A new family of polymeric, lubricant-infused, nanostructured wrinkled surfaces was designed that effectively retains inert nontoxic silicone oil, after draining by spin-coating and vigorous shear for 2 weeks. The wrinkled surfaces were fabricated using three different polymers (Teflon AF, polystyrene, and poly(4-vinylpyridine)) and two shrinkable substrates (Polyshrink and shrinkwrap), and Teflon on Polyshrink was found to be the most effective system. The volume of trapped lubricant was quantified by adding Nile red to the silicone oil before infusion and then extracting the oil and Nile red from the surfaces in heptane and measuring by fluorimetry. Higher volumes of lubricant induced lower roll-off angles for water droplets, and in turn induced better antifouling performance. The infused surfaces displayed stability in seawater and inhibited growth of Pseudoalteromonas spp. bacteria up to 99%, with as little as 0.9 μL cm of the silicone oil infused. Field tests in the waters of Sydney Harbor over 7 weeks showed that silicone oil infusion inhibited the attachment of algae, but the algal attachment increased as the silicone oil was slowly depleted over time. The infused wrinkled surfaces have high transparency and are moldable, making them suited to protect the windows of underwater sensors and cameras.

Advancements in the fabrication and study of superhydrophobic surfaces have been significant over the past 10years, and some 20years after the discovery of the lotus effect, the study of special wettability surfaces can be considered mainstream. While the fabrication of superhydrophobic surfaces is well advanced and the physical properties of superhydrophobic surfaces well-understood, the robustness of these surfaces, both in terms of mechanical and thermodynamic properties, are only recently getting attention in the literature. In this review we cover publications that appeared over the past ten years on the thermodynamic and mechanical robustness of superhydrophobic surfaces, by which we mean the long term stability under conditions of wear, shear and pressure. The review is divided into two parts, the first dedicated to thermodynamic robustness and the second dedicated to mechanical robustness of these complex surfaces. Our work is intended as an introductory review for researchers interested in addressing longevity and stability of superhydrophobic surfaces, and provides an outlook on outstanding aspects of investigation.

We report the fabrication of both single-scale and hierarchical superhydrophobic surfaces, created by exploiting the spontaneous wrinkling of a rigid Teflon AF film on two types of shrinkable plastic substrates. Sub-100 nm to micrometric wrinkles were reproducibly generated by this simple process, with remarkable control over the size and hierarchy. Hierarchical Teflon AF wrinkled surfaces showed extremely high water repellence (contact angle 172°) and very low contact angle hysteresis (2°), resulting in droplets rolling off the surface at tilt angles lower than 5°. The wrinkling process intimately binds the Teflon AF layer with its substrate, making these surfaces mechanically robust, as revealed by macroscale and nanoscale wear tests: hardness values were close to that of commercial optical lenses and aluminum films, resistance to scratch was comparable to commercial hydrophobic coatings, and damage by extensive sonication did not significantly affect water repellence. By this fabrication method the size of the wrinkles can be reproducibly tuned from the nanoscale to the microscale, across the whole surface in one step; the fabrication procedure is extremely rapid, requiring only 2 min of thermal annealing to produce the desired topography, and uses inexpensive materials. The very low roll-off angles achieved in the hierarchical surfaces offer a potentially up-scalable alternative as self-cleaning and drag-reducing coatings.

Lubricant-infused surfaces have attracted great attention recently and are described as slippery (SLIPS). Here we measured hydrodynamic drainage forces on SLIPS by colloid probe atomic force microscopy (AFM) and quantified the effective slip length over a nano-thin silicone oil layer on hydrophobized (OTS-coated) silicon wafers. The thickness of a stable silicone oil film on OTS-Si under sucrose solution was determined to be 1.8 ± 1.3 nm, and found to induce an average effective slip length of 29 ± 3 nm, very close to that of an uninfused OTS substrate. These relatively low values of effective slip are confirmed by the relatively large macroscopic roll-off angle values of water droplets on the same substrates. Both the nano-and macro-scale results reflect the immobilized nature of a silicone oil layer of thickness around 2 nm within an underlying Page 4 of 25 ACS Paragon Plus Environment Langmuir monolayer. These results have important implications for the design of drag-reducing coatings using lubricant infusion.

Direct force measurements between negatively charged silica microparticles are carried out in suspensions of like-charged nanoparticles with the atomic force microscope (AFM). In agreement with previous studies, oscillatory force profiles are observed at larger separation distances. At smaller distances, however, soft and strongly repulsive forces are present. These forces are caused by double layer repulsion between the like-charged surfaces and can be quantitatively interpreted with the Poisson-Boltzmann (PB) model. However, the PB model must be adapted to a strongly asymmetric electrolyte to capture the non-exponential nature of these forces. Thereby, the nanoparticles are modeled as highly charged co-ions, while the counter ions are monovalent. This model permits to extract the effective charge of the nanoparticles, which is well comparable to the one obtained from electrophoresis. The PB model also explains the presence of a particle-free layer close to the interface.This dependence originates from the double layer repulsion between the nanoparticles, which forces them into a liquid-like structure. Equation ( 1) follows from geometrical considerations by identifying the wavelength with the spacing between nearest neighbors in a close-packed structure. The same scaling dependence was observed with X-rays or neutrons, in particular, with small angle scattering through the position of the structural peak in bulk suspensions 12,18 or with reflectivity near an isolated interface. 19,20 Similar oscillatory forces were also observed in concentrated polyelectrolyte [21][22][23][24] and micellar solutions. 25,26 While this similarity is not surprising for spherical micelles, an analogous structuring also occurs in polyelectrolyte solutions. In the latter case, the wavelength follows the scaling law given in eq.(1) only at lower concentrations. A similar scaling law remains applicable at higher concentrations, albeit with a different exponent. 23,27,28 Less information is available on double layer forces acting in these systems. In the standard experiments the nanoparticles and the planar (or quasi-planar) surfaces are negatively charged. Therefore, the presence of strongly repulsive double layer forces is expected too. Additional exponential repulsive forces were found recently in such systems, but these forces seem relatively weak. 29 In solutions of polyelectrolytes, strongly repulsive double layer forces were recently reported by some of us. 28 In the latter case, these forces were non-exponential, but compatible with predictions of Poisson-Boltzmann (PB) theory.Structuring and forces in colloidal suspensions of charged particles have also attracted much attention in the theoretical community. [30][31][32][33] Thereby, a more rigorous description beyond the PB theory was pursued, typically with computer simulations or integral equation theories. Some researchers have considered the so-called primitive model, where the macroions and counter ions are treated on equal footing. Another approach is to only treat the na...