Capillary-Driven Water Transport by Contrast Wettability-Based Durable Surfaces.

Controlling water transport and management is crucial for continuous and reliable system operation in harsh weather conditions. Passive strategies based on nonwetting surfaces are desirable, but so far, the implementation of superhydrophobic coatings into real-world applications has been limited by durability issues and, in some cases, lack of compliance with environmental regulations. Inspired by surface patterning observed on living organisms, in this study we have developed durable surfaces based on contrast wettability for capillary-driven water transport and management. The surface fabrication process combines a hydrophobic coating with hard-anodized aluminum patterning, using a scalable femtosecond laser microtexturing technique. The concept targets heavy-duty engineering applications; particularly in aggressive weather conditions where corrosion is prevalent and typically the anodic aluminum oxide-based coating is used to protect the surface from corrosion, the concept has been validated on anodic aluminum oxide coated aluminum alloy substrates. Such substrates with contrast wettable characteristics show long-term durability in both natural and lab-based artificial UV and corrosion tests where superhydrophobic coatings tend to degrade.

Internal and interfacial microstructure characterization of ice droplets on surfaces by X-ray computed tomography.

HYPOTHESIS: Characterizing the microstructure of an ice/surface interface and its effect on the icephobic behavior of surfaces remains a significant challenge. Introducing X-ray Computed Tomography (XCT) can provide unprecedented insights into the internal (porosity) and interfacial structures, i.e. wetting regime, between (super)hydrophobic surfaces and ice by visualizing these optically inaccessible regions. EXPERIMENTS: Frozen droplets with controlled volume were deposited on top of metallic and polymeric substrates with different levels of wettability. Different modes of XCT (3D and 4D) were utilized to obtain information on the internal and interfacial structure of the ice/surface system. The results were supplemented by conventional surface analysis techniques, including optical profilometry and contact angle measurements. FINDINGS: Using XCT on ice/surface systems, the 3D and 4D (imaging with temporal resolution) structural information can be visualized. From these datasets, qualitative and quantitative results were obtained, not only for characterizing the interface but also for analyzing the entire droplet/surface system, e.g., measurement of porosity size, shape, and location. These results highlight the potential of XCT in the characterization of both droplets and substrates and proves that the technique can aid to develop hydrophobic surfaces for use as icephobic materials.

Porous micropillar structures for retaining low surface tension liquids.

The ability to manipulate fluid interfaces, e.g., to retain liquid behind or within porous structures, can be beneficial in multiple applications, including microfluidics, biochemical analysis, and the thermal management of electronic systems. While there are a variety of strategies for controlling the disposition of liquid water via capillarity, such as the use of chemically modified porous adhesive structures and capillary stop valves or surface geometric features, methods that work well for low surface tension liquids are far more difficult to implement. This study demonstrates the microfabrication of a silicon membrane that can retain exceptionally low surface tension fluorinated liquids against a significant pressure difference across the membrane via an array of porous micropillar structures. The membrane uses capillary forces along the triple phase contact line to maintain stable liquid menisci that yield positive working Laplace pressures. The micropillars have inner diameters and thicknesses of 1.5-3 µm and ∼1 µm, respectively, sustaining Laplace pressures up to 39 kPa for water and 9 kPa for Fluorinert™ (FC-40). A theoretical model for predicting the change in pressure as the liquid advances along the porous micropillar structure is derived based on a free energy analysis of the liquid meniscus with capped spherical geometry. The theoretical prediction was found to overestimate the burst pressure compared with the experimental measurements. To elucidate this deviation, transient numerical simulations based on the Volume of Fluid (VOF) were performed to explore the liquid pressure and evolution of meniscus shape under different flow rates (i.e., Capillary numbers). The results from VOF simulations reveal strong dynamic effects where the anisotropic expansion of liquid along the outer micropillar edge leads to an irregular meniscus shape before the liquid spills along the micropillar edge. These findings suggest that the analytical prediction of burst Laplace pressure obtained under quasi-static condition (i.e., equilibrium thermodynamic analysis under low capillary number) is not applicable to highly dynamic flow conditions, where the liquid meniscus shape deformation by flow perturbation cannot be restored by surface tension force instantaneously. Therefore, the critical burst pressure is dependent on the liquid velocity and viscosity under dynamic flow conditions. A numerical simulation using Surface Evolver also predicts that surface defects along the outer micropillar edge can yield up to 50% lower Laplace pressures than those predicted with ideal feature geometries. The liquid retention strategy developed here can facilitate the routing and phase management of dielectric working fluids for application in heat exchangers. Further improvements in the retention performance can be realized by optimizing the fabrication process to reduce surface defects.

Detergency and Its Implications for Oil Emulsion Sieving and Separation.

Separating petroleum hydrocarbons from water is an important problem to address in order to mitigate the disastrous effects of hydrocarbons on aquatic ecosystems. A rational approach to address the problem of marine oil-water separation is to disperse the oil with the aid of surfactants in order to minimize the formation of large slicks at the water surface and to maximize the oil-water interfacial area. Here we investigate the fundamental wetting and transport behavior of such surfactant-stabilized droplets and the flow conditions necessary to perform sieving and separation of these stabilized emulsions. We show that, for water-soluble surfactants, such droplets are completely repelled by a range of materials (intrinsically underwater superoleophobic) due to the detergency effect; therefore, there is no need for surface micro-/nanotexturing or chemical treatment to repel the oil and prevent fouling of the filter. We then simulate and experimentally investigate the effect of emulsion flow rate on the transport and impact behavior of such droplets on rigid meshes to identify the minimum pore opening (w) necessary to filter a droplet with a given diameter (d) in order to minimize the pressure drop across the mesh-and therefore maximize the filtering efficiency, which is strongly dependent on w. We define a range of flow conditions and droplet sizes where minimum droplet deformation is to be expected and therefore find that the condition of w ≈ d is sufficient for efficient separation. With this new understanding, we demonstrate the use of a commercially available filter-without any additional surface engineering or functionalization-to separate oil droplets (d < 100 µm) from a surfactant-stabilized emulsion with a flux of ∼11,000 L m-2 h-1 bar-1. We believe these findings can inform the design of future oil separation materials.

One-Step Fabrication of Microchannels Lined with a Metal Oxide Coating.

We demonstrate a simple, single-step method for metal/metal oxide coating on interior walls of microchannels in an elastomeric material like PDMS, which is the mainstay of microfluidic devices. The fabrication process involves electrodeposition of cuprous oxide on a metallic wire or a sheet, embedding it inside a PDMS matrix along with the cross-linker, curing and then swelling the PDMS elastomer, and finally pulling out the template metal wire or the metal sheet from the PDMS matrix. Stronger attachment of the metal oxide layer to PDMS allows the transfer of the metal oxide coating originally present on the template surface (wire or sheet) to the channel wall resulting in a microchannel/microslit lined with the metal/metal oxide layer. In view of the catalytic activity associated with transition metal oxides, this simple method offers a cost-effective and versatile technique to fabricate microfluidic and lab-on-a-chip devices which can be utilized as microcatalytic reactors or chemical filters. As a proof of concept, we have successfully tested the metal oxide coated microchannels and microslits as active sites for adsorption of iodide ions.

On the shedding of impaled droplets: The role of transient intervening layers.

Maintaining the non-wetting property of textured hydrophobic surfaces is directly related to the preservation of an intervening fluid layer (gaseous or immiscible liquid) between the droplet and substrate; once displaced, it cannot be recovered spontaneously as the fully penetrated Wenzel wetting state is energetically favorable. Here, we identify pathways for the "lifting" of droplets from the surface texture, enabling a complete Wenzel-to-Cassie-Baxter wetting state transition. This is accomplished by the hemiwicking of a transient (limited lifetime due to evaporation) low surface tension (LST) liquid, which is capable of self-assembling as an intervening underlayer, lifting the droplet from its impaled state and facilitating a skating-like behavior. In the skating phase, a critical substrate tilting angle is identified, up to which underlayer and droplet remain coupled exhibiting a pseudo-Cassie-Baxter state. For greater titling angles, the droplet, driven by inertia, detaches itself from the liquid intervening layer and transitions to a traditional Cassie-Baxter wetting state, thereby accelerating and leaving the underlayer behind. A model is also presented that elucidates the mechanism of mobility recovery. Ultimately, this work provides a better understanding of multiphase mass transfer of immiscible LST liquid-water mixtures with respect to establishing facile methods towards retaining intervening layers.

Spontaneous droplet trampolining on rigid superhydrophobic surfaces.

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.

An efficient and robust RSA-based remote user authentication for telecare medical information systems.

It is not always possible for a patient to go to a doctor in critical or urgent period. Telecare Medical Information Systems (TMIS) provides a facility by which a patient can communicate to a doctor through a medical server via internet from home. To hide the secret information of both parties (a server and a patient), an authentication mechanism is needed in TMIS. In 2013, Khan and Kumari proposed the authentication schemes for TMIS. In this paper, we have shown that Khan and Kumari's scheme is insecure against off-line password guessing attack. We have also shown that Khan and Kumari's scheme does not provide any security if the password of a patient is compromised. To improve the security and efficiency, a new authentication scheme for TMIS has been proposed in this paper. Further, the proposed scheme can resist all possible attacks and has better performance than the related schemes published earlier.

Physics of icing and rational design of surfaces with extraordinary icephobicity.

Icing of surfaces is commonplace in nature and technology, affecting everyday life and sometimes causing catastrophic events. Understanding (and counteracting) surface icing brings with it significant scientific challenges that requires interdisciplinary knowledge from diverse scientific fields such as nucleation thermodynamics and heat transfer, fluid dynamics, surface chemistry, and surface nanoengineering. Here we discuss key aspects and findings related to the physics of ice formation on surfaces and show how such knowledge could be employed to rationally develop surfaces with extreme resistance to icing (extraordinary icephobicity). Although superhydrophobic surfaces with micro-, nano-, or (often biomimetic) hierarchical roughnesses have shown in laboratory settings (under certain conditions) excellent repellency and low adhesion to water down to temperatures near or below the freezing point, extreme icephobicity necessitates additional important functionalities. Other approaches, such as lubricant-impregnated surfaces, exhibit both advantages and serious limitations with respect to icing. In all, a clear path toward passive surfaces with extreme resistance to ice formation remains a challenge, but it is one well worth undertaking. Equally important to potential applications is scalable surface manufacturing and the ability of icephobic surfaces to perform reliably and sustainably outside the laboratory under adverse conditions. Surfaces should possess mechanical and chemical stability, and they should be thermally resilient. Such issues and related research directions are also addressed in this article.

An efficient biometric and password-based remote user authentication using smart card for Telecare Medical Information Systems in multi-server environment.

The medical organizations have introduced Telecare Medical Information System (TMIS) to provide a reliable facility by which a patient who is unable to go to a doctor in critical or urgent period, can communicate to a doctor through a medical server via internet from home. An authentication mechanism is needed in TMIS to hide the secret information of both parties, namely a server and a patient. Recent research includes patient's biometric information as well as password to design a remote user authentication scheme that enhances the security level. In a single server environment, one server is responsible for providing services to all the authorized remote patients. However, the problem arises if a patient wishes to access several branch servers, he/she needs to register to the branch servers individually. In 2014, Chuang and Chen proposed an remote user authentication scheme for multi-server environment. In this paper, we have shown that in their scheme, an non-register adversary can successfully logged-in into the system as a valid patient. To resist the weaknesses, we have proposed an authentication scheme for TMIS in multi-server environment where the patients can register to a root telecare server called registration center (RC) in one time to get services from all the telecare branch servers through their registered smart card. Security analysis and comparison shows that our proposed scheme provides better security with low computational and communication cost.

Supercooled water drops impacting superhydrophobic textures.

Understanding the interaction of supercooled metastable water with superhydrophobic surface textures is of fundamental significance for unraveling the mechanisms of icing as well as of practical importance for the rational development of surface treatment strategies to prevent icing. We investigate the problem of supercooled water drops impacting superhydrophobic textures for drop supercooling down to -17 °C and find that increased viscous effects significantly influence all stages of impact dynamics, in particular, the impact and meniscus impalement behavior, with severe implications to water retention by the textures (sticky versus rebounding drop) and possible icing. Viscous effects in water supercooling conditions cause a reduction of drop maximum spreading (∼25% at an impact speed of 3 m/s for a millimetric drop) and can significantly decrease the drop recoil speed when the meniscus partially penetrates into the texture, leading to an increase of the contact time up to a factor of 2 in supercooling conditions compared to room temperature. We also show that meniscus penetration upon drop impact occurs with full penetration at the center, instead of ring shape, common to room temperature drop impact. To this end, we describe an unobserved mechanism for superhydrophobicity breakdown: unlike for room temperature drops, where transition from bouncing to sticky (impaled) behavior occurs sharply at the condition of full texture penetration, with a bubble captured at the point of impact, under supercooled conditions, the full penetration velocity threshold is increased markedly (increasing by ∼25%, from 2.8 to 3.5 m/s) and no bubble is entrapped. However, even though only partial texture penetration takes place, failure to completely dewet because of viscous effects can still prohibit complete supercooled drop rebound.

Hierarchically nanotextured surfaces maintaining superhydrophobicity under severely adverse conditions.

Superhydrophobic surfaces are highly desirable for a broad range of technologies and products affecting everyday life. Despite significant progress in recent years in understanding the principles of hydrophobicity, mostly inspired by surface designs found in nature, many man-made surfaces employ readily processable materials, ideal to demonstrate principles, but with little chance of survivability outside a very limited range of well-controlled environments. Here we focus on the rational development of robust, hierarchically nanostructured, environmentally friendly, metal-based (aluminum) superhydrophobic surfaces, which maintain their performance under severely adverse conditions. Based on their functionality, we superpose selected hydrophobic layers (i.e. self-assembled monolayers, thin films, or nanofibrous coatings) on hierarchically textured aluminum surfaces, collectively imparting high level robustness of superhydrophobicity under adverse conditions. These surfaces simultaneously exhibit chemical stability, mechanical durability and droplet impalement resistance. They impressively maintained their superhydrophobicity after exposure to severely adverse chemical environments like strong alkaline (pH ∼ 9-10), acidic (pH ∼ 2-3), and ionic solutions (3.5 weight% of sodium chloride), and could simultaneously resist water droplet impalement up to an impact velocity of 3.2 m s(-1) as well as withstand standard mechanical durability tests.

Multifunctional superhydrophobic polymer/carbon nanocomposites: graphene, carbon nanotubes, or carbon black?

Superhydrophobic surfaces resisting water penetration into their texture under dynamic impact conditions and offering simultaneously additional functionalities can find use in a multitude of applications. We present a facile, environmentally benign, and economical fabrication of highly electrically conductive, polymer-based superhydrophobic coatings, with impressive ability to resist dynamic water impalement through droplet impact. To impart electrical conductivity, the coatings were prepared by drop casting suspensions with loadings of different kinds of carbon nanoparticles, namely, carbon black (CB), carbon nanotubes (CNT), graphene nanoplatelets (GNP) and their combinations, in a fluoropolymer dispersion. At 50 wt % either CB or CNT, the nanocomposite coatings resisted impalement by water drops impacting at 3.7 m/s, the highest attainable speed in our setup. However, when tested with 5 vol % isopropyl alcohol-water mixture, i.e., a lower surface tension liquid posing a stiffer challenge with respect to impalement, only the CB coatings retained their impalement resistance behavior. GNP-based surfaces featured very high conductivity ∼1000 S/m, but the lowest resistance to water impalement. The optimal performance was obtained by combining the carbon fillers. Coatings containing CB:GNP:polymer = 1:1:2 showed both excellent impalement resistance (up to 3.5 m/s with 5 vol % IPA-water mixture drops) and electrical conductivity (∼1000 S/m). All coatings exhibited superhydrophobic and oleophilic behavior. To exemplify the additional benefit coming from this property, the CB and the optimal, combined CB/GNP coatings were used to separate mineral oil and water through filtration of their mixture.

Rational nanostructuring of surfaces for extraordinary icephobicity.

Icing of surfaces is commonplace in nature, technology and everyday life, bringing with it sometimes catastrophic consequences. A rational methodology for designing materials with extraordinary resistance to ice formation and adhesion remains however elusive. We show that ultrafine roughnesses can be fabricated, so that the ice nucleation-promoting effect of nanopits on surfaces is effectively counteracted in the presence of an interfacial quasiliquid layer. The ensuing interface confinement strongly suppresses the stable formation of ice nuclei. We explain why such nanostructuring leads to the same extremely low, robust nucleation temperature of ∼-24 °C for over three orders of magnitude change in RMS size (∼0.1 to ∼100 nm). Overlaying such roughnesses on pillar-microtextures harvests the additional benefits of liquid repellency and low ice adhesion. When tested at a temperature of -21 °C, such surfaces delayed the freezing of a sessile supercooled water droplet at the same temperature by a remarkable 25 hours.

On the nanoengineering of superhydrophobic and impalement resistant surface textures below the freezing temperature.

The superhydrophobic behavior of nano- and microtextured surfaces leading to rebound of impacting droplets is of great relevance to nature and technology. It is not clear however, if and under what conditions this behavior is maintained when such surfaces are severely undercooled possibly leading to the formation of frost and icing. Here we elucidate key aspects of this phenomenon and show that the outcome of rebound or impalement on a textured surface is affected by air compression underneath the impacting drop and the time scale allowing this air to escape. Remarkably, drop impalement occurred at identical impact velocities, both at room and at very low temperatures (-30 °C) and featured a ringlike liquid meniscus penetration into the surface texture with an entrapped air bubble in the middle. At low temperatures, the drop contact time and receding dynamics of hierarchical surfaces were profoundly influenced by both an increase in the liquid viscosity due to cooling and a partial meniscus penetration into the texture. For hierarchical surfaces with the same solid fraction in their roughness, minimizing the gap between the asperities (both at micro- and nanoscales) yielded the largest resistance to millimetric drop impalement. The best performing surface impressively showed rebound at -30 °C for drop impact velocity of 2.6 m/s.