Surface tension driven transport of liquids on open substrates offers an enabling tool for open micro total analysis systems that are becoming increasingly popular for low-cost biomedical diagnostic devices. The present study uses a facile wettability patterning method to produce open microfluidic tracks that - due to their shape, surface texture and chemistry - are capable of transporting a wide range of liquid volumes (~1-500 μL) on-chip, overcoming viscous and other opposing forces (e.g., gravity) at the pertinent length scales. Small volumes are handled as individual droplets, while larger volumes require repeated droplet transport. The concept is developed and demonstrated with coatings based on TiO2 filler particles, which, when present in adequate (~80 wt.%) quantities within a hydrophobic fluoroacrylic polymer matrix, form composites that are intrinsically superhydrophobic. Such composite coatings become superhydrophilic upon exposure to UV light (390 nm). A commercial laser printer-based photo-masking approach is used on the coating for spatially selective wettability conversion from superhydrophobic to superhydrophilic. Carefully designed wedge-patterned surface tension confined tracks on the open-air devices move liquid on them without power input, even when acting against gravity. Simple designs of wettability patterning are used on versatile substrates (e.g., metals, polymers, paper) to demonstrate complex droplet handling tasks, e.g., merging, splitting and metered dispensing, some of which occur in 3-D geometries. Fluid transport rates of up to 350 μL s(-1) are attained. Applicability of the design on metal substrates allows these devices to be used also for other microscale engineering applications, e.g., water management in fuel cells.

Spontaneous removal of condensed matter from surfaces is exploited in nature and in a broad range of technologies to achieve self-cleaning, anti-icing and condensation control. But despite much progress, our understanding of the phenomena leading to such behaviour remains incomplete, which makes it challenging to rationally design surfaces that benefit from its manifestation. Here we show that water droplets resting on superhydrophobic textured surfaces in a low-pressure environment can self-remove through sudden spontaneous levitation and subsequent trampoline-like bouncing behaviour, in which sequential collisions with the surface accelerate the droplets. These collisions have restitution coefficients (ratios of relative speeds after and before collision) greater than unity despite complete rigidity of the surface, and thus seemingly violate the second law of thermodynamics. However, these restitution coefficients result from an overpressure beneath the droplet produced by fast droplet vaporization while substrate adhesion and surface texture restrict vapour flow. We also show that the high vaporization rates experienced by the droplets and the associated cooling can result in freezing from a supercooled state that triggers a sudden increase in vaporization, which in turn boosts the levitation process. This effect can spontaneously remove surface icing by lifting away icy drops the moment they freeze. Although these observations are relevant only to systems in a low-pressure environment, they show how surface texturing can produce droplet-surface interactions that prohibit liquid and freezing water-droplet retention on surfaces.

Among numerous challenges to meet the rising global energy demand in a sustainable manner, improving phase change heat transfer has been at the forefront of engineering research for decades. The high heat transfer rates associated with phase change heat transfer are essential to energy and industry applications; but phase change is also inherently associated with poor thermodynamic efficiencies at low heat flux, and violent instabilities at high heat flux. Engineers have tried since the 1930's to fabricate solid surfaces that improve phase change heat transfer. The development of micro and nanotechnologies has made feasible the high-resolution control of surface texture and chemistry over length scales ranging from molecular levels to centimeters. This paper reviews the fabrication techniques available for metallic and siliconbased surfaces, considering sintered and polymeric coatings. The influence of such surfaces in multiphase processes of high practical interest, e.g. boiling, condensation, freezing, and the associated physical phenomena are reviewed. The case is made that while engineers are in principle able to manufacture surfaces with optimum nucleation or thermofluid transport characteristics, more theoretical and experimental efforts are needed to guide the design and cost-effective fabrication of surfaces that not only satisfy the existing technological needs, but also catalyze new discoveries.

It is generally accepted that the hydrophilic property of graphene can be affected by the underlying substrate. However, the role of intrinsic vs substrate contributions and the related mechanisms are vividly debated. Here, we show that the intrinsic hydrophilicity of graphene can be intimately connected to the position of its Fermi level, which affects the interaction between graphene and water molecules. The underlying substrate, or dopants, can tune hydrophilicity by modulating the Fermi level of graphene. By shifting the Fermi level of graphene away from its Dirac point, via either chemical or electrical voltage doping, we show enhanced hydrophilicity with experiments and first principle simulations. Increased vapor condensation on graphene, induced by a simple shifting of its Fermi level, exemplifies applications in the area of interfacial transport phenomena.

Inspired by manifestations in nature, microengineering and nanoengineering of synthetic materials to achieve superhydrophobicity has been the focus of much work. Generally, hydrophobicity is enhanced through the combined effects of surface texturing and chemistry; being durable, rigid materials are the norm. However, many natural and technical surfaces are flexible, and the resulting effect on hydrophobicity has been largely ignored. Here, we show that the rational tuning of flexibility can work synergistically with the surface microtexture or nanotexture to enhance liquid repellency performance, characterized by impalement and breakup resistance, contact time reduction, and restitution coefficient increase. Reduction in substrate areal density and stiffness imparts immediate acceleration and intrinsic responsiveness to impacting droplets (∼350 × g), mitigating the collision and lowering the impalement probability by ∼60% without the need for active actuation. Furthermore, we exemplify the above discoveries with materials ranging from man-made (thin steel or polymer sheets) to nature-made (butterfly wings).droplet impact | superhydrophobicity | flexible | wetting transition | biomimicry H ydrophobic surfaces have gained much attention in recent years (1) for their unique attributes, such as self-cleaning behavior (2), extreme repellency to liquids (3, 4), and resistance to surface icing (5). For practical applications, repellency to impacting liquid droplets is of great importance, and numerous studies have investigated the physics of droplet impact on rigid surfaces and the diverse outcome of such events for a broad range of liquid properties and impact conditions [liquid viscosity (6, 7), surface tension (3), environmental pressure (8, 9), etc.]. Additionally, extensive work has been done on the role surface morphology plays in determining the outcome of such eventswith the goal being full rebound of an impacting droplet from the surface (10-17). In these studies, the emphasis was on texturing rigid materials to impart enhanced properties. On the other hand, there is a broad palette of surfaces in nature and technology that is characterized by some degree of flexibility [leaves (18), construction materials, textiles (19), etc.]. Studies have been reported with respect to dynamic wetting on hydrophilic, flexible materials (20-23); however, little work has addressed the interweaving effects of wetting behavior and material flexibility. In addition, the work that has been reported (24) did not focus on the role of surface compliance or flexibility in influencing the physics of the droplet collision process.Here, we investigate the effect of substrate flexibility on superhydrophobicity through the outcome of droplet impact events with respect to impalement resistance, droplet−substrate contact time, maximum droplet deformation, and restitution coefficient. We demonstrate, through appropriate modeling and experiments, that, by rational tuning of the substrate stiffness and areal density, flexibility can actually w...

Atmospheric water vapor is ubiquitous and represents a promising alternative to address global clean water scarcity. Sustainably harvesting this resource requires energy neutrality, continuous production, and facility of use. However, fully passive and uninterrupted 24-hour atmospheric water harvesting remains a challenge. Here, we demonstrate a rationally designed system that synergistically combines radiative shielding and cooling—dissipating the latent heat of condensation radiatively to outer space—with a fully passive superhydrophobic condensate harvester, working with a coalescence-induced water removal mechanism. A rationally designed shield, accounting for the atmospheric radiative heat, facilitates daytime atmospheric water harvesting under solar irradiation at realistic levels of relative humidity. The remarkable cooling power enhancement enables dew mass fluxes up to 50 g m−2 hour−1, close to the ultimate capabilities of such systems. Our results demonstrate that the yield of related technologies can be at least doubled, while cooling and collection remain passive, thereby substantially advancing the state of the art.

Inhibiting ice accumulation on surfaces is an energy-intensive task and is of significant importance in nature and technology where it has found applications in windshields, automobiles, aviation, renewable energy generation, and infrastructure. Existing methods rely on on-site electrical heat generation, chemicals, or mechanical removal, with drawbacks ranging from financial costs to disruptive technical interventions and environmental incompatibility. Here we focus on applications where surface transparency is desirable and propose metasurfaces with embedded plasmonically enhanced light absorption heating, using ultrathin hybrid metal-dielectric coatings, as a passive, viable approach for de-icing and anti-icing, in which the sole heat source is renewable solar energy. The balancing of transparency and absorption is achieved with rationally nanoengineered coatings consisting of gold nanoparticle inclusions in a dielectric (titanium dioxide), concentrating broadband absorbed solar energy into a small volume. This causes a > 10 °C temperature increase with respect to ambient at the air-solid interface, where ice is most likely to form, delaying freezing, reducing ice adhesion, when it occurs, to negligible levels (de-icing) and inhibiting frost formation (anti-icing). Our results illustrate an effective unexplored pathway toward environmentally compatible, solar-energy-driven icephobicity, enabled by respectively tailored plasmonic metasurfaces, with the ability to design the balance of transparency and light absorption.

A solution-based, large-area coating procedure is developed to produce conductive polymer composite films consisting of hollow-core carbon nanofibers (CNFs) and a fluoroacrylic copolymer available as a water-based dispersion. CNFs (100nm dia., length ~130µm) were dispersed by sonication in a formic acid/acetone co-solvent system, which enabled colloidal stability and direct blending of the CNFs and aqueous fluoroacrylic dispersions in the absence of surfactants. The dispersions were sprayed on smooth and microtextured surfaces, thus forming conformal coatings after drying. Nanostructured composite films of different degrees of oil and water repellency were fabricated by varying the concentration of CNFs. The effect of substrate texture and CNF content on oil/water repellency was studied. Water and oil static contact angles (CA) ranged from 98° to 164° and from 61° to 164°, respectively. Some coatings with the highest water/oil CAs displayed self-cleaning behavior (droplet roll-off angles < 10°).