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The interfacial energy γWe perform static and advancing contact-line experiments with aqueous and organic liquids on different hydrophilic surfaces (Al, Cu, Si) with varying Wenzel roughnesses in the range 1-2. These parameters are combined with the liquid's known surface tension to determine FF

Study of nucleation and growth dynamic events of cubic-phase ice crystals at TiO2–water nanointerface.

Surfaces with extreme wettability (too low, superhydrophobic; too high, superhydrophilic) have attracted considerable attention over the past two decades. Titanium dioxide (TiO2) has been one of the most popular components for generating superhydrophobic/hydrophilic coatings. Combining TiO2 with ethanol and a commercial fluoroacrylic copolymer dispersion, known as PMC, can produce coatings with water contact angles approaching 170°. Another property of interest for this specific TiO2 formulation is its photocatalytic behavior, which causes the contact angle of water to be gradually reduced with rising timed exposure to UV light. While this formulation has been employed in many studies, there exists no quantitative guidance to determine or tune the contact angle (and thus wettability) with the composition of the coating and UV exposure time. In this article, machine learning models are employed to predict the required UV exposure time for any specified TiO2/PMC coating composition to attain a certain wettability (UV-reduced contact angle). For that purpose, eight different coating compositions were applied to glass slides and exposed to UV light for different time intervals. The collected contact-angle data was supplied to different regression models to designate the best method to predict the required UV exposure time for a prespecified wettability. Two types of machine learning models were used: (1) parametric and (2) nonparametric. The results showed a nonlinear behavior between the coating formulation and its contact angle attained after timed UV exposure. Nonparametric methods showed high accuracy and stability with general regression neural network (GRNN) performing best with an accuracy of 0.971, 0.977, and 0.933 on the test, train, and unseen data set, respectively. The present study not only provides quantitative guidance for producing coatings of specified wettability, but also presents a generalized methodology that could be employed for other functional coatings in technological applications requiring precise fluid/surface interactions.

The persisting outbreak of SARS-CoV-2 has posed an enormous threat to global health. The sustained human-to-human transmission of SARS-CoV-2 via respiratory droplets makes the medical procedures around the perioral area vulnerable to the spread of the disease. Such procedures include the ultrasonic dental cleaning method, which occurs within the oral cavity and involves cavitation-induced sprays, thus increasing the risk of pathogen transmission via advection. To understand the associated health and safety risks for patients and clinicians, it is critical to understand the flow pattern of the spray cloud around the operating region, the size and velocity distribution of the emitted droplets, and the extent of fluid dispersion until ultimate deposit on surfaces or escape through air vents. In this work, the droplet size and velocity distributions of the spray emerging from the tip of a free-standing common ultrasonic dental cleaning device were characterized via high-speed imaging. Deionized water and 1.5% and 3% aqueous hydrogen peroxide (H2O2) solutions were used as working fluids, with the H2O2—an established oxidizing agent—intended to curb the survival of virus released in aerosols generated from dental procedures. The measurements reveal that the presence of H2O2 in the working fluid increases the mean droplet size and ejection velocity. Detailed computational fluid dynamic simulations with multiphase flow models reveal benefits of adding small amounts of H2O2 in the feed stream of the ultrasonic cleaner; this practice causes larger droplets with shorter residence times inside the clinic before settling down or escaping through air vents. The results suggest optimal benefits (in terms of fluid spread) of adding 1.5% H2O2 in the feed stream during dental procedures involving ultrasonic tools. The present findings are not specific to the COVID-19 pandemic but should also apply to future outbreaks caused by airborne droplet transmission.

Hypothesis: Quantitative characterization of surface wettability through contact angle (CA) measurement using the sessile droplet (SD) or captive bubble (CB) methods is often limited by the intrinsic wetting properties of the substrate. Situations may arise when an extreme surface wettability may preclude using one of the two methods for predicting the behaviors of droplets or bubbles on the surface. This warrants a relationship between the dynamic CAs measured via the SD and CB methods. While the two dynamic CAs (e.g., the advancing CA of SD and receding CA of CB) add up to 180° on a smooth surface, the simple geometric supplementary principle may not apply for rough surfaces. Experiments: We perform a systematic wettability characterization of solid substrates with varying degrees of roughness using the sessile-droplet and captive-bubble methods, and interpret the experimental observations using a theoretical model. Findings: The dynamic contact angles measured by the sessile-droplet and captive-bubble methods deviate from the supplementary principle as the surface roughness is increased. We present a theoretical explanation for this disparity and predict the values of the contact angles using prevalent thermodynamic models of wetting and contact-angle hysteresis on rough substrates. The theoretical prediction is in good agreement with the experimental observations.

The interaction of rising gas bubbles with submerged air-repelling or air-attracting surfaces is relevant to various technological applications that rely on gas-microvolume handling or removal. This work demonstrates how submerged metal meshes with super air-attracting/repelling properties can be employed to manipulate microvolumes of air, rising buoyantly in the form of bubbles in water. Superaerophobic meshes are observed to selectively allow the passage of air bubbles depending on the mesh pore size, the bubble volume-equivalent diameter, and the bubble impact velocity on the mesh. On the other hand, superaerophilic meshes reduce or amplify the volume captured from a train of incoming bubbles. Finally, a spatial wettability pattern on the mesh is used to control the size of the outgoing bubble, and an empirical relation is formulated to predict the released gas volume. The study demonstrates how porous materials with controlled wettability can be used to precisely modulate and control the outcome of bubble/mesh interactions.

Background Nanoscale surface roughness has been suggested to have antibacterial and antifouling properties. Several existing models have attempted to explain the antibacterial mechanism of nanoscale rough surfaces without direct observation. Here, conventional and liquid-cell TEM are implemented to observe nanoscale bacteria/surface roughness interaction. The visualization of such interactions enables the inference of possible antibacterial mechanisms. Methods and results Nanotextures are synthesized on biocompatible polymer microparticles (MPs) via plasma etching. Both conventional and liquid-phase transmission electron microscopy observations suggest that these MPs may cause cell lysis via bacterial binding to a single protrusion of the nanotexture. The bacterium/protrusion interaction locally compromises the cell wall, thus causing bacterial death. This study suggests that local mechanical damage and leakage of the cytosol kill the bacteria first, with subsequent degradation of the cell envelope. Conclusion Nanoscale surface roughness may act via a penetrative bactericidal mechanism. This insight suggests that future research may focus on optimizing bacterial binding to individual nanoscale projections in addition to stretching bacteria between nanopillars. Further, antibacterial nanotextures may find use in novel applications employing particles in addition to nanotextures on fibers or films.

The extreme liquid transport properties of carbon nanotubes present new opportunities for surpassing conventional technologies in water filtration and purification. We demonstrate that carbon nanotubes with wettability surface patterns act as nanopumps for the ultrafast transport of picoliter water droplets without requiring externally imposed pressure gradients. Large-scale molecular dynamics simulations evidence unprecedented speeds and accelerations on the order of 1010 g of droplet propulsion caused by interfacial energy gradients. This phenomenon is persistent for nanotubes of varying sizes, stepwise pattern configurations, and initial conditions. We present a scaling law for water transport as a function of wettability gradients through simple models for the droplet dynamic contact angle and friction coefficient. Our results show that patterned nanotubes are energy-efficient nanopumps offering a realistic path toward ultrafast water nanofiltration and precision drug delivery.

Surfaces with extreme wettability (too low, superhydrophobic; too high, superhydrophilic) have attracted considerable attention over the past two decades. Titanium dioxide (TiO

The characterization “thermal diode” (TD) has been used to portray systems that spread heat very efficiently in a specific direction but obstruct it from flowing in the opposite direction. In this study, a planar vapor chamber (VC) with a wickless, wettability-patterned side and an opposing wick-lined side is fabricated and tested as a thermal diode. When the chamber operates in the forward mode, heat is naturally driven away from the heat source; in the reverse mode, the system blocks heat flow, thus acting as a thermal diode. The low-profile assembly takes advantage of the phase-changing properties of water inside a sealed chamber. The wettability-patterned plate—when on the cooled side, e.g., forward operation mode—enables spatially controlled dropwise condensation (high heat transfer rate) and filmwise condensation (high drainage rate), thus facilitating an efficient transport mechanism of the condensed medium on superhydrophilic wedge tracks by way of Laplace pressure-driven capillary forces. The same chamber acts as a thermal blocker when the wick-covered plate is on the cooled side (reverse operating mode), trapping the condensate in the wick pores and blocking heat flow to the opposite side. The system's thermal behavior is similar to the theoretical electrical diode. This work explores the effect of the condenser's wettability pattern design and the chamber's fluid charging ratio (CR). With this system, thermal diodicities exceeding 20 have been achieved, and are tunable by altering the wettability pattern. The thermal rectification concept and its proper quantification in terms of possible definitions are discussed. The present vapor chamber—thermal diode design could be well-suited for an extensive range of thermal-management applications, ranging from aerospace, spacecraft, and smart-building construction materials, to electronics protection, electronics packaging, refrigeration, thermal control during energy harvesting, thermal isolation, etc.

Condensation of water vapor on metal surfaces has a plethora of engineering applications in the energy sector. This study demonstrates an approach of estimating the condensation heat transfer performance of a hydrophobic copper plate utilizing Artificial Intelligence (AI) techniques. The situation under study is an isothermal vertical cooled plate with an attached thin copper sheet exposed to a water-vapor environment devoid of non-condensable gases. Water was chosen due to its high latent heat of vaporization and its extensive industrial use, while copper was selected because of its high thermal conductivity (385 W/m-K). A thin layer of Teflon coating was applied to the copper surface to render it hydrophobic, allowing dropwise condensation (DwC) to be investigated. The experimental setup featured a glass condensation chamber, which allowed visualization of the condensation phenomena on the condenser; a cold plate attached to a chiller, to hold and cool down the condenser; a vacuum pump to maintain low-pressure conditions; a heat source along with pressure and temperature measurement instrumentation. The condensation setup allows to directly calculate the heat transfer coefficient of dropwise condensation and, at the same time, offers visual access to the condenser. The overall efficiency of the dropwise condensation mechanism depends on many key factors, such as nucleation density and rate, maximum droplet size and efficient condensate drainage. The surface-condensation phenomena can be video-recorded while using the instrumentation to measure the heat-transfer efficiency in real time. Computer Vision algorithms were employed to process the raw video files and generate data on the basic aspects of condensing droplet dynamics (number, diameter, area coverage, etc.). The data was evaluated and cross-compared with the heat-transfer coefficients determined from instrumentation, allowing to consider how the droplet characteristics change with experimental operating conditions. Machine learning algorithms, such as Artificial Neural Network (ANN), Multi-Layer Perceptron (MLP), General Regression Neural Network (GRNN), and one ensemble model, Gradient Boosting Regression (GBR) can help to directly determine the heat transfer performance of a specific surface (with known wettability characteristics) from its condensation characteristics. The hypothesis is explored that the visual characteristics of the condensate droplet dynamics can be used in conjunction with trained machine learning models to forecast the DwC heat transfer efficiency of this system without directly measuring the corresponding variables (e.g., temperatures, flow rate, etc.).

In this study, a vapor chamber - thermal diode apparatus is fabricated and tested. The system’s components are laser-functionalized, wickless, and wettability-patterned copper plates. This is the first thermal management component that is completely wick-free and operates due to surface modification and wettability patterning instead of utilizing wicks or any kind of wicking structures. Our approach takes advantage of the phase-changing property of water in a closed loop composed of two initially identical copper plates, one acting as an evaporator, and the other designed as a condenser. The plates have different wettability patterns, each especially designed/engineered to transform the simple flat rectangular copper plate to a functional component of a vapor chamber. The apparatus, while operating as a vapor chamber, displays a thermal resistance as low as 0.07 ± 0.01 K/W, and while operating as a thermal diode, it shows a diodicity, defined as the heat flux ratio in forward and reverse operation, as high as 9. This value demonstrates the system’s ability to effectively remove the heat from a heat source (i.e., electronic chip) while, in parallel, protecting it from damage by excessive heat backflow. The diodicity of the diode is ascribed to the different wettability-pattern designs on the two copper plates comprising the system. On the forward mode, the two patterns work as designed to enable heat transfer, but on the reverse mode, the patterns no longer function harmoniously with each other, and the heat transfer is hindered. The simplicity of the design and its moderate dimensions are important advantages that make this new thermal management component attractive for engineering applications.

Heat pipe heat spreaders are metal-wick based cooling devices which circulate a phase-changing liquid to transfer heat more efficiently than solid-metal heat sinks. Although metal wicks are excellent fluid transporters, they also have capillary limitations because of their high pressure drop. On the other hand, wettability-patterned surfaces do not face these limitations and as shown in this work, could be a viable substitute for metal wicks in heat pipes. The use of wettability-patterned surfaces facilitates the efficient transport of condensate and improves heat transfer from the hot (evaporator) to the cold (condenser) side of the device. In this study, a flat heat pipe of 10 cm operating length, with copper-wick evaporator and wickless wettability-patterned condenser is assembled and tested. Although in traditional heat pipes a fully wick-lined device is of interest, in this apparatus, the condensate’s size and type are being controlled via spatially designed superhydrophilic/hydrophobic juxtaposed areas to regulate both Dropwise Condensation (DwC) and Filmwise Condensation (FwC) towards enhancing the heat transfer capability while reducing the thermal resistance of the device. In this design, low-capillary pressure domains are designated to collect the condensate and return it back (via wick) to the evaporator’s wick. Variable-porosity copper wicks are incorporated on the heater and the rest of the heat pipe to facilitate liquid transport from the cold to the hot side. Two charging loads are examined with the best performance achieved for a thermal resistance 0.16 K/W at 105 W. In addition, a heat pipe with a uniform hydrophilic condenser was fabricated and compared to the wettability-patterned featured heat pipes.

Water is ubiquitous in natural systems, ranging from the vast oceans to the nanocapillaries in the earth crust or cellular organelles. In bulk or in intimate contact with solid surfaces, water molecules arrange themselves according to their hydrogen (H) bonding, which critically affects their short- and long-range molecular structures. Formation of H-bonds among water molecules designates the energy levels of certain nonbonding molecular orbitals of water, which are quantifiable by spectroscopic techniques. While the molecular architecture of water in nanoenclosures is of particular interest to both science and industry, it requires fine spectroscopic probes with nanometer spatial resolution and sub-eV energy sensitivity. Graphene liquid cells (GLCs), which feature opposing closely spaced sheets of hydrophobic graphene, facilitate high-resolution transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) measurements of attoliter water volumes encapsulated tightly in the GLC nanovessels. We perform in situ TEM and EELS analysis of water encased in thin GLCs exposed to room and cryogenic temperatures to examine the nanoscale arrangement of the contained water molecules. Simultaneous quantification of GLC thickness leads to the conclusion that H-bonding strengthens under increased water confinement. The present results demonstrate the feasibility of nanoscale chemical characterization of aqueous fluids trapped in GLC nanovessels and offer insights on water molecule arrangement under high-confinement conditions.

We study experimentally the cooling of a mm-thick, heat-spreading metal plate of O(10 cm2) area, which is heated locally by an embedded heat source with area O(1 cm2). A liquid jet impinges orthogonally on the plate several hydraulic diameters away from the localized heat source, and gets diverted on a surface track passing over the heated region, thus cooling the plate. Capillary-driven, directional transport of the cooling liquid (water) is achieved by rendering the metallic substrate hydrophobic and laying a wettable, diverging track on it, connecting the jet impact point at its narrow end with the heated region, which is either at the wider end of the track or around the half-way point. Cooling performance is evaluated in terms of sensible heat transfer at various flow rates for different track wedge angles and relative position of the heat source, a situation that emulates cooling of an integrated circuit (chip). The effect of thermocapillary stresses, which oppose the flow under certain conditions, is analyzed; effective strategies to overcome such effects are also devised and implemented. Finally, a thermocapillarity-resilient, multi-track design that exhibits superior performance (as compared to single-track designs) is rationalized and demonstrated. We observe that maintaining the track width below the capillary length of the working fluid is vital for improved cooling performance. Cooling can be improved by using multiple, narrow tracks fed by individual impinging jets or a track design that splits the jet into multiple streams laid over the heated domain. Heat removal rates of the order of 100 W/cm2 are attained without phase change at coolant flow rates as low as ∼1 mL/s and chip superheats of 65 °C. The approach opens up new opportunities for heat removing devices that rely on advective cooling facilitated by wettability-patterned metal substrates.

Liquid-jet impact on porous, relatively thin solids has a variety of applications in heat transfer, filtration, liquid-fuel atomization, incontinence products, and solid-substrate erosion, among others. Many prior studies focused on liquid-jet impact on impermeable substrates, and some have investigated the hydraulic jump phenomenon. In the present work, the liquid jet strikes a superhydrophobic, permeable, metal mesh orthogonally, and the radial spreading and throughflow of the liquid are characterized. The prebreakthrough hydraulic jump, the breakthrough velocity, and the postbreakthrough spatial distributions of the liquid are investigated by varying the liquid properties (density, surface tension, and viscosity) and the openness of the metal mesh. The hydraulic jump radius in the prebreakthrough regime increases with jet velocity and is independent of the liquid properties and mesh geometry (pore size, wire diameter and pitch). The breakthrough velocity increases with surface tension of the liquid and decreases with the mesh opening diameter and liquid viscosity. A simple analytical model predicts the jet breakthrough velocity; its predictions are in accordance with the experimental observations. In the postbreakthrough regime, as the jet velocity increases, the liquid flow rate penetrating the mesh shows an initially steep increase, followed by a plateau, which is attributed to a Cassie-Baxter-to-Wenzel transition at the impact area of the mesh.

COVID-19, caused by the SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) virus, has been rapidly spreading worldwide since December 2019, causing a public health crisis. Recent studies showed SARS-CoV-2's ability to infect humans via airborne routes. These motivated the study of aerosol and airborne droplet transmission in a variety of settings. This study performs a large-scale numerical simulation of a real-world dentistry clinic that contains aerosol-generating procedures. The simulation tracks the dispersion of evaporating droplets emitted during ultrasonic dental scaling procedures. The simulation considers 25 patient treatment cubicles in an open plan dentistry clinic. The droplets are modeled as having a volatile (evaporating) and nonvolatile fraction composed of virions, saliva, and impurities from the irrigant water supply. The simulated clinic's boundary and flow conditions are validated against experimental measurements of the real clinic. The results evaluate the behavior of large droplets and aerosols. We investigate droplet residence time and travel distance for different droplet diameters, surface contamination due to droplet settling and deposition, airborne aerosol mass concentration, and the quantity of droplets that escape through ventilation. The simulation results raise concerns due to the aerosols' long residence times (averaging up to 7.31 min) and travel distances (averaging up to 24.45 m) that exceed social distancing guidelines. Finally, the results show that contamination extends beyond the immediate patient treatment areas, requiring additional surface disinfection in the clinic. The results presented in this research may be used to establish safer dental clinic operating procedures, especially if paired with future supplementary material concerning the aerosol viral load generated by ultrasonic scaling and the viral load thresholds required to infect humans.

Spreading of liquid droplets on wettability-confined paths has attracted considerable attention in the past decade. On the other hand, the inverse scenario of a gas bubble spreading on a submerged, wettability-confined track has rarely been studied. In the present work, an experimental investigation of the spreading of millimetric gas bubbles on horizontally submerged, textured, wettability-confined tracks is carried out. The width of the track is kept fixed along its entire length, and the spreading behavior of a gas bubble, dispensed at one end of the track, is studied. The effects of varying track width, bubble diameter, and ambient liquid are investigated. Post-contact, the gas bubble spreads along the track at a linear rate with time, while remaining pinned at its back end; the recorded spreading speed is O(0.5 m/s). An inertio-capillary force balance describes the experimentally observed spreading dynamics with excellent agreement.

© 2020 American Physical Society. Numerical simulations have unexplored potential in the study of droplet impact on nonuniform wettability surfaces. In this paper, we compare numerical and experimental results to investigate the application potential of a volume-of-fluid method utilized in OpenFOAM. The approach implements the Kistler model for the dynamic contact angle of impacting droplets. We begin with an investigation into the influence of the most important solver parameters to optimize the computational setup and reach the best compromise between computational cost and solution errors, as assessed in comparison to experimental results. Next, we verify the accuracy of the predictions for droplet impact on uniformly hydrophilic or superhydrophobic surfaces. Benchmarking the maximal spreading factor, contact, and spreading times, as well as contact-line behavior, we show strong agreement between the present numerical results and the models of Pasandideh-Fard, Phys. Fluids 8, 650 (1996)PHFLE61070-663110.1063/1.868850 and Clanét, J. Fluid Mech. 517, 199 (2004)JFLSA70022-112010.1017/S0022112004000904. Lastly, we demonstrate the capability of the model to accurately predict outcome behaviors of droplets striking distributed-wettability surfaces, which introduce 3D outcome characteristics, even in orthogonal impact. The model successfully predicts droplet splitting and vectoring, as reported in the experiments of Schutzius, Sci. Rep. 4, 7029 (2014)2045-232210.1038/srep07029. Finally, we demonstrate a configuration wherein a droplet centrally strikes a circular disk of different wettability than its surrounding domain. The main contribution of the present paper is a numerical model capable of accurately simulating droplet impact on spatially nonuniform wettability patterns of any foreseeable design.

The term "vapor chamber" has been used to describe a device that spreads heat, as opposed to the term "thermal diode" that has been assigned to a device technically designed to prevent heat flow along a specific direction. In this study, a vapor chamber with a wickless, wettability-patterned condenser is fabricated and tested. The device takes advantage of the phase-changing properties of water, inside a closed loop comprised of a classical wick evaporator opposing a wickless wettability-patterned condenser. The wettability pattern facilitates spatially-controlled dropwise and filmwise condensation, and provides an efficient way to transport the condensate with specially-designed wedge tracks using capillary forces. The vapor chamber can also act as a thermal diode if the wickless condenser has a higher temperature than the wick, thus blocking heat flow in the opposite direction. When the device is performing as a vapor chamber (VC), the working medium evaporates from the hot superhydrophilic copper wick and condenses on the cold wickless condenser. The condensed water returns to the hot side of the device from strategically-placed superhydrophilic wells on the condenser, where condensate droplets accumulate, grow and ultimately bridge between the evaporator and the condenser. However, when the device is performing as a thermal diode (reverse mounted), evaporation must be initiated on the wickless plate and condensation occurs on the opposing wick of the condenser. In this way, the fluid circulation in the device is choked and heat transfer is impeded. The device diodicity is tunable by changing the wettability pattern and can be adjusted as needed for different thermal applications. When the device operates as a VC, heat is effortlessly pumped out of the heat source; at the same time, the device blocks the undesirable heat backflow while working as a thermal diode. The present VC thermal diode apparatus could prove beneficial in a wide spectrum of thermal-management applications, such as aerospace, spacecraft, building materials, protection of electronics, packaging, refrigeration, thermal regulation during energy harvesting, thermal isolation, etc.

With the rapid spread of COVID-19 worldwide, the demand for appropriate face masks in the market has also skyrocketed. To ease strain on the supply of masks to the essential healthcare sector, it has become imperative that ordinary people rely more on home-made masks that can be easily put together using commonly available materials, while at the same time performing reasonably at arresting the ingress or egress of airborne droplets. Here, we propose a simple do-it-yourself (DIY) method for preparing a three-layered face mask that deploys two hydrophobic polypropylene nonwoven layers interspaced with a hydrophilic cellulosic cloth. The first hydrophobic layer, facing the user, allows high-momentum droplets (e.g., expelled by a sneeze or cough) to pass through and get absorbed in the next hydrophilic layer, thereby keeping the skin in contact with the mask dry and comfortable. The third (outermost) hydrophobic layer prevents penetration of the liquids from the middle layer to the outside, and also arrests any airborne droplets on its exterior. Simple tests show that our masks perform better in arresting the droplet transmission as compared to surgical masks available in the market. Electronic supplementary material The online version of this article (10.1007/s41403-020-00115-9) contains supplementary material, which is available to authorized users.

The outbreak of unknown viral pneumonia in Wuhan China in December 2019 led to a new coronavirus (SARS-CoV-2), which attracted worldwide attention, with the related COVID-19 disease quickly becoming a global pandemic. In about 5 months, this disease has led to ~ 4 million cases and claimed more than 200 k deaths as a result of its highly contagious nature. The present understanding is that SARS-CoV-2 is a type of influenza virus that can be transmitted through respiratory droplets and aerosols; Lewis (Nature 580:175, 2020). The primary methodology to prevent the spreading of this disease has been “social distancing” and usage of personal protective equipment (PPE) at the front lines of healthcare and other critical operations. The scale of the disease has led to unprecedented demand for PPEs and increased functionality of the same. This paper focuses on improving PPE functionality in a scalable manner by surface treatment and coating with appropriate materials and other functional enhancements, such as exposure to UV rays or other sterilizing agents (e.g., hydrogen peroxide).

In the modern construction industry, large gypsum (CaSO4·2H2O) boards are manufactured through a two-step procedure, which features the heating of fine gypsum powders to form the intermediate plast...

Porous substrates have the ability to transport liquids not only laterally on their open surfaces but also transversally through their thickness. Directionality of the fluid transport can be achieved through spatial wettability patterning of these substrates. Different designs of wettability patterns are implemented herein to attain different schemes (modes) of three-dimensional transport in a high-density paper towel, which acts as a thin porous matrix directing the fluid. All schemes facilitate precise transport of metered liquid microvolumes (dispensed as droplets) on the surface and through the substrate. One selected mode features lateral fluid transport along the bottom surface of the substrate, with the top surface remaining dry, except at the initial droplet dispension point. This configuration is investigated in further detail, and an analytical model is developed to predict the temporal variation of the penetrating drop shape. The analysis and respective measurements agree within the experimental error limits, thus confirming the model's ability to account for the main transport mechanisms.

Liquid jet impingement on porous materials is particularly important in many applications of heat transfer, filtration, or in incontinence products. Generally, it is desired that the liquid not penetrate the substrate at or near the point of jet impact, but rather be distributed over a wider area before reaching the back side. A facile wettability-patterning technique is presented, whereby a water jet impinging orthogonally on a wettability-patterned nonwoven substrate is distributed on the top surface and through the porous matrix, and ultimately dispensed from prespecified points underneath the sample. A systematic approach is adopted to identify the optimum design that allows for a uniform distribution of the liquid on horizontally mounted substrates of ∼50 cm2 area, with minimal or no spilling over the sample edges at jet flow rates exceeding 1 L/min. The effect of the location of jet impingement on liquid distribution is also studied, and the design is observed to perform well even under offset jet i...

Spontaneous pumpless transport of droplets on wettability-confined tracks is important for various applications, such as rapid transport and mixing of fluid droplets, enhanced dropwise condensation, biomedical devices, and so forth. Recent studies have shown that on an open surface, a superhydrophilic track of diverging width, laid on a superhydrophobic background, facilitates the transport of water from the narrower end to the wider end at unprecedented rates (up to 40 cm/s) without external actuation. The spreading behavior on such surfaces, however, has only been characterized for water. Keeping in mind that such designs play a key role for a diverse range of applications, such as handling organic liquids and in point-of-care devices, the importance of characterizing the spreading behavior of viscous liquids on such surfaces cannot be overemphasized. In the present work, the spreading behavior on the aforementioned wettability-patterned diverging tracks was observed for fluids of different viscosities. Two dimensionless variables were identified, and a comprehensive relationship was obtained. Three distinct temporal regimes of droplet spreading were established: I), a Washburn-type slow spreading, II) a much faster Laplace pressure-driven spreading, and III), a sluggish density-augmented Tanner-type film spreading. The results offer design guidance for tracks that can pumplessly manage fluids of various viscosities and surface tensions.

Rising environmental concerns have forced many industries to seek more environmentally-friendly processing methods and safer material alternatives. The development of water-repellent (superhydrophobic and low-adhesion) coatings is one such application. Typically, repellent coatings make use of low surface energy materials (often fluorinated, or containing organosilicon compounds) that require processing with harsh organic solvents. We report a methodology to fabricate fluorine- and silane-free, entirely water-based coating formulations with plant-based filler materials (e.g. cellulose, lycopodium) for making superhydrophobic surfaces, commonly defined as having water contact angles greater than 150°. All of the formulations discussed herein contain a plant-based filler material that imparts surface roughness, and an aqueous, polymeric dispersion providing the low surface energy coating component. More importantly, the present formulations are developed using a facile, low-cost, safe processing technique. Out of several coating formulations developed in this work, those comprising of lycopodium and a natural wax (e.g. carnauba wax, beeswax) offer the highest performance in terms of hydrophobicity and water mobility, with the added advantages of no nanoparticles and thermal processing at temperatures below 100 °C. Once applied to a substrate, these coatings comprise of ∼99.5% naturally-occurring materials, with the remaining 0.5% comprising an FDA approved cross-polymer (Pemulen 1622). The green approach presented in the current study opens new avenues for technological advancement in water-repellent coatings without adverse environmental consequences.

The impact of liquid droplets on permeable substrates is important for a number of applications, such as fog collection, liquid atomization, and interaction of liquids with filters and textiles. When a water droplet impacts a wettable mesh, it penetrates the mesh easily with a part of the liquid remaining pinned. On the other hand, when striking a superhydrophobic mesh, part of the water droplet may penetrate and detach from the parent droplet, depending upon the impact velocity and the relative length scales of the droplet and the mesh. In most cases, the remaining droplet would rebound from the top of the superhydrophobic mesh. In this work, we study the impact of a water droplet on a wettability-patterned mesh, with the droplet centrally impacting the wettability-contrast line between the superhydrophobic and superhydrophilic semi-infinite domains. Half of the droplet seeing the superhydrophobic domain responds to it in a fashion that differs from the half hitting the superhydrophilic mesh side. This creates a wide range of post-impact scenarios, depending on the impact conditions and the relative characteristics of the droplet and the mesh. The difference in mesh wettability leads to a net unbalanced surface-tension force that makes the droplet rebound with a horizontal momentum component directed from the non-wettable to the wettable side. Some part of the droplet may even detach during such directional rebounding (i.e., vectoring). Along with the experimental results, a simplified analytical model is presented, which differentiates the cases of detachment or no detachment during vectoring.

Liquid-jet impingement on flat impermeable substrates is important for a multitude of applications ranging from electronic-equipment cooling to fuel atomization and erosion of solid surfaces. On a wettable horizontal surface, where a sufficient downstream liquid depth can be sustained after axisymmetric impingement, the jet forms a thin film on the substrate up to a radial distance where the film height suddenly increases, forming a hydraulic jump. On a superhydrophobic surface, where a downstream liquid depth is not naturally sustained, the thin film expands and breaks up into droplets, which are subsequently ejected in a random fashion outward, as carried by their radial momentum. In the present work, a facile, scalable, wettability-patterning approach is presented for delaying or even eliminating droplet breakup in the case of jet impingement on horizontal superhydrophobic surfaces. Analytical expressions for predicting the hydraulic jump and droplet breakup locations are developed to designate the proper wettability patterns that facilitate alteration and control of the postimpingement liquid behavior. The axisymmetric model is extended to evaluate the radial variation of the competing forces responsible for film breakup, and a design criterion for the effective wettability patterns is proposed.

The directed transport of liquids at the nanoscale is of great importance for nanotechnology applications ranging from water filtration to the cooling of electronics and precision medicine. Here we demonstrate such unidirectional, pumpless transport of water nanodroplets on graphene sheets patterned with hydrophilic/phobic areas inspired by natural systems. We find that spatially varying patterning of the graphene surfaces can lead to water transport at ultrafast velocities, far exceeding macroscale estimates. We perform extensive molecular dynamics simulations to show that such high transport velocities (O(102 m/s)) are due to differences of the advancing and receding contact angles of the moving droplet. This contact angle hysteresis and the ensuing transport depend on the surface pattern and the droplet size. We present a scaling law for the driving capillary and resisting friction forces on the water droplet and use it to predict nanodroplet trajectories on a wedge-patterned graphene sheet. The present results demonstrate that graphene with spatially variable wettability is a potent material for fast and precise transport of nanodroplets with significant potential for directed nanoscale liquid transport and precision drug delivery.

Frost formation is omnipresent when suitable environmental conditions are met. A good portion of research on combating frost formation has revolved around the passive properties of superhydrophobic (SHPO) and slippery lubricant-impregnated porous (SLIP) surfaces. Despite much progress, the need for surfaces that can effectively combat frost formation over prolonged periods still remains. In this work, we report, for the first time, the use of electrically conductive SHPO/SLIP surfaces for active mitigation of frost formation. First, we demonstrate the failure of these surfaces to passively avert prolonged (several hours) frosting. Next, we make use of their electroconductive property for active Joule heating, which results in the removal of any formed frost. We study the role of the impregnating lubricant in the heat transfer across the interface, the surface, and the ambient. We show that, even though the thermal properties of the impregnating lubricant may vary drastically, the lubricant type does not noticeably affect the defrosting behavior of the surface. We attribute this outcome to the dominant thermal resistance of the thick frost layer formed on the cooled surface. We support this claim by drawing parallels between the present system and heat transfer through a one-dimensional (1D) composite medium, and solving the appropriate transient transport equations. Lastly, we propose periodic thermal defrosting for averting frost formation altogether. This methodology utilizes the coating's passive repellent capabilities, while eliminating the dominant effect of thick deposited frost layers. The periodic heating approach takes advantage of lubricants with higher thermal conductivities, which effectively enhance heat transfer through the porous multiphase surface that forms the first line of defense against frosting.

Jet impingement finds application in high-rate cooling because of its numerous merits, which, however, do not include selective directionality. The present study introduces a new configuration employing a wettability-patterning approach to divert an orthogonally-impinging laminar water jet onto a pre-determined portion of the target surface. Diverging wettable tracks on a superhydrophobic background provide the means to re-direct the impinging jet and enable spatially-selective cooling on the heated surface. An open-surface heat exchanger is constructed using this approach, and its heat transfer performance is characterized. Sensible heat transfer is quantified in terms of the extracted cooling flux and the heat transfer coefficient. Since this approach facilitates prolonged liquid contact with the underlying heated surface through thin-film spreading, evaporative cooling is also promoted. Thus, phase-change heat transfer is also facilitated, and results in the extraction of 12.4 W/cm 2 at water flow rate of ∼1.5 mL/min. By comparing with other jet-impingement cooling approaches, the present method provides roughly four times more efficient cooling by using less amount of coolant. The reduced coolant use, combined with the gravity-independent character of this technique, offer a new paradigm for compact heat transfer devices designed to operate in reduced- or zero-gravity environments. Multiple hot spot cooling is also demonstrated using a single jet to feed two different tracks by minimally displacing or splitting the impinging jet.

Condensation of vapor on metal surfaces has many engineering applications. We demonstrate a facile and scalable approach for accelerated removal of condensate on a vertical plate during condensation of water vapor in the presence of non-condensable gases. We use wettability-patterned superhydrophilic tracks (filmwise condensing domains) laid strategically on a mirror-finish (hydrophilic) aluminum surface that promotes dropwise condensation (DWC). The design facilitates capillary-driven condensate drainage and enhanced DWC heat transfer by reducing the departing droplet size on the DWC regions. The study offers quantitative insight on the promoting and retarding influences of the dropwise and filmwise condensation zones and their ramifications on overall heat transfer on this substrate. An optimal design has been achieved by changing the fractional area of superhydrophilic tracks with respect to the overall condenser plate surface, and optimizing rapid drainage by altering the track spatial layout. The study indicates that heat transfer improvements in excess of 30% can be achieved in this metal system even in the presence of non-condensable gases that are well known to hinder energy transport.