Droplet motion driven by humidity gradients during evaporation and condensation.

The motion of droplets on solid surfaces in response to an external gradient is a fundamental problem with a broad range of applications, including water harvesting, heat exchange, mixing and printing. Here we study the motion of droplets driven by a humidity gradient, i.e. a variation in concentration of their own vapour in the surrounding gas phase. Using lattice-Boltzmann simulations of a diffuse-interface hydrodynamic model to account for the liquid and gas phases, we demonstrate that the droplet migrates towards the region of higher vapour concentration. This effect holds in situations where the ambient gradient drives either the evaporation or the condensation of the droplet, or both simultaneously. We identify two main mechanisms responsible for the observed motion: a difference in surface wettability, which we measure in terms of the Young stress, and a variation in surface tension, which drives a Marangoni flow. Our results are relevant in advancing our knowledge of the interplay between gas and liquid phases out of thermodynamic equilibrium, as well as for applications involving the control of droplet motion.

Ambient-mediated wetting on smooth surfaces.

A consensus was built in the first half of the 20th century, which was further debated more than 3 decades ago, that the wettability and condensation mechanisms on smooth solid surfaces are modified by the adsorption of organic contaminants present in the environment. Recently, disagreement has formed about this topic once again, as many researchers have overlooked contamination due to its difficulty to eliminate. For example, the intrinsic wettability of rare earth oxides has been reported to be hydrophobic and non-wetting to water. These materials were subsequently shown to display dropwise condensation with steam. Nonetheless, follow on research has demonstrated that the intrinsic wettability of rare earth oxides is hydrophilic and wetting to water, and that a transition to hydrophobicity occurs in a matter of hours-to-days as a consequence of the adsorption of volatile organic compounds from the ambient environment. The adsorption mechanisms, kinetics, and selectivity, of these volatile organic compounds are empirically known to be functions of the substrate material and structure. However, these mechanisms, which govern the surface wettability, remain poorly understood. In this contribution, we introduce current research demonstrating the different intrinsic wettability of metals, rare earth oxides, and other smooth materials, showing that they are intrinsically hydrophilic. Then we provide details on research focusing on the transition from wetting (hydrophilicity) to non-wetting (hydrophobicity) on somooth surfaces due to adsorption of volatile organic compounds. A state-of-the-art figure of merit mapping the wettability of different smooth solid surfaces to ambient exposure as a function of the surface carbon content has also been developed. In addition, we analyse recent works that address these wetting transitions so to shed light on how such processes affect droplet pinning and lateral adhesion. We then conclude with objective perspectives about research on wetting to non-wetting transitions on smooth solid surfaces in an attempt to raise awareness regarding this surface contamination phenomenon within the engineering, interfacial science, and physical chemistry domains.

Nanorough Is Not Slippery Enough: Implications on Shedding and Heat Transfer.

Lowering droplet-surface interactions via the implementation of lubricant-infused surfaces (LISs) has received important attention in the past years. LISs offer enhanced droplet mobility with low sliding angles and the recently reported slippery Wenzel state, among others, empowered by the presence of the lubricant infused in between the structures, which eventually minimizes the direct interactions between liquid droplets and LISs. Current strategies to increase heat transfer during condensation phase-change relay on minimizing the thickness of the coating as well as enhancing condensate shedding. While further surface structuring may impose an additional heat transfer resistance, the presence of micro-structures eventually reduces the effective condensate-surface intimate interactions with the consequently decreased adhesion and enhanced shedding performance, which is investigated in this work. This is demonstrated by macroscopic and optical microscopy condensation experimental observations paying special attention at the liquid-lubricant and liquid-solid binary interactions at the droplet-LIS interface, which is further supported by a revisited force balance at the droplet triple contact line. Moreover, the occurrence of a condensation-coalescence-shedding regime is quantified for the first time with droplet growth rates one and two orders of magnitude greater than during condensation-coalescence and direct condensation regimes, respectively. Findings presented here are of great importance for the effective design and implementation of LISs via surface structure endowing accurate droplet mobility and control for applications such as anti-icing, self-cleaning, water harvesting, and/or liquid repellent surfaces as well as for condensation heat transfer.

Tuning contact line dynamics on slippery silicone oil grafted surfaces for sessile droplet evaporation.

Controlling the dynamics of droplet evaporation is critical to numerous fundamental and industrial applications. The three main modes of evaporation so far reported on smooth surfaces are the constant contact radius (CCR), constant contact angle (CCA), and mixed mode. Previously reported methods for controlling droplet evaporation include chemical or physical modifications of the surfaces via surface coating. These often require complex multiple stage processing, which eventually enables similar droplet-surface interactions. By leveraging the change in the physicochemical properties of the outermost surface by different silicone oil grafting fabrication parameters, the evaporation dynamics and the duration of the different evaporation modes can be controlled. After grafting one layer of oil, the intrinsic hydrophilic silicon surface (contact angle (CA) ≈ 60°) is transformed into a hydrophobic surface (CA ≈ 108°) with low contact angle hysteresis (CAH). The CAH can be tuned between 1° and 20° depending on the fabrication parameters such as oil viscosity, volume, deposition method as well as the number of layers, which in turn control the duration of the different evaporation modes. In addition, the occurrence and strength of stick-slip behaviour during evaporation can be additionally controlled by the silicone oil grafting procedure adopted. These findings provide guidelines for controlling the droplet-surface interactions by either minimizing or maximising contact line initial pinning, stick-slip and/or constant contact angle modes of evaporation. We conclude that the simple and scalable silicone oil grafted coatings reported here provide similar functionalities to slippery liquid infused porous surfaces (SLIPSs), quasi-liquid surfaces (QLS), and/or slippery omniphobic covalently attached liquid (SOCAL) surfaces, by empowering pinning-free surfaces, and have great potential for use in self-cleaning surfaces or uniform particle deposition.

Binary Mixture Droplet Evaporation on Microstructured Decorated Surfaces and the Mixed Stick-Slip Modes.

The interactions between liquid droplets and solid surfaces during wetting and phase change are important to many applications and are related to the physicochemical properties of the substrate and the fluid. In this work, we investigate experimentally the evaporation of pure water, pure ethanol, and their binary mixture droplets, accessing a wide range of surface tensions, on hydrophobic micro-pillared surfaces varying the spacing between the pillars. Results show that on structured surfaces, droplets evaporate following three classical evaporative behaviors: constant contact radius/pinning, stick-slip, or mixed mode. In addition, we report two further droplet evaporation modes, which are a mixed stick-slip mode where the contact angle increases while the contact radius decreases in a stick-slip fashion and a mixed stick-slip mode where both the contact angle and the contact radius decrease in a stick-slip fashion. We name these evaporation modes not yet reported in the literature as the increasing and decreasing contact angle mixed stick-slip modes, respectively. The former ensues because the fluid surface tension increases as the most volatile fluid evaporates coupled to the presence of structures, whereas the latter is due to the presence of structures for either fluid. The duration of each evaporation mode is dissimilar and depends on the surface tension and on the spacing between structures. Pure water yields longer initial pinning times on all surfaces before stick-slip ensues, whereas for binary mixtures and pure ethanol, initial pinning ensues mainly on short spacing structures due to the different wetting regimes displayed. Meanwhile, mixed stick-slip modes ensue mainly for high ethanol concentrations and/or pure ethanol independent of the solid fraction and for low ethanol concentrations on large spacing. Contact line jumps, changes in contact angle and pinning forces are also presented and discussed. This investigation provides guidelines for tailoring the evaporation of a wide range of surface tension fluids on structured surfaces for inkjet printing, DNA patterning, or microfluidics applications.

3D Solar Evaporation Enhancement by Superhydrophilic Copper Foam Inverted Cone and Graphene Oxide Functionalization Synergistic Cooperation.

Solar evaporation has become a promising and sustainable technique for harvesting freshwater from seawater and wastewater. However, the applicability and efficacy of solar evaporation need further improvement to achieve high production closer to theoretical limits in compact systems. A 3D (three-dimensional) hierarchical inverted conical solar evaporation is developed, which consists of a 3D copper foam skeleton cone decorated with micro-/nano-structures functionalized with graphene oxide, fabricated via easy and scalable wet oxidation, impregnation, and drying at room temperature. The proposed configuration empowers high-efficiency solar absorption, continuous liquid film spreading and transport, enhanced interfacial local evaporation, and rapid vapor diffusion through the pores. More notably, the 3D conical evaporator realizes thermal localization at the skeleton interface and allows evaporation to occur along the complete structure with unimpeded liquid and vapor rapid diffusion. The solar-thermal evaporation efficiency under 1-Sun is as high as 93% with a maximum evaporation rate per unit area of 1.71 kg·m-2 ·h-1 . This work highlights the benefits of synergistic cooperation of an easily scalable 3D hierarchical functiomicro-/nano-structured copper foam skeletons and functionalized graphene oxide for high-efficient solar evaporation of interest to numerous applications.

Suppression of crystallization in saline drop evaporation on pinning-free surfaces.

For sessile droplets of pure liquid on a surface, evaporation depends on surface wettability, the surrounding environment, contact angle hysteresis, and surface roughness. For non-pure liquids, the evaporation characteristics are further complicated by the constituents and impurities within the droplet. For saline solutions, this complication takes the form of a modified partial vapor pressure/water activity caused by the increasing salt concentration as the aqueous solvent evaporates. It is generally thought that droplets on surfaces will crystallize when the saturation concentration is reached, i.e., 26.3% for NaCl in water. This crystallization is initiated by contact with the surface and is thus due to surface roughness and heterogeneities. Recently, smooth, low contact angle hysteresis surfaces have been created by molecular grafting of polymer chains. In this work, we hypothesize that by using these very smooth surfaces to evaporate saline droplets, we can suppress the crystallization caused by the surface interactions and thus achieve constant volume droplets above the saturation concentration. In our experiments, we used several different surfaces to examine the possibility of crystallization suppression. We show that on polymer grafted surfaces, i.e., Slippery Omniphobic Covalently Attached Liquid-like (SOCAL) and polyethyleneglycol(PEGylated) surfaces, we can achieve stable droplets as low as 55% relative humidity at 25 °C with high reproducibility using NaCl in water solutions. We also show that it is possible to achieve stable droplets above the saturation concentration on other surfaces, including superhydrophobic surfaces. We present an analytical model, based on water activity, which accurately describes the final stable volume as a function of the initial salt concentration. These findings are important for heat and mass transfer in relatively low humidity environments.

Silicone Oil-Grafted Low-Hysteresis Water-Repellent Surfaces.

Wetting plays a major role in the close interactions between liquids and solid surfaces, which can be tailored by modifying the chemistry as well as the structures of the surfaces' outermost layer. Several methodologies, such as chemical vapor deposition, physical vapor deposition, electroplating, and chemical reactions, among others, have been adopted for the alteration/modification of such interactions suitable for various applications. However, the fabrication of low-contact line-pinning hydrophobic surfaces via simple and easy methods remains an open challenge. In this work, we exploit one-step and multiple-step silicone oil (5-100 cSt) grafting on smooth silicon substrates (although the technique is suitable for other substrates), looking closely at the effect of viscosity as well as the volume and layers (one to five) of oil grafted as a function of the deposition method. Remarkably, the optimization of grafting of silicone oil fabrication results in non-wetting surfaces with extremely low contact angle hysteresis (CAH) below 1° and high contact angles (CAs) of ∼108° after a single grafting step, which is an order of magnitude smaller than the reported values of previous works on silicone oil-grafted surfaces. Moreover, the different droplet-surface interactions and pinning behavior can additionally be tailored to the specific application with CAH ranging from 1 to 20° and sliding angles between 1.5 and 60° (for droplet volumes of 3 µL), depending on the fabrication parameters adopted. In terms of roughness, all the samples (independent of the grafting parameters) showed small changes in the root-mean-square roughness below 20 nm. Lastly, stability analysis of the grafting method reported here under various conditions shows that the coating is quite stable under mechanical vibrations (bath ultrasonication) and in a chemical environment (ultrasonication in a bath of ethanol) but loses its low-pinning characteristics when exposed to saturated steam at T ∼ 99 °C. The findings presented here provide a basis for selecting the most appropriate and suitable method and parameters for silicone oil grafting aimed at low pinning and low hysteresis surfaces for specific applications.

Binary mixture droplet wetting on micro-structure decorated surfaces.

Liquid surface tension as well as solid structure play a paramount role on the intimate wetting and non-wetting regimes and interactions between liquids droplets and solid substrates. We hypothesise that the coupling of these two variables, independently addressed in the past, eventually offer a wider range of understanding to the surface science and interfacial communities. In this work, intrinsically hydrophobic micro-pillared surfaces varying in the spacing between structures, and pure ethanol, pure water and their binary mixtures (as well as acetone-water and ethylene glycol-water mixtures) are utilised, accessing a wide range of substrate solid fractions and liquid surface tensions experimentally. Wettability measurements are carried out at different azimuthal directions to exemplify the wetting/non-wetting behaviour as well as the droplet asymmetry function of both liquid composition and structure spacing. Our findings reveal that high water concentration droplets, i.e., high surface tension fluids, sit in the Cassie-Baxter regime while partial non-wetting Wenzel or mixed-mode regimes with enhanced droplet asymmetry ensuing for medium and high ethanol concentrations, i.e., low surface tension fluids, below certain micropillar spacing. Beyond micropillar spacing s ≥ 40 µm, the impact of the surface structure on the droplet shape is negligible, and droplets adopt a similar contact angle and circular shape as on a flat smooth hydrophobic surface. Wetting and non-wetting regimes are then supported by classical wetting theories and equations. A wetting regime map for a wide range of surface tension fluids and/or their mixtures on a wide domain of solid fractions is then proposed.

The apparent surface free energy of rare earth oxides is governed by hydrocarbon adsorption.

The surface free energy of rare earth oxides (REOs) has been debated during the last decade, with some reporting REOs to be intrinsically hydrophilic and others reporting hydrophobic. Here, we investigate the wettability and surface chemistry of pristine and smooth REO surfaces, conclusively showing that hydrophobicity stems from wettability transition due to volatile organic compound adsorption. We show that, for indoor ambient atmospheres and well-controlled saturated hydrocarbon atmospheres, the apparent advancing and receding contact angles of water increase with exposure time. We examined the surfaces comprehensively with multiple surface analysis techniques to confirm hydrocarbon adsorption and correlate it to wettability transition mechanisms. We demonstrate that both physisorption and chemisorption occur on the surface, with chemisorbed hydrocarbons promoting further physisorption due to their high affinity with similar hydrocarbon molecules. This study offers a better understanding of the intrinsic wettability of REOs and provides design guidelines for REO-based durable hydrophobic coatings.

Bioinspired functional SLIPSs and wettability gradient surfaces and their synergistic cooperation and opportunities for enhanced condensate and fluid transport.

Bioinspired smart functional surfaces have received increasing attention in recent years owed to their tunable wettability and enhanced droplet transport suggesting them as excellent candidates for industrial and nanotechnology-related applications. More specifically, bioinspired slippery lubricant infused porous surfaces (SLIPSs) have been proposed for their low adhesion enabling continuous dropwise condensation (DWC) even of low-surface tension fluids. In addition, functional surfaces with chemical and/or structural wettability gradients have also been exploited empowering spontaneous droplet transport in a controlled manner. Current research has focused on the better understanding of the mechanisms and intimate interactions taking place between liquid droplets and functional surfaces or on the forces imposed by differences in surface wettability and/or by Laplace pressure owed to chemical or structural gradients. Nonetheless, less attention has been paid to the synergistic cooperation of efficiently driving droplet transport via chemical and/or structural patterns/gradients on a low surface energy/adhesion background imposed by SLIPSs, with the consequent promising potential for microfluidics and condensation heat transfer applications amongst others. This review provides a detailed and timely overview and summary on recent advances and developments on bioinspired SLIPSs and on wettability gradient surfaces with focus on their synergistic cooperation for condensation and fluid transport related applications. Firstly, the fundamental theory and mechanisms governing complex droplet transport on homogeneous, on wettability gradient surfaces and on inclined SLIPSs are introduced. Secondly, recent advances on the fabrication and characterization of SLIPSs and functional surfaces are presented. Then, the condensation performance on such functional surfaces comprising chemical or structural wettability gradients is reviewed and their applications on condensation heat transfer are summarized. Last a summary outlook highlighting the opportunities and challenges on the synergistic cooperation of SLIPSs and wettability gradient surfaces for heat transfer as well as future perspective in modern applications are presented.

Development and validation of an instrument to evaluate medical students' lifestyle. / Desarrollo y validación de un instrumento para medir el estilo de vida de estudiantes de medicina.

INTRODUCTION: It is required to have validated instruments in health science students that identify unhealthy habits and assess the impact of educational interventions and programs aimed at promoting a healthy lifestyle. OBJECTIVE: To evaluate the validity and reliability of an instrument to measure medical students' lifestyles. METHODS: A lifestyle questionnaire was developed using the Delphi technique by a group of experts. The final questionnaire was applied to 332 students of the School of Medicine of the Ricardo Palma University in 2017. A preliminary examination was carried out to assess preconditions for construct validityincluding the correlation matrix, the Kaiser Meyer Olkin statistic, and the Bartlett sphericity test. Factor analysis was used for construct validity, and the possible resulting factors were extracted through the principal component analysis. Cronbach's alpha coefficient was calculated to assess the instrument reliability. RESULTS: In this study, 41.6% of participants were men with a mean age of 20 years (standard deviation = 3). The preconditions for the factor analysis were a Kaiser Meyer Olkin coefficient = 0.773 and a significant Bartlett sphericity test. For the 47 items of the final questionnaire, the factor analysis showed an explained variance of 56.7% with eigenvalues greater than one. Cronbach's alpha was 0.78. The final questionnaire could assume values between -23 to 151 points. Based on a cut point of 71 points, the prevalence of students with an unhealthy lifestyle was 73.6%. CONCLUSION: The developed instrument has acceptable validity and reliability to measure lifestyle in medical students. For external validation, studies in other university populations are suggested.

INTRODUCCIÓN: Se requiere contar con instrumentos validados en estudiantes de ciencias de la salud que permitan identificar hábitos no saludables, junto con evaluar el impacto de intervenciones y programas educativos destinados a promover un estilo de vida saludable. OBJETIVO: Evaluar la validez y fiabilidad de un instrumento para medir el estilo de vida en estudiantes de la carrera de medicina humana. MÉTODO: Se elaboró un cuestionario sobre estilos de vida desarrollado mediante la técnica Delphi entre un grupo de expertos. El cuestionario final resultante fue aplicado a una población de 332 estudiantes de la Facultad de Medicina Humana de una universidad peruana en el año 2017. Se realizó una exploración para evaluar las condiciones previas para ejecutar la validez de constructo, incluyendo la evaluación de la matriz de correlaciones, el estadístico Kaiser Meyer Olkin y la prueba de esfericidad de Bartlett. Para la validez de constructo se empleó el análisis factorial y se extrajeron los posibles factores resultantes por medio del análisis de componentes principales. Para demostrar la fiabilidad del instrumento se calculó el coeficiente α de Cronbach. RESULTADOS: El 41,6% de los participantes fueron varones. La edad media fue de 20 años (desviación estándar 3). Las condiciones previas al análisis factorial fueron un coeficiente Kaiser Meyer Olkin de 0,773 y una prueba de esfericidad de Bartlett significativa. El análisis factorial sobre los 47 ítems del cuestionario final obtuvo una varianza explicada de 56,7% con autovalores superiores a 1. El valor del α de Cronbach fue de 0,78. El cuestionario final puede tomar valores entre -23 hasta 151 puntos. Sobre la base de un punto de corte de 71 puntos, la prevalencia de estudiantes con un estilo de vida no saludable fue de 73,6%. CONCLUSIÓN: El instrumento desarrollado tiene validez y fiabilidad aceptable para medir el estilo de vida en estudiantes de medicina. Se sugiere la validación externa en otras poblaciones universitarias.

Gas-Phase Temperature Mapping of Evaporating Microdroplets.

Evaporation is a ubiquitous and complex phenomenon of importance to many natural and industrial systems. Evaporation occurs when molecules near the free interface overcome intermolecular attractions with the bulk liquid. As molecules escape the liquid phase, heat is removed, causing evaporative cooling. The influence of evaporative cooling on inducing a temperature difference with the surrounding atmosphere as well as within the liquid is poorly understood. Here, we develop a technique to overcome past difficulties encountered during the study of heterogeneous droplet evaporation by coupling a piezo-driven droplet generation mechanism to a controlled micro-thermocouple to probe microdroplet evaporation. The technique allowed us to probe the gas-phase temperature distribution using a micro-thermocouple (50 µm) in the vicinity of the liquid-vapor interface with high spatial (±10 µm) and temporal (±100 ms) resolution. We experimentally map the temperature gradient formed surrounding sessile water droplets having varying curvature dictated by the apparent advancing contact angle (100° ≲ θ ≲ 165°). The experiments were carried out at temperatures below and above ambient for a range of fixed droplet radii (130 µm ≲ R ≲ 330 µm). Our results provide a primary validation of the centuries-old theoretical framework underpinning heterogeneous droplet evaporation mediated by the working fluid, substrate, and gas thermophysical properties, droplet apparent contact angle, and droplet size. We show that microscale droplets residing on low-thermal-conductivity substrates such as glass absorb up to 8× more heat from the surrounding gas compared to droplets residing on high-thermal-conductivity substrates such as copper. Our work not only develops an experimental understanding of the heat transfer mechanisms governing droplet evaporation but also presents a powerful platform for the study and characterization of liquid-vapor transport at curved interfaces wetting and nonwetting advanced functional surfaces.

Phase-Change Slippery Liquid-Infused Porous Surfaces with Thermo-Responsive Wetting and Shedding States.

Slippery liquid-infused porous surfaces (SLIPSs) prepared with phase invariant materials (e.g., Krytox GPL oil) have been increasingly researched as low-adhesion engineered functional surfaces in the last decade. However, phase change materials (PCMs) have been scarcely adopted, although they are potential candidates because of their inherent lubricant characteristics as well as temperature-dependent phases empowering unique thermo-responsive switchable wettability. Here, paraffin wax (an organic PCM) has been applied on a hydrophobized nanoporous copper substrate to realize the phase-change SLIPSs (PC-SLIPSs) fabricated via spin-coating followed by thermal annealing, which overcomes earlier limitations encountered on the PC-SLIPSs. Advantages of these PC-SLIPSs are the prompting of a low-adhesion Wenzel state as opposed to the earlier completely pinned Wenzel state in the solid phase and the optimized slippery state without excess of PCM in the liquid phase. Further, in order to characterize the intimate interactions between liquid droplets and the different phases of the PC-SLIPSs, that is, solid, mush, and liquid phases, the contact line dynamics have been comprehensively investigated, unveiling the water droplet adhesion and depinning phenomenon as the function of the thermo-responsive wetting states. Lastly, the PC-SLIPSs have also been tested for water vapor condensation, demonstrating the feasibility of dropwise condensation and the shift of the droplet size distribution in both the solid and liquid phases. The results suggest that such engineered surfaces have great potential to prompt and tune dropwise condensation via thermo-responsive switchable wettability for heat transfer and water harvesting applications.

Effect of Substrate Conductivity on the Transient Thermal Transport of Hygroscopic Droplets during Vapor Absorption.

In all kinds of liquid desiccant dehumidification systems, the temperature increase of the desiccant solution due to the effect of absorptive heating is one of the main reasons of performance deterioration. In this study, we look into the thermal effects during vapor absorption into single hygroscopic liquid desiccant droplets. Specifically, the effect of substrate conductivity on the transient heat and mass transfer process is analyzed in detail. The relative strength of the thermal effect and the solutal effect on the rate of vapor absorption is investigated and compared to the thermal effect by evaporative cooling taking place in pure water droplets. In the case of liquid desiccants, results indicate that the high thermal conductivity of copper substrates ensures more efficient heat removal, and the temperature at the droplet surface decreases more rapidly than that on Polytetrafluoroethylene (PTFE) substrates. As a result, the initial rate of vapor absorption on copper substrates slightly outweighs that on PTFE substrates. Further analysis by decomposing the vapor pressure difference indicates that the variation of vapor pressure caused by the temperature change during vapor absorption is much weaker than that induced by the concentration change. The conclusions demonstrate that a simplified isothermal model can be applied to capture the main mechanisms during vapor absorption into hygroscopic droplets even though it is evidenced to be unreliable for droplet evaporation.

Dropwise Condensation on Multiscale Bioinspired Metallic Surfaces with Nanofeatures.

Nonwetting surfaces engineered from intrinsically hydrophilic metallic materials are promising for self-cleaning, anti-icing, or condensation heat transfer applications where the durability of commonly applied hydrophobic coatings is an issue. In this work, we fabricate and study the wetting behavior and the condensation performance on two metallic nonwetting surfaces with varying number and size of roughness tiers without the need for further hydrophobic coating procedure. On one hand, the surface resembling a rose petal exhibits a sticky nonwetting behavior as drops wet the microscopic roughness features with consequent enhanced drop adhesion, which leads to filmwise condensation. On the other hand, the surface resembling a lotus leaf provides super-repellent nonwetting behavior prompting the continuous nucleation, growth, and departure of spherical drops in a dropwise condensation fashion. On a lotus leaf surface, the third nanoscale roughness tier (created by chemical oxidation) combined with ambience exposure prompts the growth of drops in the Cassie state with the benefit of minimal condensate adhesion. The two different condensation behaviors reported are well supported by a drop surface energy analysis, which accounts for the different wetting performance and the surface structure underneath the condensing drops. Further, we coated the above-mentioned surfaces with polydimethylsiloxane, which resulted in filmwise condensation due to the smoothening of the different roughness tiers. Continuous dropwise condensation on a hierarchical bioinspired lotus leaf metallic surface without the need for a conformal hydrophobic coating is hence demonstrated, which offers a novel path for the design and manufacture of noncoated metallic super-repellent surfaces for condensation phase change applications, among others.

Atmosphere-Mediated Superhydrophobicity of Rationally Designed Micro/Nanostructured Surfaces.

Superhydrophobicity has received significant attention over the past three decades owing to its significant potential in self-cleaning, anti-icing and drag reduction surfaces, energy-harvesting devices, antibacterial coatings, and enhanced heat transfer applications. Superhydrophobicity can be obtained via the roughening of an intrinsically hydrophobic surface, the creation of a re-entrant geometry, or by the roughening of a hydrophilic surface followed by a conformal coating of a hydrophobic material. Intrinsically hydrophobic surfaces have poor thermophysical properties, such as thermal conductivity, and thus are not suitable for heat transfer applications. Re-entrant geometries, although versatile in applications where droplets are deposited, break down during spatially random nucleation and flood the surface. Chemical functionalization of rough metallic substrates, although promising, is not utilized because of the poor durability of conformal hydrophobic coatings. Here we develop a radically different approach to achieve stable superhydrophobicity. By utilizing laser processing and thermal oxidation of copper (Cu) to create a high surface energy hierarchical copper oxide (CuO), followed by repeatable and passive atmospheric adsorption of hydrophobic volatile organic compounds (VOCs), we show that stable superhydrophobicity with apparent advancing contact angles ≈160° and contact angle hysteresis as low as ≈20° can be achieved. We exploit the structure length scale and structure geometry-dependent VOC adsorption dynamics to rationally design CuO nanowires with enhanced superhydrophobicity. To gain an understanding of the VOC adsorption physics, we utilized X-ray photoelectron and ion mass spectroscopy to identify the chemical species deposited on our surfaces in two distinct locations: Urbana, IL, United States and Beijing, China. To test the stability of the atmosphere-mediated superhydrophobic surfaces during heterogeneous nucleation, we used high-speed optical microscopy to demonstrate the occurrence of dropwise condensation and stable coalescence-induced droplet jumping. Our work not only provides rational design guidelines for developing passively durable superhydrophobic surfaces with excellent flooding-resistance and self-healing capability but also sheds light on the key role played by the atmosphere in governing wetting.

Water vapor uptake into hygroscopic lithium bromide desiccant droplets: mechanisms of droplet growth and spreading.

The study of vapor absorption into liquid desiccant droplets is of general relevance to a better understanding and description of vapor absorption phenomena occurring at the macroscale as well as for practical optimization of dehumidification and refrigeration processes. Hence, in the present work, we provide the first systematic experimental study on the fundamentals of vapor absorption into liquid desiccant at the droplet scale, which initiates a novel avenue for the research of hygroscopic droplet growth. More specifically we address the behavior of lithium bromide-water droplets on hydrophobic PTFE and hydrophilic glass substrates under controlled ambient conditions. Driven by the vapor pressure difference between the ambient air and the droplet interface, desiccant droplets absorb water vapor and increase in volume. To provide further insights on the vapor absorption process, the evolution of the droplet profile is recorded using optical imaging and relevant profile characteristics are extracted. Results show that, even though the final expansion ratio of droplet volume is only a function of relative humidity, the dynamics of contact line and the absorption rate are found to differ greatly when comparing data with varying substrate wettability. Droplets on hydrophilic substrates show higher absorption kinetics and reach equilibrium with the ambient much faster than those on hydrophobic substrates. This is attributed to the absorption process being controlled by solute diffusion on the droplet side and to the shorter characteristic length for the solute diffusion on hydrophilic substrates. Moreover, the apparent droplet spreading process on hydrophilic substrates when compared to hydrophobic ones is explained based on a force balance analysis near the triple contact line, by the change of liquid-vapor surface tension due to the increase in water concentration, and assuming a development of a precursor film.

Quantifying vapor transfer into evaporating ethanol drops in a humid atmosphere.

The effect of ambient temperature and relative humidity on the dynamics of ethanol drop evaporation is investigated. Although drop evaporation of mixtures and pure fluids has been extensively studied, very little is known about the transition from a pure fluid to a binary mixture following transfer of a second component present in the atmosphere. This is of importance for industrial, biological and medical applications where the purity of the solvent is paramount. Adsorption-absorption and/or condensation of water into ethanol drops during evaporation is presented through direct quantification of the drop composition in time. In particular, we combine drop profile measurements with Gas Injection Chromatography (GIC) to directly quantify the amount of ethanol evaporated and that of water intake over time. As expected, drops evaporate faster at higher temperatures since both the ethanol saturation concentration and the vapor diffusion coefficient are directly proportional to temperature. On the other hand, increases in the ethanol evaporation rate and in the water intake are observed at higher relative humidity. The increase in ethanol evaporation at higher relative humidity is interpreted by the greater diffusion coefficient of ethanol into humid air when compared to dry air. Moreover, as ethanol evaporates in a high humidity environment, the drop interfacial temperature falls below the dew point due to evaporative cooling and water condenses compared to lower humidity conditions. As a consequence of the heat released by adsorption-absorption and/or condensation, a greater temperature is reported at the liquid-vapor interface as confirmed by IR thermography, inducing a greater ethanol saturation concentration at the surface and hence a greater driving force for evaporation. By coupling the drop profile and the composition of ethanol and water within the drop, we propose a combined evaporation-adsorption/absorption and/or condensation empirical correlation. The proposed correlation accounts for: the decreases in ethanol concentration due to water adsorption-absorption and/or condensation, the diffusion coefficient dependence on relative humidity, and the amount of water intake during evaporation. The proposed empirical correlation agrees remarkably well with experimental observations.

Enhanced Coalescence-Induced Droplet-Jumping on Nanostructured Superhydrophobic Surfaces in the Absence of Microstructures.

Superhydrophobic surfaces are receiving increasing attention due to the enhanced condensation heat transfer, self-cleaning, and anti-icing properties by easing droplet self-removal. Despite the extensive research carried out on this topic, the presence or absence of microstructures on droplet adhesion during condensation has not been fully addressed yet. In this work we, therefore, study the condensation behavior on engineered superhydrophobic copper oxide surfaces with different structural finishes. More specifically, we investigate the coalescence-induced droplet-jumping performance on superhydrophobic surfaces with structures varying from the micro- to the nanoscale. The different structural roughness is possible due to the specific etching parameters adopted during the facile low-cost dual-scale fabrication process. A custom-built optical microscopy setup inside a temperature and relative humidity controlled environmental chamber was used for the experimental observations. By varying the structural roughness, from the micro- to the nanoscale, important differences on the number of droplets involved in the jumps, on the frequency of the jumps, and on the size distribution of the jumping droplets were found. In the absence of microstructures, we report an enhancement of the droplet-jumping performance of small droplets with sizes in the same order of magnitude as the microstructures. Microstructures induce further droplet adhesion, act as a structural barrier for the coalescence between droplets growing on the same microstructure, and cause the droplet angular deviation from the main surface normal. As a consequence, upon coalescence, there is a decrease in the net momentum in the out-of-plane direction, and the jump does not ensue. We demonstrate that the absence of microstructures has therefore a positive impact on the coalescence-induced droplet-jumping of micrometer droplets for antifogging, anti-icing, and condensation heat transfer applications.