Antiadhesive Properties of Oil-Infused Gels against the Universal Adhesiveness of Polydopamine.

Polydopamine (PDA) is well-known as the first material-independent adhesive, which firmly attaches to various substances, even hydrophobic materials, through strong coordinative interactions between the phenolic hydroxyl groups of PDA and the substances. In contrast, oil-infused materials such as self-lubricating gels (SLUGs) exhibit excellent antiadhesive properties against viscous liquids, ice/snow, (bio)fouling, and so on. In this study, we simply questioned: "What will happen when these two materials with contrary nature meet"? To answer this, we formed a PDA layer on a SLUG surface that exhibits thermoresponsive syneretic properties (release of liquid from the gel matrix to the outer surface) and investigated its interfacial behavior. The oil layer caused by syneresis from the SLUGs at -20 °C was found to show resistance to adhesion of universally adhesive PDA.

Smooth and Transparent Films Showing Paradoxical Surface Properties: The Lower the Static Contact Angle, the Better the Water Sliding Performance.

Smooth and transparent hydrophilic films showing excellent water sliding properties were prepared by using a sol-gel solution of 2-[methoxy (ethyleneoxy)10 propyl]trimethoxysilane and tetraethoxysilane. The resulting hybrid films were statically hydrophilic (static water contact angles (CAs) were in the range of 30-45°), but water droplets (50 µL) could move smoothly on an inclined surface (minimum sliding angle was 6°) without pinning or tailing because of low CA hysteresis (5 ± 1°). Thanks to this hybrid film formation on aluminum (Al) substrate, drainage performance during condensation and frosting/defrosting markedly improved compared to that on hydrophilic, bare Al, or hydrophobic monolayer-covered Al substrates.

"Nickel Nanoflowers" with Surface-Attached Fluoropolymer Networks by C,H Insertion for the Generation of Metallic Superhydrophobic Surfaces.

Metallic superhydrophobic surfaces (SHSs) combine the attractive properties of metals, such as ductility, hardness, and conductivity, with the favorable wetting properties of nanostructured surfaces. Moreover, they promise additional benefits with respect to corrosion protection. For the modification of the intrinsically polar and hydrophilic surfaces of metals, a new method has been developed to deposit a long-term stable, highly hydrophobic coating, using nanostructured Ni surfaces as an example. Such substrates were chosen because the deposition of a thin Ni layer is a common choice for enhancing corrosion resistance of other metals. As the hydrophobic coating, we propose a thin film of an extremely hydrophobic fluoropolymer network. To form this network, a thin layer of a fluoropolymer precursor is deposited on the Ni substrate which includes a comonomer that is capable of C,H insertion cross-linking (CHic). Upon UV irradiation or heating, the cross-linker units become activated and the thin glassy film of the precursor is transformed into a polymer network that coats the surface conformally and permanently, as shown by extensive extraction experiments. To achieve an even higher stability, the same precursor film can also be transformed into a chemically surface-attached network by depositing a self-assembled monolayer of an alkane phosphonic acid on the Ni before coating with the precursor. During cross-linking, by the same chemical process, the growing polymer network will simultaneously attach to the alkane phosphonic acid layer at the surface of the metal. This strategy has been used to turn fractal Ni "nanoflower" surfaces grown by anisotropic electroplating into SHSs. The wetting characteristics of the obtained nanostructured metallic surfaces are studied. Additionally, the corrosion protection effect and the significant mechanical durability are demonstrated.

Extending the Lotus Effect: Repairing Superhydrophobic Surfaces after Contamination or Damage by CHic Chemistry.

Superhydrophobic surfaces have gained a reputation to show a self-cleaning behavior ("Lotus effect") as drops rolling off the surface take along loosely adhering dust particles. However, this self-cleaning process reaches its limits when such surfaces are brought in contact with sticky contaminants such as oils and smaller particles. Once intimate contact is established between the surface and a small particle, it will be almost impossible to remove it because of strong surface interactions. Such contaminations, however, lead to contact line pinning and destroy the superhydrophobic effect. Because the fragility of the micro- and nanostructures prohibits any mechanical cleaning, the sample is usually doomed. Here, we report a universal method for restoring superhydrophobicity: by simple dip-coating, a conformal ultrathin layer (≈10 nm) of a highly hydrophobic and photoreactive fluoropolymer is deposited. Through short UV irradiation (5 min), this thin layer is cross-linked and chemically attached to the underlying surface by C,H-insertion cross-linking, thus covering the contaminant like a thin veil. We use this "cover-up" strategy of masking the contaminants to restore superhydrophobicity. We demonstrate this principle by deliberately soiling the surface with various model contaminants, such as oily substances and particles, and study the repair process.

Omniphobic Metal Surfaces with Low Contact Angle Hysteresis and Tilt Angles.

Various metal (Al, Ti, Fe, Ni, and Cu) surfaces with native oxide layers were rendered "omniphobic" by a simple thermal treatment of neat liquid trimethylsiloxy-terminated polymethylhydrosiloxanes (PMHSs) with a range of different molecular weights (MWs). Because of this treatment, the PMHS chains were covalently attached to the oxidized metal surfaces, giving 2-10 nm thick PMHS layers. The resulting surfaces were fairly smooth, liquid-like, and showed excellent dynamic omniphobicity with both low contact angle hysteresis (≲5°) and substrate tilt angles (≲8°) toward small-volume liquid drops (5 µL) with surface tensions ranging from 20.5 to 72.8 mN/m. Droplet mobility was improved overall as a result of heating the substrates to 70 °C. The reaction kinetics and final dynamic dewetting properties were found to be not dependent of the types of metals employed or MWs of PMHS, but mainly dominated by both reaction temperatures and reaction times.

Molting Materials: Restoring Superhydrophobicity after Severe Damage via Snakeskin-like Shedding.

The nanostructures that are required to generate superhydrophobic surfaces are always sensitive to shear and are easily damaged, especially by scratching with sharp objects. As a result of this destruction, the water repellency will be lost. We introduce a novel approach to restoring the original surface properties after mechanical damage. In this approach, the damaged layer is shed like the skin of a snake. This is demonstrated with a three-layer stack as a proof-of-principle system: when the original, superhydrophobic surface layer is damaged, this leads to the dissolution of a sacrificial layer below it. Thus, the damaged layer is shed, a new unscathed surface is uncovered, and superhydrophobicity can easily be restored after a short washing.

Photochemistry between a ruthenium(II) pyridylimidazole complex and benzoquinone: simple electron transfer versus proton-coupled electron transfer.

A ruthenium(II) complex with two 4,4'-bis(trifluoromethyl)-2,2'-bipyridine chelates and a 2-(2'-pyridyl)imidazole ligand was synthesized and characterized by electrochemical and optical spectroscopic means. The respective complex has the potential to act as a combined electron-proton donor when promoted to its long-lived (3)MLCT excited state with visible light. The possibility of proton-coupled electron transfer (PCET) between the ruthenium(II) complex and 1,4-benzoquinone as an electron/proton acceptor was explored by steady-state and time-resolved luminescence spectroscopy, as well as by transient absorption spectroscopy in the nanosecond time regime. Excited-state deactivation is found to occur predominantly via simple oxidative quenching involving no proton motion, but a minor fraction of the photoexcited complex appears to react via PCET since there is spectral evidence for semiquinone as a photoproduct. Presumably, PCET is not kinetically competitive with simple electron transfer because the latter process is sufficiently exergonic and because there is little thermodynamic benefit from coupling proton transfer to the photoinduced electron transfer.