Superhydrophobicity

This article studies the anti-ice performance of several micro/nano-rough hydrophobic coatings with different surface chemistry and topography. The coatings were prepared by spin-coating or dip coating and used organosilane, fluoropolymer or silicone rubber as a top layer. Artificially created glaze ice, similar to the naturally accreted one, was deposited on the nanostructured surfaces by spraying supercooled water microdroplets (average size ˜80 μm) in a wind tunnel at subzero temperature (-10 °C). The ice adhesion strength was evaluated by spinning the samples in a centrifuge at constantly increasing speed until ice delamination occurred. The results show that the anti-icing properties of the tested materials deteriorate, as their surface asperities seem to be gradually broken during icing/de-icing cycles. Therefore, the durability of anti-icing properties appears to be an important point for further research. It is also shown that the anti-icing efficiency of the tested superhydrophobic surfaces is significantly lower in a humid atmosphere, as water condensation both on top and between surface asperities takes place, leading to high values of ice adhesion strength. This implies that superhydrophobic surfaces may not always be ice-phobic in the presence of humidity, which can limit their wide use as anti-icing materials.

Many plant surfaces like, for instance, the leaves of lotus (Nelumbo nucifera) exhibit superhydrophobic and water-repellent properties, which are attributed to their textured structures with hierarchical micrometer- and nanometer-sized surface protrusions. Αs rainwater drops move on these surfaces, they can remove dust particles. Consequently, these surfaces have self-cleaning properties. Lotus leaves afford an impressive demonstration of this effect, which is, therefore, called the “lotus effect'”.
Water-repellent films have potential for numerous applications, as described in several recently published articles, including their use as surface protective coatings for the preservation of monuments and other outdoor objects of the cultural heritage. Water that originates from atmospheric humidity or rain is an important decay factor for stone monuments by freezing-thawing cycles, crystallization of salts and chemical reactions initiated by atmospheric pollutants that are carried by water into the pores of the stone. The application/deposition of water-repellent coatings on the surfaces of the monuments may drastically decrease the deteriorative activity of water.
We recently developed simple and low-cost methods that can be easily used to induce the desired water-repellency to large surfaces, including stones which have been used in historical buildings and monuments. In particular, water-repellent nanoparticle/polymer composites and nanostructured siloxanes were produced through various methods and deposited on stone specimens. The efficiency of these coatings to be used for monument protection and conservation was evaluated through various appropriate tests. The goal of the paper is to present and discuss the results of this research activity.


IOANNIS KARAPANAGIOTIS, Assistant Professor, Department of Management and Conservation of Ecclesiastical Cultural Heritage Objects, University Ecclesiastical Academy of Thessaloniki. Hellas.
Rev. GRIGORIOS - TILEMACHOS STAMKOPOULOS, Associate Professor, Chairman of Board of University Ecclesiastical Academy of Thessaloniki. Hellas

Superhydrophobic surfaces were prepared using a very simple and low-cost method by spray coating. A high static water contact angle of about 154° was obtained by deposition of stearic acid on an aluminium alloy. However, this coating demonstrated a high contact angle hysteresis (∼ 30°). On the other hand, superhydrophobic surfaces with a static contact angle of about 162° and 158°, and a low contact angle hysteresis of about 3° and 5° were respectively obtained by incorporating nanoparticles of SiO 2 and CaCO 3 in stearic acid. The excellent resulting hydrophobicity is attributed to the synergistic effects of micro/nanoroughness and low surface energy. A study of the wettability of these surfaces at temperatures ranging from 20 to -10 °C showed that the superhydrophobic surface becomes rather hydrophobic at supercooled temperatures.

This paper describes casting-based microfabrication of metal microstructures and nanostructures. The metal was cast into flexible silicone molds which were themselves cast from microfabricated silicon templates. Microcasting is demonstrated in two metal alloys of melting temperature 70 • C or 138 • C. Many structures were successfully cast into the metal with excellent replication fidelity, including ridges with periodicity 400 nm and holes or pillars with diameter in the range 10–100 μm and aspect ratio up to 2:1. The flexibility of the silicone mold permits casting of curved surfaces, which we demonstrate by fabricating a cylindrical metal roller of diameter 8 mm covered with microstructures. The metal microstructures can be in turn used as a reusable molding tool. (Some figures in this article are in colour only in the electronic version)

A method for the preparation of inorganic superhydrophobic silica coatings using sol–gel processing with tetramethoxysilane and isobutyltrimethoxysilane as precursors is described. Incorporation of isobutyltrimethoxysilane into silica layers resulted in the existence of hydrophobic isobutyl surface groups, thereby generating surface hydrophobicity. When combined with the surface roughness that resulted from sol–gel processing, a superhydrophobic surface was achieved. This surface showed improved UV and thermal stability compared to superhydrophobic surfaces generated from polybutadiene by plasma etching. Under prolonged UV tests (ASTM D 4329), these surfaces gradually lost superhydrophobic character. However, when the as-prepared superhydrophobic surface was treated at 500 °C to remove the organic moieties and covered with a fluoroalkyl layer by a perfluorooctylsilane treatment, the surface regained superhydrophobicity. The UV and thermal stability of these surfaces was maintained upon exposure to temperatures up to 400 °C and UV testing times of 5500 h. Contact angles remained >160° with contact angle hysteresis ∼2°.The UV stability was significantly improved after the hydrophobic surface groups (isobutyl) were replaced by fluoroalkyl groups.

Nanocomposite polymer electrolytes (NCPE) composed of poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) and chitin for different concentrations of LiClO4 have been prepared by a hot-press technique. The prepared NCPE films were subjected to XRD, SEM, FTIR and tensile analyses. The thermal stability of NCPE membrane was investigated by TG-DTA. Ionic conductivity studies have also been made as a function of lithium salt concentration for different temperatures ranging from 0 to 80 °C. The polymeric membrane comprising PVDF-HFP/chitin/LiClO4 of ratio 75:20:5 (wt.%) offered maximum ionic conductivity. Thermal study reveals that these membranes are stable up to 260 °C.

Birds in the cormorant (Phalacrocoracidae) family dive tens of metres into water to prey on fish while entraining a thin layer of air (a plastron film) within the microstructures of their feathers. In addition, many species within the family spread their wings for long periods of time upon emerging from water. To investigate whether wetting and wing-spreading are related to feather structure, microscopy and photographic studies have previously been used to extract structural parameters for barbs and barbules. In this work, we describe a systematic methodology to characterize the quasi-hierarchical topography of bird feathers that is based on contact angle measurements using a set of polar and non-polar probing liquids. Contact angle measurements on dip-coated feathers of six aquatic bird species (including three from the Phalacrocoracidae family) are used to extract two distinguishing structural parameters, a dimensionless spacing ratio of the barbule ( D *) and a characteristic len...

When nanoparticles are incorporated into surfaces to generate roughness, adhesion of the particles is critical to achieve a durable superhydrophobic surface. In this investigation, we explored the use of bis-phenol A based epoxy and silica nanoparticles to form a composite layer on substrates. After an ${\rm O}_{2}$ plasma treatment of the surface layer, the epoxy was etched away and silica nanoparticles exposed on the surface, thereby generating roughness. The plasma etching time was examined to correlate the resulting surface morphology and water droplet contact angles after a fluoroalkyl silane treatment. Surface mechanical stability was studied by an abrasion test. Water vapor condensation on the surface was also assessed by investigation of the contact angle, which offers insight into the applicability of the surfaces to use under hot and humid conditions where degradation of the superhydrophobic surfaces may occur.

Insects and plants are two types of organisms that are widely separated on the evolutionary tree; for example, plants are mostly phototrophic organisms whilst insects are heterotrophic organisms. In order to cope with environmental stresses, their surfaces have developed cuticular layers that consist of highly sophisticated structures. These structures serve a number of purposes, and impart useful properties to these surfaces. These two groups of organisms are the only ones identified thus far that possess truly superhydrophobic and self-cleaning properties. These properties result from their micro- and nano-scale structures, comprised of three-dimensional wax formations. This review analyzes the surface topologies and surface chemistry of insects and plants in order to identify the features common to both organisms, with particular reference to their superhydrophobic and self-cleaning properties. This information will be valuable when determining the potential application of these ...