Takeo Moriya

Controllable Broadband Optical Transparency and Wettability Switching of Temperature-Activated Solid/Liquid-Infused Nanofibrous Membranes

Inspired by biointerfaces, such as the surfaces of lotus leaves and pitcher plants, researchers have developed innovative strategies for controlling surface wettability and transparency. In particular, great success has been achieved in obtaining low adhesion and high transmittance via the introduction of a liquid layer to form liquid-infused surfaces. Furthermore, smart surfaces that can change their surface properties according to external stimuli have recently attracted substantial interest. As some of the best-performing smart surface materials, slippery liquid-infused porous surfaces (SLIPSs), which are super-repellent, demonstrate the successful achievement of switchable adhesion and tunable transparency that can be controlled by a graded mechanical stimulus. However, despite considerable efforts, producing temperature-responsive, super-repellent surfaces at ambient temperature and pressure remains difficult because of the use of nonreactive lubricant oil as a building block in previously investigated repellent surfaces. Therefore, the present study focused on developing multifunctional materials that dynamically adapt to temperature changes. Here, we demonstrate temperature-activated solidifiable/liquid paraffin-infused porous surfaces (TA-SLIPSs) whose transparency and control of water droplet movement at room temperature can be simultaneously controlled. The solidification of the paraffin changes the surface morphology and the size of the light-transmission inhibitor in the lubricant layer; as a result, the control over the droplet movement and the light transmittance at different temperatures is dependent on the solidifiable/liquid paraffin mixing ratio. Further study of such temperature-responsive, multifunctional systems would be valuable for antifouling applications and the development of surfaces with tunable optical transparency for innovative medical applications, intelligent windows, and other devices.

Slippery Liquid-Immobilized Coating Films Using in Situ Oxidation–Reduction Reactions of Metal Ions in Polyelectrolyte Films

We fabricated slippery liquid-immobilized coating (SLIC) films by reacting a slippery liquid (polymethylhydrosiloxane) near the surface of a polyelectrolyte film containing silver ions prepared by the layer-by-layer method. The obtained films maintained their slipperiness after chemical and physical treatments, in contrast to slippery liquid-infused porous surfaces. The high chemical and physical stabilities of the films were attributable to gelation and immobilization of the lubricant owing to an oxidation-reduction reaction with subsequent dehydration condensation of Si-OH on the outer surface of the polyelectrolyte film and the bonding of Si-H with NH2 groups within the polyelectrolyte film, respectively. Moreover, the SLIC films exhibited a high degree of slipperiness with respect to low-surface-tension liquids. To the best of our knowledge, this technique of lubricant immobilization using silver ions has not been reported previously. The films should be suitable for use in various applications where contamination must be prevented under extreme conditions, such as those requiring high physical durability and organic solvent use.

Porous Transition of Polyelectrolyte Film through Reaction-Induced Phase Separation Caused by Interaction with Specific Metal Ions.

We describe a novel method for the simple and eco-friendly fabrication of porous polyelectrolyte films. A polyelectrolyte with many amine groups undergoes structural transformation from a dense to a porous structure upon immersion in a specific metal ion solution. The porous transition was the result of a reaction-induced phase separation, which was caused by the formation of new bonds between the polyelectrolyte and metal ions. This method enables control of the pore size of the porous structure in the nanoscale (54 nm) to microscale (1.63 μm) range through variation of the concentration or type of metal ions in the solution. To the best of our knowledge, this is the first report illustrating wide-range control of the pore size of a porous polyelectrolyte structure achieved by metal ions. These porous polyelectrolyte films with adjustable pore size and metastable metal ions can be employed in applications such as adsorption and catalysis.

A Fluorine-Free Slippery Surface with Hot Water Repellency and Improved Stability against Boiling

Inspired by natural living things such as lotus leaves and pitcher plants, researchers have developed many excellent antifouling coatings. In particular, hot-water-repellent surfaces have received much attention in recent years because of their wide range of applications. However, coatings with stability against boiling in hot water have not been achieved yet. Long-chain perfluorinated materials, which are often used for liquid-repellent coatings owing to their low surface energy, hinder the potential application of antifouling coatings in food containers. Herein, we design a fluorine-free slippery surface that immobilizes a biocompatible lubricant layer on a phenyl-group-modified smooth solid surface through OH-π interactions. The smooth base layer was fabricated by modification of phenyltriethoxysilane through a sol-gel method. The π-electrons of the phenyl groups interact with the carboxyl group of the oleic acid used as a lubricant, which facilitates immobilization on the base layer. Water droplets slid off the surface in the temperature range from 20 to 80 °C at very low sliding angles (<2°). Furthermore, we increased the π-electron density in the base layer to strengthen the OH-π interactions, which improved long-term boiling stability under hot water. We believe that this surface will be applied in fields in which the practical use of antifouling coatings is desirable, such as food containers, drink cans, and glassware.

A superrepellent coating with dynamic fluorine chains for frosting suppression: effects of polarity, coalescence and ice nucleation free energy barrier

Ice formation on surfaces is a serious issue in many different fields in terms of function, safety, and cost of operation in human life. Hydrophobic coating technology is one of the effective ways to prevent ice formation. Previous studies focused on the effects of surface structure and surface chemical modification on anti-icing ability. However, only a few studies have clarified a method to inhibit the initial formation of ice on surfaces; in addition, an effective mechanism for anti-frosting has not been identified as yet. Here, hydrophobic smooth surface coatings using three coupling agents with low surface energy and different molecular chain dynamics were fabricated. The surface roughnesses were lower than 1 nm. The fluorocarbon-based coatings delayed frost formation compared with the uncoated surface until −6 °C. We explored why the coating surface prevented frost formation and the effects of surface chemical modification on frost resistance from the viewpoint of heat exchange contact area during droplet coalescence, ice nucleation free energy barrier, polarity and polarizability of the coated surface.

Improvement of heat transfer by promoting dropwise condensation using electrospun polytetrafluoroethylene thin films

Vapor condensation is a crucial part of a broad range of industrial applications including power generation, water harvesting, and air conditioning. Hydrophobic and superhydrophobic surfaces promote dropwise condensation in vapor-filled environments and increase their heat transfer coefficients more than filmwise condensation on hydrophilic surfaces. Although dropwise condensation can lead to energy-efficient transfer, it is hard to achieve stable dropwise condensation in high-temperature environments. To decide the best conditions for achieving higher heat transfer is also difficult because the heat transfer coefficient is influenced by not only surface wettability but also surface structures of thin films and substrates. Herein, we fabricated thin films with different wettabilities and surface structures using polytetrafluoroethylene (PTFE) which show high heat resistance to determine the best conditions for heat transfer. Several different films were prepared by electrospinning a mixed solution of PTFE and polyvinyl alcohol on aluminum (Al) and copper (Cu) tubes. After annealing them, the PTFE thin films enhanced heat transfer performance and showed stable dropwise condensation in high-temperature environments. The films fabricated by electrospinning a solution containing 66 wt% PTFE displayed the highest heat transfer coefficients, with heat transfer coefficients 64% and 61% greater than those of uncoated Al and Cu tubes, respectively. That is because homogeneous superhydrophobic surfaces that showed the highest departure frequency of condensed water droplets were fabricated using 66 wt% PTFE. The results suggest that these electrospun PTFE thin films would demonstrate excellent potential for use on the surface of heat exchangers in various industries.