Lena Mammen

Candle Soot as a Template for a Transparent Robust Superamphiphobic Coating

In the Stick of It If a coating makes a surface nonstick, how do you stick the coating to the surface in the first place? For many nonstick coatings, this involves procedures to ensure good adhesion to the underlying surface though the use of surface roughening or intermediary layers. Deng et al. (p. 67, published online 1 December; see the cover) found a very simple route using little more than candle soot as a temporary sublayer that is coated with a silica shell and subsequently removed via calcination. Once top-coated with a semifluorinated silane, the resulting material possessed a low surface energy for water and also repelled oils, alchohols, and alkanes. While the coating could be damaged through mechanical wear, the remaining material continued to show superhydrophobic and superoleophobic behavior. Coatings that are highly resistant to water and to hydrocarbons can be made starting from candle soot. Coating is an essential step in adjusting the surface properties of materials. Superhydrophobic coatings with contact angles greater than 150° and roll-off angles below 10° for water have been developed, based on low-energy surfaces and roughness on the nano- and micrometer scales. However, these surfaces are still wetted by organic liquids such as surfactant-based solutions, alcohols, or alkanes. Coatings that are simultaneously superhydrophobic and superoleophobic are rare. We designed an easily fabricated, transparent, and oil-rebounding superamphiphobic coating. A porous deposit of candle soot was coated with a 25-nanometer-thick silica shell. The black coating became transparent after calcination at 600°C. After silanization, the coating was superamphiphobic and remained so even after its top layer was damaged by sand impingement.

Transparent, Thermally Stable and Mechanically Robust Superhydrophobic Surfaces Made from Porous Silica Capsules

Superhydrophobic surfaces are advantageous for a cost-effective maintenance of a variety of surfaces. The combination of micro and nano-sized roughness increases the contact angle of water such that water droplets cannot adhere but roll off. Therefore, superhydrophobic coatings are self-cleaning and anticorrosive. If the superhydrophobic surface were even transparent, the range of possible applications could be expanded to glass-based substrates such as goggles or windshields and, equally important, prevent an efficiency degradation of solar cells by pollution accumulation. Moreover mechanical robustness is also particularly critical because the dual scale roughness can easily be destroyed irreversibly leading to a rapid decrease of the contact angle and an increase of contact angle hysteresis. We use porous silica capsules as key components to build lotus leaf-like superhydrophobic surfaces. The latter are highly transparent as well as mechanically and thermally stable, see Fig. 1, left. When used as transparent coatings for organic solar cells they leave their performance unaffected, see Fig. 1, right.

How superhydrophobicity breaks down

A droplet deposited or impacting on a superhydrophobic surface rolls off easily, leaving the surface dry and clean. This remarkable property is due to a surface structure that favors the entrainment of air cushions beneath the drop, leading to the so-called Cassie state. The Cassie state competes with the Wenzel (impaled) state, in which the liquid fully wets the substrate. To use superhydrophobicity, impalement of the drop into the surface structure needs to be prevented. To understand the underlying processes, we image the impalement dynamics in three dimensions by confocal microscopy. While the drop evaporates from a pillar array, its rim recedes via stepwise depinning from the edge of the pillars. Before depinning, finger-like necks form due to adhesion of the drop at the pillar’s circumference. Once the pressure becomes too high, or the drop too small, the drop slowly impales the texture. The thickness of the air cushion decreases gradually. As soon as the water–air interface touches the substrate, complete wetting proceeds within milliseconds. This visualization of the impalement dynamics will facilitate the development and characterization of superhydrophobic surfaces.

Superhydrophobic surfaces by hybrid raspberry-like particles.

Surface roughness on different length scales is favourable for superhydrophobic behaviour of surfaces. Here we report (i) an improved synthesis for hybrid raspberry-like particles and (ii) a novel method to obtain superhydrophobic films of good mechanical stability. Polystyrene spheres with a diameter of 400 nm-1 microm are decorated with silica colloids < 100 nm in size, thus introducing surface asperities on a second length scale. To improve mechanical resistance, we then coated the polystyrene core and attached silica colloids with a smooth silica shell of 10 nm to 40 nm thickness. All three steps of this synthesis procedure can be sensitively tuned so that the average size and number of the silica colloids as well as the morphology of the resulting raspberry particles can be predicted. As the particles disperse in water, either monolayers can be prepared by dip coating or multilayers by drop casting. Although mechanically stable, the shells are porous enough to allow for leakage of molten or dissolved polystyrene from the core. In tetrahydrofuran vapour polystyrene bridges form between the particles that render the multilayer-film stable. Leaked polystyrene that masks some asperities can be removed by plasma cleaning. Surface roughness on larger scales can be tuned by the drying procedure. The films are hydrophobized by silanization with a semi-fluorinate silane.

Effect of Nanoroughness on Highly Hydrophobic and Superhydrophobic Coatings

The effect of nanoroughness on contact angles and pinning is investigated experimentally and numerically for low-energy surfaces. Nanoroughness is introduced by chemical vapor deposition of tetraethoxysilane and was quantified by scanning force microscopy. Addition of a root-mean-square roughness of 2 nm on a flat surface can increase the contact angle after fluorination by a semifluorinated silane by up to 30°. On the other hand, nanoroughness can improve or impair the liquid repellency of superhydrophobic surfaces that were made from assembled raspberry particles. Molecular dynamics simulations are performed in order to gain a microscopic understanding on how the length and the surface coating density of semifluorinated silanes influence the hydrophobicity. Solid-liquid surface free energy computations reveal that the wetting behavior strongly depends on the density and alignment of the semifluorinated silane. At coating densities in the range of experimental values, some water molecules can penetrate between the semifluorinated chains, thus increasing the surface energy. Combining the experimental and numerical data exhibits that a roughness-induced increase of the contact angle competes with increased pinning caused by penetration of liquid into nanopores or between neighboring semifluorinated molecules.

Optimization of superamphiphobic layers based on candle soot

Abstract Liquid repellent layers can be fabricated by coating a fractal-like layer of candle soot particles with a silicon oxide layer, combusting the soot at 600 °C and subsequently silanizing with perfluoroalkylsilanes. Drops of different liquids deposited on these so called “superamphiphobic” layers easily roll off thanks to the low liquid-solid adhesion. The lower value of the surface tension of liquids that can be repelled depends on details of the processing. Here, we analyze the influence of the soot deposition duration and height with respect to the flame on the structure and wetting properties of the superamphiphobic layer. The mean diameter of the soot particles depends on the distance from the wick. Close to the wick, the average diameter of the particles varies between 30 and 50 nm as demonstrated by scanning electron microscopy (SEM). Close to the top of the flame, the particles size decreases to 10–20 nm. By measuring the mass of superamphiphobic layers and their thickness by laser scanning confocal microscopy (LSCM) in reflection mode, we could determine that the average porosity is 0.91. The height-dependent structural differences affect the apparent contact and roll-off angles. Lowest contact angles are measured when soot is deposited close to the wick due to wax that is not completely burnt, smearing out the required overhanging structures. The small particle size close to the top of the flame also reduces contact angles, again due to decreasing size of overhangs. Sooting in the middle of the flame led to optimal liquid repellency. Furthermore, for sooting times longer than 45 s the properties of the layer did not change with sooting time, verifying the self-similarity of the layer.

Functional superhydrophobic surfaces made of Janus micropillars.

Particle coated micropillar arrays having hydrophobic sidewalls and hydrophilic silica tops are fabricated, enabling the top sides to be selectively post-functionalized. The so termed Janus pillars remain in the Cassie state even after chemical modification of the top faces.

CHAPTER 8:Challenges and Opportunities of Superhydrophobic/Superamphiphobic Coatings in Real Applications

Contamination of surfaces with organic compounds and biological residues still represents a broad challenge, ranging from industry and medicine to our daily lives. Superhydrophobic coatings are exceptionally water repellent and have self-cleaning properties. Water drops roll off when tilting the surface by a few degrees. However, low surface tension liquids like oils and other organic contaminants easily adhere to superhydrophobic surfaces. Recently developed superamphiphobic coatings may prevent this problem. Superamphiphobic coatings could not only prevent wetting of surfaces by oil but also delay the deposition of biological material such as cells, proteins and bacteria. In this chapter we discuss the wetting behaviour of superhydrophobic and superamphiphobic surfaces. We address topics beyond the fabrication, characterization and optimization process of super-liquid repellent surfaces and present possible applications, ranging from industry to medicine. The durability and the long-term stability of superhydrophobicity/superamphiphobicity still present major challenges, limiting their industrial use.