Super Water-repellent Surfaces Research Articles

Super water-repellent surfaces with fractal structures and their potential application to biological studies

Super water repellency of solid surfaces can be realized by formation of fractal structures, which were verified and explained by various materials, such as alkylketene dimer (AKD), platinum–palladium alloy-covered AKD, and poly(alkylpyrrole), and also by comparison of fractal surfaces with smooth ones. Novel potential applications of the fractal solid surfaces to unique biological studies are suggested based on our previous and the present biological studies on fractal AKD surfaces.

Preparation of super water-repellent membrane by radiation-induced copolymerization

Super water-repellent surfaces are generally introduced by controlling the surface chemistry and surface roughness, which are applied by means of complex time-consuming processes. We describe a simple method for forming super water-repellent surfaces using 60Coγ irradiation induced hexafluoropropylene/ethyl methacrylate (HFP/EMA) vapor phase copolymerization under atmospheric pressure conditions. The resulting coral-reef-like microtexture surface has a water contact angle of 153°. The method described here could easily be extended to preparing superhydrophobic surface from a wide variety of materials.

Fabrication of Super Water-Repellent Surfaces by Nanosphere Lithography

Abstract: Inspired by the water-repellent behavior of the micro- and nano-structured plant surfaces, superhydrophobic materials, with a water contact larger than 150° superhydrophobic surfaces using a combination of nanosphere lithography and plasma etching. It has been found that the water contact angle on these surfaces can be systematically tuned from 132° to 168° by trimming the diameters of polystyrene nanospheres using oxygen plasma. The water contact angles measured on these surfaces can be modeled by the Cassie's formulation without any adjustable parameter.

Dynamic Plasma CVD and Preparation of Functional Organic Thin Films

This paper describes the chemical and morphological properties of super water-repellent surface of solid materials prepared by a 13.56 MHz inductively coupled plasma using fluorohydrocarbons such as vinylidene fluoride and 1, 1, 1, 2-tetrafluoroethane. Through the reaction initiated by the electron impact into fluorohydrocarbon molecules, fluorine-containing and hydrophobic polymers grew in the gas phase to deposit on substrates. The morphology of deposits seems to depend on hydrophilicity or hydrophobicity of substrates. The more hydrophilic substrate may maintain the smaller granular structure than ca. 100-nm diam. However, polymer granules may be expanded radially by surface tension and get into flat on hydrophobic substrates. Dynamic plasma CVD we proposed enabled us to control the size of deposited granules on the surface to be smaller and improve the fluorine content to be richer. Thus, we could easily obtain super water-repellent thin film with gradation in granular size and chemical compositions.

Drag Reduction on Super Water Repellent Surface with Air Injection Method. 1st Report. Flow Pattern of Air Film and Stability.

A frictional drag reduction using a specific coating surface (Super Water-Repellent Surface) has been studied. Experiments were carried out by injecting a small amount of air to a rectangular horizontal duct with super water-repellent wall. Three kinds of flow patterns, i.e., bubble, slug and stable air film flow, were defined based on the observation of flow behaviors. It was shown that the friction factors with super water-repellent wall duct were smaller than those of the smooth acrylic wall when the thin air film was formed. The maximum drag reduction ratio was about 32% at Reynolds number around 24000. At higher Reynolds number, the drag reduction ratio decreased due to air film-bubble flow transition occurring.

Drag Reduction on Super Water Repellent Surface with Air Injection Method. 2nd Report. Drag Reduction Mechanism.

Experiments were conducted to investigate the slit width effect on the frictional drag reduction using the Super water repellent surface (SWR) and air injection method. The results showed that the friction factor of slit width 2.0 mm was smaller than that of 0.5 mm at low Reynolds number. It was also observed that the stable plane air film was formed for slit width 2.0 mm. The drag reduction mechanism, considering fluid slip at the air/water interface, was also studied both numerically and theoretically. As a result, the drag reduction by the thin air film was maximally 95% at Re=24×104 was obtained.

軽金属材料の表面処理技術 電気化学的処理と化学吸着によるアルミニウム表面の超はっ水化

In order to decrease frost on an air conditioner's fins, super water-repellent aluminum is required. Two requirements for the super water-repellent surface are reported, i.e., (1) roughness surface (large surface area) and (2) water-repellent functional group on the surface. Using both electrolytic etching and anodic oxidation made the aluminum rougher. During anodic oxidation, small pits were made in the large pits due to electrolytic etching. During the process of water-repellent treatment, a silane surfactant was used. The surfactant was attached to the aluminum by the Al–O–Si unit. For water-repellency, a fluoroalkyl group is better than an alkyl group. Fluoroalkylsilane, whose fluoroalkyl group is longer than C = 6, show super water-repellency, because many CF2 functional groups appear on the surface.

Resistance Tests of Tanker and High Length-to-beam-ratio Ship Models

In the 1 st report, a new technique for reducing frictional drag using a super water repellent surface and air-injection (SWR & A method) was proposed, and its effectiveness for two-dimensional flow was confirmed by carrying out pressure loss tests in a rectangular tube and resistance tests on a horizontal flat plate.This paper presents the results of resistance tests applying the new technique to a tanker model of 7.2 m long and a high length-to-beam-ratio ship model of 12 m long. The results of these tests show that the new technique can significantly reduce frictional drag of the tanker model and the high length-to-beam-ratio ship model, and that frictional drag on the SWR surface of the 12.0 m high length-to-beam-ratio ship model was reduced by 75% at a speed of 6 m/sec, which is the same reduction rate obtained with the flat plate.

Super Water- and Oil-Repellent Surfaces Resulting from Fractal Structure

Super water- and oil-repellent surfaces have been made utilizing the fractal structure of the solid surfaces. Super water-repellent surfaces showing a contact angle of 160° for water droplets have been made of anodically oxidized aluminum surfaces treated with hydrophobic surface coupling agent (1H,1H,2H,2H-perfluorooctyltrichlorosilane). The fractal dimension of this surface was evaluated to be 2.19 by box counting fractal analysis. The anodically oxidized aluminum surface itself was superwettable for water (and also other solvents), and the surface hydrophobized with the surface coupling agent was altered to a super water-repellent surface. The super oil-repellent surfaces are also prepared by treating the same aluminum surface with fluorinated monoalkyl phosphates (1H,1H,2H,2H-perfluorodecyl- and -perfluorododecyl-phosphate). A rapeseed oil droplet having a surface tension of ∼35 mN/m rolls around on the surface without attaching. The contact angle of the rapeseed oil on the super oil-repellent surface was 150°. Most of the oils having a surface tension greater than 23 mN/m show greater contact angle than 120° on the super-oil-repellent surfaces. The critical surface tension, γc, was estimated to be 14.8 mN/m from the Zisman plot of the contact angles on the flat surface treated with 1H,1H,2H,2H-perfluorododecyl phosphate. This value is between the γcof Teflon (18.5 mN/m) and that of –CF3group (∼6 mN/m).

Super Water-repellent Surfaces Resulting from Fractal Structure. II.

A super water-repellent surface with maximum contact angle of 174° was previously obtained by solidifing a wax, alkyl ketene dimer, from a melt. This surface was fractal with dimension of D≈2.3. The surface area magnification factor became quite large due to the fractal structure and then the contact angle of a water droplet also very large. This paper is an extension of the study for clarifying why the alkyl ketene dimer forms a fractal surface. Crystallization of the alkyl ketene dimer from the melt was observed by polarizing optical microscope and DSC measurement. Attempt was also made to develop a super water-repellent paint containing the alkyl ketene dimer. Flake-like crystals of the alkyl ketene dimer were observed on the painted surface following drying of the paint-layer, and super water-repellency of the surface was noted. Through the use of triacylglycerols and dialkyl ketones, the super-water repellent surfaces have been also obtained. It has been experimentally shown that any wax with a contact angle exceeding 90° become super water repellent by making the surface sufficiently rough.

Super Water-Repellent Surfaces Resulting from Fractal Structure

Super water-repellent surfaces showing a contact angle of 174° for water droplets have been made of alkylketene dimer (AKD). Water droplets roll around without attachment on the super water-repellent surfaces when tilted slightly. The AKD is a kind of wax and forms spontaneously a fractal structure in its surfaces by solidification from the melt. The fractal surfaces of AKD repel a water droplet completely and show a contact angle larger than 170° without any fluorination treatments. Theoretical prediction of the wettability of the fractal surfaces has been given in the previous paper.3 The relationship between the contact angle of the flat surface θ and that of the fractal surface θf is expressed by the equation cos θf = (L/l)D-2 cos θ where (L/l)D-2 is the surface area magnification factor. The fractal dimension of the solid AKD surface was determined to be D ≈ 2.3 applying the box-counting method to the SEM images of the AKD cross section. L and l, which are the largest and the smallest size limits of ...