Super Water-repellent Surfaces Research Articles

Processing conditions of microfibers on surface of poly（lactic acid）and their wettability

Technology controlling of material surface wettability is important for imparting functionality to the material, and is thought to lead to the development of useful devices in the medical and environmental fields. Devices using Poly（lactic acid）（PLA）are sustainable, because PLA is derived from renewable resources like corn starch. Therefore, by examining the conditions processing the super water-repellent surface of PLA, the development of devices with low environmental impact can be expected. The purpose of this study was to clarify the relationship between the PLA microstructure and the processing conditions necessary to produce its super water-repellent surface. Therefore, the relationship between the processing conditions and the contact angle of water droplets on the processed PLA surface was evaluated. As a result, it was found that the conditions for fabricating the super water-repellent surface due to the pinning effect were clarified.

Measurement of Water Distribution on Micro-structured Surface Buried in Water as a Model of Super Water Repellent Surface by Stimulated Raman Scattering Interferometer.

Water repellency of surfaces is realized by the roughness of the surfaces and the coating compounds on the surfaces. It is known that the water repellent surfaces are due to invasion of water into the trench structures on the surface, or due to air remaining in the trench structures. However, for more effective designing and fabrication of water-repellent surfaces, it is essential to measure the spatial distribution of water around the microstructures. In the present study, the SRS interferometer developed by the authors is applied to distinguish whether water or air is present in the trenches on the water-substrate interface buried by water. By obtaining the SRS interference signal generated from water while scanning the sample position, the phase shift of the SRS interference signal due to the microstructure on the interface was observed. From the quantitative analysis of the phase difference, it was revealed that the trench was filled with water. It is concluded that SRS interferometry is an effective method to observe experimentally the interface condition with chemical contrast along microstructures.

Morphology and functions of astrocytes cultured on water-repellent fractal tripalmitin surfaces

In the brain, astrocytes play an essential role with their multiple functions and sophisticated structure, as surrounded by a fractal environment which has not been available in our traditional cell culture. Water-repellent fractal tripalmitin (PPP) surfaces can imitate the fractal environment in vivo, so the morphology and biochemical characterization of astrocytes on these surfaces are examined. Water-repellent fractal PPP surface can induce astrocytes to display sophisticated morphology with smaller size of cell area, longer and finer filopodium-like processes, and higher morphological complexity. The super water-repellent fractal PPP surface with water contact angle of 150°∼160° produces the maximal effects compared with other surfaces at lower water contact angles. The trends of characteristic protein expression, including that of nestin, vimentin, GFAP and glutamine synthetase, for astrocytes cultured on super water-repellent fractal PPP surfaces approximate more to in vivo pattern. The super water-repellent PPP surface also render astrocytes to perform more pronounced promotion of neurogenesis by increasing the release of nerve growth factor in a co-culture system. Altogether, our results suggest that the super water-repellent fractal PPP surface facilitates the astrocytes to mimic their in vivo performance, thus provides a closer-to-natural culture environment for experimental assessment of glial structure and functions.

Super Water-Repellent Surfaces: Potential Application to Drag Reduction of Yachts

We fabricated miniature yachts whose bottoms are coated with different water contact angle-materials, and investigated on the direct correlation between the water contact angle and the sailing velocity. As a result, the sailing velocity increased with increasing in the water contact angle of the yacht bottom. We considered that the rough surface of the yacht bottom, coated with the dispersion containing super water-repellent silica nanoparticles, may decrease contact area with water, resulting in drag reduction and subsequently increasing the sailing velocity. We also briefly reviewed recent patents (US0330340 (2010); US8258206 (2012); US6994045 (2006); US20120247383 (2012); CN200780042283 (2011); CN201110396837 (2011); CN201110167924 (2011); CN201010555247 (2011); WO2012015700 (2012)) on drag reduction by super water-repellent surfaces in this paper. Keywords: Drag reduction, fractal structure, friction, super water-repellency, wettability, yacht.

Deposition of carbon based materials by continuous and pulsed discharges

This review surveys the recent developments in the plasma deposition of polycrystalline diamond (PCD) films from highly diluted (1%CH4 in H2) gas mixture and superhydrophobic fluorocarbon films from C2F4 gas. Specifically, the pulsed plasmas are also used and examined at different duty cycles and pulse periods. Emphasis is given to the role of pulsed plasmas with respect to continuous ones in controlling the gas surface interaction and the growth chemistry and in determining the material properties. The obtained materials have been characterised using a wide range of methods: scanning electron microscopy, atomic force microscopy, Raman spectroscopy, X-ray diffractometry, spectroscopic ellipsometry, water contact angle, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Power modulation affects the PCD film morphology but deteriorates their optical and structural properties: the refractive index and the quality factor of continuous films are better than pulsed ones. As for fluorocarbon films, the control of both the plasma phase and the film deposited is improved under pulsed conditions. The pulsed plasmas, contrary to the continuous ones, allow to tune the CF2 growth precursor and correspondingly the composition, structure and morphology of the film, exhibiting a superwater repellent surface with contact angles of up to 170°.

Kinetics on formation of super water repellent surfaces from phase transformation in binary mixtures of trimyristin and tripalmitin

Triglyceride surfaces were prepared by solidification from the melting of binary mixtures of trimyristin (MMM) and tripalmitin (PPP) with different mole fraction (xPPP) to further investigate the formation mechanism of such super water-repellent surfaces. The hydrophobicity change of the MMM/PPP surfaces was monitored by the measurements on time-dependent contact angle. It has been found that all the mixture samples can form super water-repellent surfaces spontaneously under appropriate heat treatment conditions. However, the time (t150) needed to exhibit super water-repellency for an MMM/PPP surface, incubated at a specific temperature, decreases slightly along with xPPP at the range from 0 to 0.2, while the t150 value increases steadily at the xPPP range from 0.2 to 1.0. The plot of t150 versus xPPP has the similar trend to the solid-liquid phase diagram with a eutectic point of the binary system determined by differential scanning calorimetry (DSC). The fractal dimensions (FDs) describing roughness of these surfaces were calculated by the box-counting method from the scanning electron microscopy (SEM) images of different water-repellent surfaces and their cross sections. Kinetic constants of the formation of super water-repellent surfaces are further obtained and quantitatively compared by the correlation on the basis of the time-dependent contact angles.

Are Superhydrophobic Surfaces Best for Icephobicity?

Ice formation can have catastrophic consequences for human activity on the ground and in the air. Here we investigate water freezing delays on untreated and coated surfaces ranging from hydrophilic to superhydrophobic and use these delays to evaluate icephobicity. Supercooled water microdroplets are inkjet-deposited and coalesce until spontaneous freezing of the accumulated mass occurs. Surfaces with nanometer-scale roughness and higher wettability display unexpectedly long freezing delays, at least 1 order of magnitude longer than typical superhydrophobic surfaces with larger hierarchical roughness and low wettability. Directly related to the main focus on heterogeneous nucleation and freezing delay of supercooled water droplets, the observed ensuing crystallization process consisted of two distinct phases: one very rapid recalescent partial solidification phase and a subsequent slower phase. Observations of the droplet collision process employed for the continuous liquid mass accumulation up to the point of ice formation reveal a previously unseen atmospheric-pressure, subfreezing-temperature regime for liquid-on-liquid bounce. On the basis of the entropy reduction of water near a solid surface, we formulate a modification to the classical heterogeneous nucleation theory, which predicts the observed freezing delay trends. Our results bring to question recent emphasis on super water-repellent surface formulations for ice formation retardation and suggest that anti-icing design must optimize the competing influences of both wettability and roughness.

Superhydrophobic alumina surface based on stearic acid modification

A novel superhydrophobic alumina surface is fabricated by grafting stearic acid layer onto the porous and roughened aluminum film. The chemical and phase structure, morphology, and the chemical state of the atoms at the superhydrophobic surface were investigated by techniques as FTIR, XRD, FE-SEM, and XPS, respectively. Results show that a super water-repellent surface with a contact angle of 154.2° is generated. The superhydrophobic alumina surface takes on an uneven flowerlike structure with many nanometer-scale hollows distribute in the nipple-shaped protrusions, and which is composed of boehmite crystal and γ-Al2O3. Furthermore, the roughened and porous alumina surface is coated with a layer of hydrophobic alkyl chains which come from stearic acid molecules. Therefore, both the roughened structure and the hydrophobic layer endue the alumina surface with the superhydrophobic behavior.

Preparation of Highly Water-repellent Surface by Spontaneous Formation of Double Scale Roughness Pattern

Hydrophobic organic-inorganic hybrid composite suspensions were prepared by mixing hydrophobic octylsilyl titanium dioxide particles having average diameter of 35 nm with drying oil or moisture cure room temperature vulcanization silicone gum in volatile silicone. They were spread on a glass plate by using a linear motor coater and an applicator. Spatially periodic stripe patterns parallel to the direction of dragging the applicator were usually generated. The phenomenon is called directional viscous fingering, i.e. spontaneous pattern formation by the growth of fluctuation in morphology of mobile interface during the dragging coat. The pattern spontaneously formed on the surface became double scale when stored samples were coated. In this case, the large scale spatially periodic pattern was formed by the directional viscous fingering and the small ragged random pattern may be due to the giant molecules formed by cross-linking of silicone gum. Double scale roughness patterns were also generated by double dragging coat. The large and small scale pattern was formed by the first and second dragging coat, respectively. The formation of double scale roughness enhanced the water-repellent property of the hydrophobic surface. In some cases, water contact angle increased by 20 degrees to realize super water-repellent surface with a value exceeding 150 degrees.

Immersed superhydrophobic surfaces: Gas exchange, slip and drag reduction properties

Superhydrophobic surfaces combine high aspect ratio micro- or nano-topography and hydrophobic surface chemistry to create super water-repellent surfaces. Most studies consider their effect on droplets, which ball-up and roll-off. However, their properties are not restricted to modification of the behaviour of droplets, but potentially influence any process occurring at the solid-liquid interface. Here, we highlight three recent developments focused on the theme of immersed superhydrophobic surfaces. The first illustrates the ability of a superhydrophobic surface to act as a gas exchange membrane, the second demonstrates a reduction in drag during flow through small tubes and the third considers a macroscopic experiment demonstrating an increase in the terminal velocity of settling spheres.

Terminal velocity and drag reduction measurements on superhydrophobic spheres

Super water-repellent surfaces occur naturally on plants and aquatic insects and are created in the laboratory by combining micro- or nanoscale surface topographic features with hydrophobic surface chemistry. When such types of water-repellent surfaces are submerged they can retain a film of air (a plastron). In this work, we report measurements of the terminal velocity of solid acrylic spheres with various surface treatments settling under the action of gravity in water. We observed increases in terminal velocity corresponding to drag reduction of between 5% and 15% for superhydrophobic surfaces that carry plastrons.

Super Water-Repellent Fractal Surfaces of a Photochromic Diarylethene Induced by UV Light

Photochromic diarylethene forms super water-repellent surfaces upon irradiation with UV light. Microfibril-like crystals grow on the solid diarylethene surface after UV irradiation, and the contact angle of water on the surface becomes larger with increasing surface roughness with time. The fractal analysis was made by the box-counting method for the rough surfaces. There are three regions in the roughness size having the fractal dimension of ca. 2.4 (size of roughness smaller than 5 µm), of ca. 2.2 (size of roughness between 5–40 µm), and of ca. 2.0 (size of roughness larger than 40 µm). The fractal dimension of ca. 2.4 was due to the fibril-like structures generated gradually by UV irradiation on diarylethene surfaces accompanied with an increase in the contact angle. The surface structure with larger fractal dimension mainly contributes to realizing the super water-repellency of the diarylethene surfaces. This mechanism of spontaneous formation of fractal surfaces is similar to that for triglyceride and alkylketene dimer waxes.

Spontaneous Formation of Super Water-Repellent Fractal Surfaces in Mixed Wax Systems

Alkylketene dimers (AKDs) and triglyceride waxes form fractal surfaces spontaneously and show super water-repellent property. Spontaneous formation of fractal structures on their surfaces takes place when the meta-stable crystalline phase of the waxes transforms to the thermodynamically stable form of crystal. The triglyceride waxes form the meta-stable alpha-phase in whole specimen when solidified from their melt. In the case of AKD, on the other hand, only a small portion of the specimen solidifies in the meta-stable form of crystal. The surface of the AKD, however, becomes fractal in the whole part. We have, thus, examined the fractal structure formation in the mixed wax systems in which one wax forms fractal surfaces and the other one does not. In the stearic acid/tristearin mixed system as a typical one, the super water-repellent fractal surfaces form in the higher composition region of tristearin than that of the eutectic point in their mixture.

Fractal Materials and Their Functional Properties

Fractal, a mathematical concept, has been utilized to develop some functional materials by the present author and his coworkers. Fractal surfaces show super water- and even oil-repellent property due to their very large surface area. In order to realize the super water-repellent fractal surfaces some waxes (alkylketene dimer and triglycerides) were employed. These waxes interestingly form the fractal surfaces spontaneously. The formation mechanism of the fractal structures on the wax surfaces has been made clear. The waxes which form the fractal surfaces have a meta-stable crystalline phase when solidified from their melts, and then transform to the thermodynamically stable crystal. During this phase transition process, the wax surfaces become fractal. For practical applications in future, durable super water-repellent and highly oil-repellent fractal surfaces have been synthesized by the electrochemical polymerization of an alkylpyrrole and the coating them with a fluorinated alkylsilane-coupling agents. In the final part of this article, the creation of a fractal body, not the surfaces, with a novel template method is mentioned. Such a fractal body is expected to show some interesting and unique functional properties.

Formation mechanism of fractal structures on wax surfaces with reference to their super -repellency.

Alkylketene dimer (AKD: a kind of wax) spontaneously forms a fractal structure and its surfaces show super water-repellency (the contact angle = 174°). However, the formation mechanism of the fractal surfaces of AKD is still unclear. In this work, surface structures, wettability and phase behaviors of various waxes have been investigated in order to understand the mechanism for spontaneous formation of super water-repellent fractal surfaces. We have found an empirical general rule without any exceptions at least for the wax samples tested. First, the wax must form a meta-stable crystalline phase when solidified from its melt. Then, the super water-repellent fractal surfaces form spontaneously during the phase transition from a meta-stable to a stable crystalline form. The tempering method also supported the above rule for the waxes showing the fractal structure formation on their surfaces.

Importance of pinning effect of wetting in super water-repellent surfaces

Abstract We have first found that the pinning effect of wetting plays an important role as the 4th mechanism for the super water-repellent surfaces in some cases, particularly in the surfaces consisting of pillar structures. In such cases the plot of cos θR (θR: contact angle of a liquid on a rough surface) versus cos θ (θ: that on the flat surface) deviates remarkably from the expected curves of the Wenzel and the Cassie/Baxter theories, and one can judge the dominance of pinning effect in the super water-repellency from this plot. In addition the super water-repellent surfaces resulting from the pinning effect is not in the thermodynamically equilibrium state, but in the metastable one when the contact angle on the flat surface is smaller than 90°.

Formation mechanism of super water-repellent fractal surfaces of alkylketene dimer

Alkylketene dimer (AKD), a kind of wax, has been known to form fractal surfaces spontaneously, and to show super water-repellent property. In order to understand further the super water-repellency of the AKD surfaces and to elucidate the mechanism of spontaneous formation of the fractal structures on the wax surfaces, a pure and a mixed AKD samples were employed to make the surfaces by the melt-solidification method. Time-dependent contact angles of water droplets on the AKD surfaces cured at several temperatures were systematically measured. It was found that spontaneous formation of super water-repellent surfaces was thermally induced quite effectively. For example, it took about 6 days for the pure AKD surface to show the super water-repellency at 40 °C but did just 1 h at 50 °C. The fractal dimension of the mixed AKD surface with super water-repellency was calculated to be 2.26 from the SEM images by the box-counting method which should be reasonably compared with 2.29 of pure AKD surface. The mechanism for the spontaneous formation of fractal structures was discussed from the results of the time-dependent contact angles, differential scanning calorimetric (DSC) curves and X-ray diffractometric (XRD) patterns. It has been made clear that the fractal AKD surface and its super water-repellency result from the phase transformation from the metastable state to the stable one of the AKD crystals.

Recent Progress in Unique Solid Surfaces for Biological Applications

This review briefly describes three unique types of solid surfaces, i.e., honeycomb polymer films, thermo-responsive polymergrafted glass surfaces and super water-repellent surfaces with a fractal structure. Details of preparation and physical properties are presented first followed by highlights of recent progress in cell culture on the solid surfaces.

Spontaneous Formation of Fractal Structures on Triglyceride Surfaces with Reference to Their Super Water-Repellent Properties

Alkylketene dimer (AKD), a kind of wax, has been known to form fractal surfaces spontaneously and show super water-repellency. Such formation of water-repellent and fractal surfaces was also found in this work for triglycerides. Since the crystal phase transitions of these waxes were well studied, we studied the formation of their fractal surfaces through contact angle measurements, differential scanning calorimetry (DSC), and X-ray diffraction (XRD). From time-dependent contact angle measurements, it was found that the formation of super water-repellent surfaces with fractal structures occurred spontaneously also on the triglyceride surfaces at different temperatures. The freshly solidified triglyceride surfaces were almost transparent, and their initial contact angles of water were close to 110 degrees. The surfaces then became rough and clouded after being incubated for a certain time at a specified temperature. The super water-repellent surfaces were quite rough and showed fractal structures with the dimension of ca. 2.2 calculated from the scanning electron microscopic (SEM) images by the box-counting method. The phase transformation from a metastable state to a stable cystalline one after the solidification from the melt of triglycerides was clearly observed by DSC and XRD measurements. The fractal crystalline structures and the super water-repellency resulted from this phase transformation and the crystal growth. Ensuring the initial sample solidified into the metastable state and curing the surface at an appropriate temperature are key factors for the successful preparation of fractal triglyceride surfaces by the solidification method.

Boiling Feature on a Super Water-Repellent Surface

The boiling feature on a super water-repellent (SWR) surface has been studied. The SWR surface has a coating layer of fine particles of nickel and PTFE. Its contact angle to water is 152°in room temperature. The heat transfer surface is facing upward, and the diameter of the heated section is 17 mm. The boiling feature of this surface is completely different from that of usual surfaces. The stable film boiling occurs in very small superheating, and there is no nucleate boiling region. The bubbles generated on the surface coalesce into a vapor film without departing from the surface. The stable vapor film exists even at a surface temperature below the saturation temperature.