooling effects, which weakens the effects of chemical reaction. This is reflected in the decrease in the mean values of density-weighted displacement speed with decreasing water droplet diameter. The weakening of flame wrinkling as a result of injection of small water droplets is explained through the curvature dependence of the density-weighted displacement speed. The combined influence of cooling induced by the latent heat extraction of water droplets and flame surface flattening leads to a decrease in volume-integrated burning rate with decreasing water droplet diameter in the laminar cases, whereas the cooling effects are primarily responsible for the drop in burning rate with decreasing water droplet diameter in the turbulent cases.

Study on the mechanism of sessile droplets evaporation enhanced by the electric field

The evaporation characteristics of a sessile droplet under a direct current electric field were numerically investigated, and a numerical model was developed based on the arbitrary Lagrange-Euler method, combining the flow, heat transfer, vapour transport, and electrostatic equations. The simulations were validated to be in good agreement with previously reported experiments. The results demonstrate that the electric field can enhance the evaporation of the sessile droplet with different evaporative cooling numbers (initial electric field intensity E0 = 280 kV·m−1; 32.9% decrease in the evaporation time for water) by the following three methods: increasing the surface area of the droplet evaporation (2.6%) through the electric-field-induced deformation, improving the heat transfer efficiency between the droplet and substrate (droplet average temperature increases by 2.3 K) through the internal vortex, and by enhancing the shearing effect on the vapour layer (450.3% increase in vapour diffusion distance dc) through the external vortex. Additionally, the evaporation of a sessile water droplet undergoing thermocapillary convection is further enhanced under the electric field (E0 = 280 kV·m−1; 20.0% decrease in the evaporation time). As the inhomogeneous temperature distribution inside the droplet was eliminated by thermocapillary convection, the evaporation efficiency was dominated by the increased surface area of the droplet evaporation and the enhanced shearing effect on the vapour layer. These results will provide a better understanding of the electro-hydrodynamic enhanced droplet evaporation.

Drying microcharacteristics of volatile and non-volatile droplets on smooth and structured surfaces

In order to analyze the dynamics of droplet evaporation from solid surfaces and to study the liquid-solid interactions, liquid droplets were deposited on substrates and examined under a microscope. Due to the variation of interfacial tensions at liquid-gas and at liquid-solid the contact line changes depending on the type of solid surface and liquid. In this study, two liquids with different surface tensions (water and ethanol) are employed to examine the effects of evaporation time and to elucidate the underlying mechanisms of solid-liquid interactions. In the case of rapidly evaporating ethanol droplets, the formation of residual droplets and internal currents was observed. In contrast, the evaporation of water droplets resulted in only a change in the shape of the contact line. The paper presents a dependence of the Ohnesorge number on the dimensionless diameter, which characterizes all the considered types of interaction. Additionally, an analysis of the contact angle and contact diameter is presented.

(Digital Presentation) Porous Silicon Formed by Electrochemical, Chemical and Combined Etching

This work is devoted the thermophysical characteristics (TPC) of PS formed by Electrochemical, Metal-assisted chemical and combined etching methods. The TPC of PS/MACE+EC formed by the MACE+EC– method is higher than those of the PS/EC and PS/MACE samples formed by the EC and MACE methods. The energy activity of water splitting on the surface of PS/MACE+EC is higher than the energy activity of the PS/EC and PS/MACE samples. The combined (MACE+EC) method is the best method for obtaining PS, which has the highest energy activity in water splitting processes. The activation energy of water splitting processes Ea(PS/MACE+EC) is higher than the energy activity of the Ea(PS/EC) and Ea(PS/MACE) samples. The presence of Ni particles in the pores of PS<Ni> stabilizes the them TPC. The TPC of PS were studied in comparison with the TPC of crystalline (c-Si) and powdered silicon (powder-Si). It is known that PS has a high catalytic activity of hydrogen evolution during photoelectrochemical splitting of water. For example, the use of a surface enriched Ni layer on the surface of a PS photocathode PS<Ni> made it possible to increase its catalytic activity two times higher than the catalytic activity of the Pt- electrode in the reaction of hydrogen evolution [1]. The TPC of PS samples formed by EC, MACE and combined (MACE+EC)- etching methods are studied in comparison with the TPC of c-Si and powder-Si samples, Table. Method Electrolyte Composition Marked I Et1 HF:H2O2=1:1 EC II Et2 HF:H2O2: Ni(NO3)2 =1:1:1 МАСЕ III Et3 HF:H2O2: Ni(NO3)2 =1:1:1 EC+МАСЕ According of electron microscopy research the highest porosity is observed on the PS/MACE+EC samples, the smallest - on the PS/MACE sample formed by the MACE- method. The Raman spectra testify this, PS/MACE+EC(Et3) samples have a higher intensity of the Raman peak at a high-frequency shift due to electron-phonon scattering, the intensity and frequency of which depend on the concentration of scattering centers and, consequently, on the conditions for the formation of pores [2]. The TPC of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) spectra record changes in the mass Δm of the sample and heat flux ΔQ with an isobaric temperature change ΔT at the points of T-characteristic. An analysis of the calorimetric TPC and the TGA and DSC spectra shows that c-Si and powder have almost the same patterns of change in thermal conductivity, in contrast to the PS-spectra. The PS/EC, PS/MACE samples have the maximum change in heat flux Q(PS/EC)=187.6 mW, Q(PS/MACE)=231.6 mW, in contrast to powder-Si Q(powder-Si)=107.1 mW and c-Si ΔQ(c-Si)=65.02 mW. This dependence, which is responsible for the heat flux ΔQ change in the high T-range is associated with the maximum developed surface PS. Due to the fact that the presence of the Ni in the pores of PS<Ni>/MACE and PS<Ni>/MACE+EC stabilizes the TPC of PS, and thereby prevents significant changes in the TPC of Ni-containing PS<Ni>, in contrast to PS that does not Ni-contain in pores. Also have carried out experiments on the evaporation of water droplets at room temperature from the surface of c-Si and PS/EC, PS/MACE PS/MACE+EC samples. Experiments on the evaporation of water droplets at Troom from the surface of crystalline and PS samples obtained by various etching methods showed that the highest energy activity is observed in PS/MACE+EC samples, Ea(PS/MACE+EC) = 0.2087 eV, which is much higher than Ea(PS/EC) = 0.070 eV, Ea(PS/MACE) = 0.050 eV and Еа(c-Si) = 0.025 eV,The porous surface of the PS/MACE+EC sample has the highest energy activity compared to the PS/EC and PS/MACE samples, which is due to the high porosity of PS/MACE+EC. The PS/MACE and PS/MACE+EC samples showed high energy stability over 1 year in water evaporation experiments. The values of the heat capacities of various EC-, MACE-, EC+MACE - etching methods are established: - Cp(c -Si) = 20.7 KJ/gK, Cp(powder-Si) = 13.11 KJ/gK, Cp(PS/MACE) = 12.79 KJ/gK, Cp(PS/AE) = 5.62 KJ/gK.Combination of electrochemical etching with Ni-stimulated and chemical etching (EC+MACE) makes the possibility Ni-containing nanoporous silicon with high energy activity in water splitting processes: - Еа(PS/MACE+EC) = 0.2087 eV, Еа(PS/MACE) = 0.070 eV , Еа(PS/АЕ) =0.050 eV.The Ni- presence in the pores stabilizes the TPC of PS/MACE and PS/MACE+EC. The stability of Ni-containing samples PS/MACE+EC) and Ea(PS/MACE) is maintained for one year. [1] K.B. Tynyshtykbayev et al. Russian Microelectronics, 2016, Vol. 45, Nos. 8–9, pp. 603–612. https://doi.org/10.1134/S106373971608014X[2] K. Tynyshtykbayev et al. ECS Journal of Solid State Science and Technology, 2021 10 013009 DOI: 10.1149/2162-8777/abdd86

Heat and Mass Transfer Processes and Evaporation of a Liquid Droplet on a Structured Surface

The characteristics of water droplet heating and evaporation on structured hydrophobic and hydrophilic surfaces in the range of static contact angles from 73° to 155° were studied experimentally using high-speed video recording. Two fundamentally different technologies for applying coatings on a metal surface were used in comparison with the results on a polished surface. Microscopic studies were conducted to identify the features of the formed coatings. The wetting properties were characterized by means of the static contact angle and the contact angle hysteresis: on polished surface No. 1 (contact angle—73°, hysteresis—11°), on structured surface No. 2 (contact angle—125°, hysteresis—9°), and on structured surface No 3 (contact angle—155°, hysteresis—7°). The experimental dependences of the droplet evaporation rate on the different surfaces under normal conditions (ambient air temperature—293 K, atmospheric pressure, humidity—35%) were obtained. The evaporation regimes of droplets on the surfaces under study were identified. Water droplets evaporated in the pinning mode on surfaces No. 1 and No. 2. When a water droplet evaporated on surface No 3, the droplet was in the constant contact angle regime for ≈90% of its lifetime. Based on the experimental data obtained, a two-dimensional model of conjugate heat and mass transfer was developed, which describes the heating and evaporation of a liquid droplet on structured hydrophobic and hydrophilic surfaces at a wide range of contact angles. Satisfactory agreement was obtained between the numerical simulation results and experimental data. Using the model, the fields of temperature, concentration and other key characteristics were established at different points in time. Recommendations for its application in the development of gas–vapor–droplet applications were formulated.

Experimental Investigation of the Water Droplet Evaporation on Inclined Surfaces by Taguchi and ANOVA Optimization Analysis

Abstract We experimentally investigated the evaporation characteristics of a sessile water droplet on a glass substrate with different surface roughness levels. The influence of five parameters is evaluated for the evaporation process: substrate temperature (30 °C, 45 °C, and 60 °C), surface roughness (P0, P600, and P60), droplet volume (3, 5, and 8 µL), water droplets initial temperature (30 °C, 40 °C, and 60 °C), and inclination angle (0 deg, 45 deg, and 75 deg) of the glass substrate. The Taguchi orthogonal array design of L27 is utilized to establish minimal candidate trial points for experimental works, and more trials have been conducted to quantify the effects accurately. Then, analysis of variance (ANOVA) has been used to evaluate the evaporation times for the sessile droplets. The results indicate that evaporation times decrease with rising substrate temperatures, increasing substrate inclination angle, and increasing initial water droplet temperatures. In contrast, evaporation times rise with increasing surface roughness and droplet volumes. After evaluation of the ANOVA analysis, surface roughness levels and droplet volumes are considered the most influential parameters after substrate temperatures, which is the most effective parameter on the evaporation times. On the other hand, initial water droplet temperatures and substrate inclination angle are less effective considering droplet evaporation times. A linear regression fit was derived via ANOVA analysis for the evaporation time, and the best mean deviation was found to be 10% from the experiments. The experimental outcomes were compared to previous research, and correlations were derived. The proposed correlation has given good results considering experimental and literature data.

Application of acoustic levitation for studying convective heat and mass transfer during droplet evaporation

The observation of acoustically levitated droplets offers great potential for studying their evaporation characteristics under well-defined ambient conditions. In contrast to alternatives such as sessile droplets or droplets suspended by a tiny fiber, the key advantage of acoustic levitation is the contact-free nature of the experimental method. However, acoustic streaming effects or droplet deformation by acoustic forces may affect the heat and mass transfer considerably. Consequently, the droplet evaporation process is usually dominated by these intrusive effects of the acoustic field. The main objective of this study is to present an experimental setup, which minimizes the disturbing effects of the levitation technique in order to enable a solid investigation of convective heat and mass transfer during droplet evaporation. To achieve this goal, a high resonance frequency of 100kHz is chosen and the investigation is limited to substantially smaller droplets compared to other studies in the literature. Moreover, an air flow is injected through a hole pattern in the reflector in order to control the relative velocity between the droplet and the gas phase. Particle Shadow Velocimetry is used to characterize the gas flow field in a stepwise approach, initially without any levitated droplet. Subsequently, the vortex structures around non-evaporating droplets are discussed. Eventually, evaporating water droplets are studied. The applied optical diagnostics offer the distinct advantage that the droplet size can be measured simultaneously with the surrounding gas flow field. Hence, the evaporation rate can be evaluated for different combinations of droplet size and relative velocity to the gas phase. For the entire range of resulting Reynolds numbers, the experimentally obtained evaporation rates were found to agree very well with results of a numerical evaporation model when a well-established correlation is used to account for convective heat and mass transfer.

Experimental Investigation and Modeling: Considerations of Simultaneous Surface Steel Droplets’ Evaporation and Corrosion

The present work focuses on the problem of steel surface corrosion as a kinetic expression when water droplets are repeatedly deposited and evaporated on/from its surface. This process, together with the rainwater film corrosion process, belongs to the theoretical foundations of the problem of atmospheric corrosion. It was considered that the formation of water droplets on surfaces is a random but repetitive process, as well as the fact that experimental and theoretical observations show that the droplet corrosion front of a metal surface is located in its zone circumference. We thus aimed to establish how the corrosion process evolves on a steel plate when many drops are deposited and removed repeatedly. An experimental setup and working procedure were used to obtain data characterizing the simultaneous process of steel surface corrosion and water droplet evaporation. For natural convection conditions with a variable relative humidity and temperature environment, an extensive data set consisting of the dynamics of individual droplet evaporation coupled simultaneously with the corrosion of the steel surface under the droplet was obtained. The mathematical models for evaporation and corrosion under the droplet have the same dynamic transfer surface for water evaporation and oxygen supply in the droplet. An approach for determining this surface depending on the momentary droplet mass was considered. Several simultaneous measurements of evaporation–corrosion dynamics were used to calibrate the coupled models, which were then used to show their compatibility with experimental data.

Independent microscale sensing of phase interface and surface temperature during droplet evaporation

Despite the important and pervasive nature of evaporative heat transfer phenomena, fundamental questions still remain about the microscopic processes that occur in and around individual droplets on a surface. In order to understand the underlying physics behind evaporative heat transfer, it is critical to have information at the individual droplet level regarding the surface temperature distribution with time as well as the location and speed of the moving multiphase contact line (MCL). In this work, a multifunctional microscale device comprised of a resistance heater, an array of spatially distributed thin-film resistance temperature detectors, and a phase interface sensing capacitance micro-sensor array has been utilized to measure the local heat transfer characteristics and MCL behavior simultaneously for the evaporation of individual sessile water droplets on a horizontal heated surface. The resistance- and capacitance-based operating principles of the micro-device means that it is capable of detecting temperature changes and tracking MCL at the microscale in real time even for applications with limited or no visibility such as within thermal management hardware or processing equipment. Importantly, having knowledge of the MCL’s location and speed with microscale precision allows for its influence on surface temperature and heat transfer to be directly studied rather than inferred. Results show that the MCL passage precedes the change in local surface temperature and the duration of the time difference between these events depends on the MCL’s speed. In addition, the passage of the MCL accounts for more than 70% of the overall temperature change during the evaporation process.

Evaporation of water droplets and nucleation at the interface

Evaporation of water droplets and nucleation at the interface

High solar water droplet evaporation rates from Actinocyclus sp. diatom frustules decorated with silver nanoparticles

Solar vapour generation technology has emerged as a promising approach for reducing the global problem of water shortage issues via the solar water interfacial evaporation technique utilised for water purification. However, achieving superior light absorption and a high evaporation rate in a solar evaporator remains challenging. Here, we show that high solar interfacial water evaporation rates can be achieved using a natural and sustainable biomaterial, diatom frustules, decorated with silver nanoparticles made using a one-pot synthesis method. The species of diatom selected for this study was Actinocyclus sp. that has complex nano- and micro-structured surfaces and air pores that lead to multiple light scattering. The absorptance before and after silver nanoparticle coating was measured after creating a thin frustule layer on a hydrophobically treated quartz substrate. It was found that the cleaned and uncoated frustules had zero absorptance from 400 nm to 1200 nm. However, silver nanoparticles coated frustules showed a large broadband absorptance up to 90 % from 400 nm to 1200 nm, covering most of the solar spectrum, due to localised surface plasma resonance and multiple light scattering in the air pores. The frustules were floated on the surface of a water droplet for solar water interfacial evaporation rate measurements. A high evaporation rate of 5.4 kg m−2 h−1 under 1 sun irradiation was found for silver nanoparticles decorated frustules and for a droplet volume of 10 mm3. This is significantly larger than that found for frustules without silver nanoparticles or bare water where the evaporation rates were 1.7 kg m−2 h−1 and 1.2 kg m−2 h−1, respectively. Moreover, increasing the number of floating frustules per drop resulted in a factor of 1.67 enhancement. Our results show that research into silver nanoparticle decorated frustules can potentially provide a novel route for fabricating a high-performance solar vapour generator.