Superhydrophilic Materials Research Articles

Friction Reduction at a Superhydrophilic Surface: Role of Ordered Water

Low-friction but superhydrophilic materials are urgently needed in biomedical and engineering fields because of their nonfouling property and biocompatibility, particularly when the surfaces are definitely superhydrophilic, such as metal or TiO2 as the surface coatings of the intravascular stents. However, generally, there is a higher friction coefficient on the superhydrophilic surfaces than on the hydrophobic surfaces. On the basis of molecular dynamics simulations, we show that the friction on the superhydrophilic surface with appropriate charge patterns is evidently reduced, where the lower friction is similar to that of a rather hydrophobic surface with a contact angle of water droplet of ∼44°. This reduction is attributed to the existence of an ordered water monolayer on the superhydrophilic surface with appropriate charge patterns, and the friction between this ordered water monolayer and the water molecules above is small.

Hierarchically nanostructured CeO2 films with superhydrophilicity and corrosion resistance by coupling of surface topography and oxygen vacancies

Functional superhydrophilic materials have attracted increasing attention because of their potential applications such as antifogging and heat transfer/dissipation. In this study, without UV exposure or any chemical modification, a type of CeO2 films with superhydrophilicity and excellent corrosion resistance have been successfully fabricated by a simple and effective one-step electrodeposition method. It is found that hierarchically nanostructured CeO2 films consisting of islands plus microcracks provide the fast channel for water spreading based on capillary effect, while numerous surface active sites originating from oxygen vacancies improve the interfacial affinity for water molecules, together resulting in the superhydrophilicity of CeO2 surface. Based on the consistent surface morphology and work function before and after corrosion in NaCl solution, such superhydrophilic CeO2 films exhibit excellent corrosion resistance. These results will be significant to understand and design superhydrophilic materials as well as explore their potential engineering applications.

A superamphiphobic coating with an ammonia-triggered transition to superhydrophilic and superoleophobic for oil-water separation.

Superhydrophilic and superoleophobic materials are very attractive for efficient and cost-effective oil-water separation, but also very challenging to prepare. Reported herein is a new superamphiphobic coating that turns superhydrophilic and superoleophobic upon ammonia exposure. The coating is prepared from a mixture of silica nanoparticles and heptadecafluorononanoic acid-modified TiO2 sol by a facile dip-coating method. Commonly used materials, including polyester fabric and polyurethane sponge, modified with this coating show unusual capabilities for controllable filtration of an oil-water mixture and selective removal of water from bulk oil. We anticipate that this novel coating may lead to the development of advanced oil-water separation techniques.

Superhydrophobic merino fibres utilising silica nanospheres

This research describes a new generation of superhydrophobic and superhydrophilic materials based on wool, possessing a wide range of possible consumer applications such as water harvesting, stain-resistant self-cleaning and water-repellent breathable surfaces. On the basis of nature’s Stenocara beetle, superhydrophobic and superhydrophilic woven woollen textile surfaces have been developed, by creating a hierarchical surface structure in the micro- and nano-range. This surface structure was achieved through layer-by-layer assembly of functionalised silica nanospheres of different diameters, synthesised in the laboratory either via a modified Stöber process or via the Klebosol range of commercially available colloidal silica suspensions. The resulting surfaces showed superhydrophilic properties. Subsequent chemical surface modification was attained through Dynasylan, a modified fluoroalkylsiloxane. Static contact angles of 156°, with corresponding contact angle hysteresis and roll-off angles below 15° for a 10 μl droplet, have been measured. The superhydrophobic surfaces produced in this way on wool fibres and fabric have potential commercial applications in carpet and upholstery textiles, and apparel, where non-wettability, non-staining and easy cleaning properties are required.

Superhydrophilic zinc oxide film prepared by controlling ZnO microrods growth and its attractive recyclable photocatalytic performance

Superhydrophilic functional materials have been found to be of great value for a variety of practical applications in recent years. In this paper, zinc oxide (ZnO) microrod films have been directly synthesized on a large-area zinc substrate via a simple solution method. Morphological and structural observation and crystallinity of the grown products were carried out using scanning electron microscopy, X-ray diffraction, energy dispersive spectrometer, transmission electron microscopy, ultraviolet (UV)–vis diffuse reflectance spectroscopy and photoluminescence (PL) spectroscopy. The influence of reaction time on the size and shapes of the as-prepared ZnO samples was studied. It was found that superhydrophilic ZnO films at reaction time of 20h were made up of uniform pure ZnO microrods with 600nm in average diameter and 6μm in length. Room-temperature PL spectra of the ZnO products showed a UV emission and a broad green band. Photocatalytic performance and sample stability were studied. Under UV light irradiation over 95% of methylene blue was degraded by ZnO microrod films in 4h, ZnO microrod film can be easily separated from the solution, and no observable performance degradation was observed after 5cycles.

Emerging Applications of Superhydrophilic‐Superhydrophobic Micropatterns

Water on superhydrophilic surfaces spreads or is absorbed very quickly, and exhibits water contact angles close to zero. We encounter superhydrophilic materials in our daily life (e.g., paper, sponges, textiles) and they are also ubiquitous in nature (e.g., plant and tree leaves, Nepenthes pitcher plant). On the other hand, water on completely non-wettable, superhydrophobic surfaces forms spherical droplets and rolls off the surface easily. One of the most well-known examples of a superhydrophobic surface is the lotus leaf. Creating novel superhydrophobic surfaces has led to exciting new properties such as complete water repellency, self-cleaning, separation of oil and water, and antibiofouling. However, combining these two extreme states of superhydrophilicity and superhydrophobicity on the same surface in precise two-dimensional micropatterns opens exciting new functionalities and possibilities in a wide variety of applications from cell, droplet, and hydrogel microarrays for screening to surface tension confined microchannels for separation and diagnostic devices. In this Progress Report, we briefly describe the methods for fabricating superhydrophilic-superhydrophobic patterns and highlight some of the newer and emerging applications of these patterned substrates that are currently being explored. We also give an outlook on current and future applications that would benefit from using such superhydrophilic-superhydrophobic micropatterns.

Ice Adhesion Tests on Pliable Polymer Sheets for Protection Against Sea-Water Spray Icing

Heavy sea-water spray icing is a major problem in the maintenance of lighthouses situated in northern harbors facing the Sea of Japan. Moreover, low ice adhesion materials or anti-icing coatings are ineffective over long periods. In this study, we have investigated the characteristics of saline ice adhesion on hydrophilic and hydrophobic pliable polymer sheets that can be used to envelop and thereby protect small lighthouses. We conducted experiments in cold room laboratories, and further, we verified the deicing conditions. Ice adhesion strength tests using fresh water indicated that a highly hydrophobic surface exhibited lower adhesion strength than other test materials. The results of saline ice adhesion tests at −20°C indicated that the adhesion strength decreases very rapidly as the salinity increases for all materials; this trend was particularly remarkable in case of superhydrophilic materials. The water spray icing tests were verified by experiments conducted using an ice model basin. Wet growth of ice occurred on all test specimens including the highly hydrophobic sheets. Furthermore, the initial icing depended significantly on the hydrophobic or hydrophilic surface properties. The icing on the hydrophobic surface was characterized by the formation of vertical streaks of ice; irregular projections could be observed on the ice surface. On the other hand, a thin fluid flow was formed on superhydrophilic surfaces. Exfoliation of pieces of the sheet ice was often observed during the initial growth. Deicing could be easily performed for both pliable polymer sheets. A combination of low ice adhesion property and rapid distortion was effective for the deicing of sea-water spray icing.

Formation of superhydrophobic/superhydrophilic patterns by combination of nanostructure-imprinted perfluoropolymer and nanostructured silicon oxide for biological droplet generation

In this letter, we report a technology for fabricating superhydrophobic/superhydrophilic patterns using a combination of a nanostructure-imprinted perfluoropolymer and nanostructured silicon oxide. In our previous study, we used a combination of hydrophobic and superhydrophilic materials. However, it was difficult to split low-surface-tension liquids such as biological liquids into droplets solely using hydrophobic/hydrophilic patterns. In this study, the contact angle of the hydrophobic region was enhanced from 109.3° to 155.6° by performing nanostructure imprinting on a damage-reduced perfluoropolymer. The developed superhydrophobic/superhydrophilic patterns allowed the splitting of even those media that contained fetal bovine serum into droplets of a desired shape.

Hydrophilic and superhydrophilic surfaces and materials

The term superhydrophilicity is only 11–12 years old and was introduced just after the explosion of research on superhydrophobic surfaces, in response to the demand for surfaces and coatings with exceptionally strong affinity to water. The definition of superhydrophilic substrates has not been clarified yet, and unrestricted use of this term to hydrophilic surfaces has stirred controversy in the last few years in the surface chemistry community. In this review, we take a close look into major definitions of hydrophilic surfaces used in the past, before we review the physics behind the superhydrophilic phenomenon and make recommendation on defining superhydrophilic surfaces and coatings. We also review chemical and physical methods used in the fabrication of substrates on surfaces of which water spreads completely. Several applications of superhydrophilic surfaces, including examples from the authors' own research, conclude this review.

Utilizing superhydrophilic materials to manipulate oil droplets arbitrarily in water

A facile method utilizing superhydrophilic materials to manipulate oil droplets freely in water is provided in this study. Due to the hydrophilic property and hierarchical structures of frosted glass plates, water was trapped in the interstices of glass microstructures. Trapped water served as supporting points to effectively prevent the penetration of oil droplets, yielding superoleophobic and low-adhesive surfaces in water. Then, oil droplets, whether heavier or lighter than water, could be manipulated arbitrarily by a pair of tweezers consisting of superoleophobic glass surfaces on the tips. This study successfully overcame adhesive problems in conventional oil droplet manipulation techniques. Besides freely on-demand manipulation of oil droplets, the coalescence of oil droplets containing different reagents led to miniature organic reactions happening inside such an oil-droplet-based miniature reactor. Most importantly, reaction products inside coalesced droplets could be collected effectively due to the ultra-low adhesive property of superoleophobic pedestals, which will be beneficial for harvesting valuable products in limited amounts. These results proposed a novel, fluoride-free and low-cost strategy for on-demand manipulation of oil droplets, which might provoke new ideas for controllable droplet motion and the design of miniature reactors.

Design and investigation of photo-induced super-hydrophilic materials for car mirrors

During the past decades, interests in various properties in titanium dioxide thin films have been growing rapidly. There have been several reports for TiO 2 thin films prepared on various media with photocatalytic and hydrophilic properties, in order to function as self-cleaning and/or anti-fogging materials. An obvious application is usually found in side-view car mirrors in the automobile industries. In this study, a number of photocatalytic TiO 2 films are prepared on soda-lime glasses for car mirrors by an electron-beam evaporation. The designs and development of the photocatalytic TiO 2 films, based on crystallinity, deposition rate, film thickness, film structure, and surface roughness are discussed. In comparison to the commercialized products, a systematic investigation procedure for the super-hydrophilic properties of the light-induced TiO 2 films for car mirrors has been developed, based on super-hydrophilicity, sustainability, self-cleaning property, and degradation of the samples. In addition, physical characterization by X-ray diffraction and surface roughness are also discussed. It has been found that most commercial products attain super-hydrophilicity only after exposed to ultraviolet and solar irradiation in less than 1 h. They can also maintain hydrophilicity after rigorous cleaning process. On the other hand, our prepared TiO 2 thin films demonstrate super-hydrophilic and photocatalytic properties after exposed to ultraviolet light for more than 2 h. According to the study, their anatase crystallinity, small grain size, and surface conditions all contributes to the excellent results. However, the prepared samples do not attain sufficient retention property to maintain their hydrophilicity. Conclusively, the designs of the TiO 2 films on car mirrors prove adequate to produce super-hydrophilic materials, which still degrade over normal usage. Nevertheless, our proposed investigation methods prove useful in quality evaluation in order to differentiate the results of super-hydrophilic materials of interests.

In vitro blood compatibility of modified PDMS surfaces as superhydrophobic and superhydrophilic materials

AbstractThe surface of polydimethylsiloxane rubber (PDMS) was irradiated by a CO2‐pulsed laser. The irradiated surfaces were grafted by hydroxyethylmethacrylate phosphatidylcholine (HEMAPC) by using the preirradiation method. The laser‐treated surfaces and HEMAPC‐grafted PDMS surfaces were characterized by using a variety of techniques including ATR‐FTIR spectroscopy, scanning electron microscopy (SEM), and wettability, which was measured by a water‐drop contact angle. Different surfaces with different wettability were prepared. These surfaces, including untreated PDMS (hydrophobic), laser‐treated PDMS (superhydrophobic), and HEMAPC‐grafted surfaces (superhydrophilic), were used for a platelet adhesion study. Results from in vitro testing indicated that chemical structures, such as negative‐charge polar groups and wettability, are important factors in blood compatibility of these surfaces and the superhydrophilic (the most wettable) and the superhydrophobic (the most unwettable) of modified PDMS surfaces have excellent blood compatibility compared to the unmodified PDMS. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91:2042–2047, 2004