Droplet viscosity effects on dynamic hydrophobicity of a solid–liquid bulk composite prepared from porous glass

Significance Statement

Hydrophobic coatings find wide applications in anti-wetting, lowering friction resistance by reducing solid-liquid interaction and anti-rusting. Super-hydrophobic surfaces with more than 150° water contact angles are desirable for anti-corona activities, anti-snow adherence and antibacterial effects owing to their superior water-shedding functionality. Super-hydrophobic surfaces can be obtained through surface roughness with reduced surface energy. However, these unique properties originate from limited area of contact between water and solid and the developed air-layer at the interface.

The lifespan of the developed air layer between water and the solid is often inadequate. The layer is easily eliminated by friction with water or collision with a droplet. Soon after the air layer is removed, a drop in the performance of the water-shedding is deteriorated due to its original surface roughness. Above all, surface roughness leads to surface staining making outdoor durability hard to achieve. Therefore, synthesizing a solid-liquid bulk composite in which the liquid droplet does not come into contact with the solid would be helpful.

In a recent paper published in  Journal of Materials Science researchers led by professor Akira Nakajima at Tokyo Institute of Technology in Japan, prepared in their study, a solid-liquid bulk composite using a silicone oil, porous glass, and fluoroalkylsilane. The team investigated the effect of droplet viscosity on the sliding of the droplet on the solid-liquid bulk composite with varying droplet viscosity while maintaining its surface energy with a water-glycerin solution.

The authors precleaned a porous glass and then coated fluoroalkylsilane on its surface. They then impregnated silicone oil into the silane-coated glass and used a sessile drop method to determine the contact angles. They used water-ethanol and water glycerin blends, and water as droplet liquids for their study. Water-ethanol blend was employed for surface energy computation while the water-glycerin one was applied for the evaluating the impact of liquid viscosity.

The team analyzed the contact angles of the silane coating using a water-ethanol blend by altering the liquid mixing ratios (surface energy) and found that the surface energy for the fluoroalkylsilane coatings was approximately 16.0 mN/m.

The practical values of the spreading coefficients computed for the developed solid-liquid bulk composite through the pendant drop method indicated that the solid surface was not in direct contact with the water and the impregnated oil was spread between water and solid surfaces. Impregnating oil reduced contact and sliding angles. The drop in the contact angle was pegged at the phase change from air to oil in the porous glass. Decrease in the sliding angle resulted from the drop of probability for contacting the pining portion for the three-phase contact line motion on the oil surface.

From the results, the authors concluded that water droplets slid down majorly by slipping mode. The effect of rolling was found to increase steadily with droplet viscosity. When the concentration of glycerin was raised to more than 65%, rolling velocity was equal to the total velocity. The researchers found that for the water glycerin mixture, the flow around the droplet perimeter was faster than around the bottom side. This indicated that the liquid viscosity affected the droplet sliding.

The researchers managed to prepare a solid-liquid bulk composite without a contact between the hydrophobic surface and the liquid droplets. The sliding mode moved from slipping to rolling with increasing viscosity. Viscous energy dissipation computation suggested that the dominant elements were affected by the droplet viscosity.

Droplet viscosity effects on dynamic hydrophobicity of a solid–liquid bulk composite prepared from porous glass-advances in engineering

About the author

Mr.Kota Yokoyama earned his B.E. and M.E. degrees in Inorganic Materials Science from Tokyo Institute of Technology in 2014 and 2016 under the guidance of Profs. Akira Nakajima, Sachiko Matsushita and Toshihiro Isobe in the field of fabrication of water repellent surface.

Currently he is working in NGK Spark Plug CO., LTD.

About the author

Dr. Munetoshi Sakai earned Ph.D. from University of Tsukuba. He then joined “Super Hydrophilic/phobic Surface Project” in Kanagawa Academy of Science and Technology as Researcher and began research on hydrophobic coatings. In 2007 and 2010, he transferred into Photocatalyst group and combined with Organic Solar Cell Assessment Project respectively. He concurrently served as temporal lecture of Meiji University and University of Tsukuba. In 2014, he move to Tokyo University of Science, Yamaguchi. In April 2016, he joined the faculty staff of Strategic Coordination of Research and Intellectual Properties (OSRI) at Meiji University. His major researches are fluid physics, physical chemistry and system engineering for Functional Surface Materials.

About the author

Dr. Toshihiro Isobe earned his B.E. and M.E. degrees in Materials Science from Nagoya Institute of Technology in 2001 and 2003, respectively. He earned his Dr. of Eng. from Tokyo Institute of Technology in 2006. He then joined National Institute of Advanced Industrial Science and Technology (AIST). In Jun. 2008, he joined the faculty staff of the Tokyo Institute of Technology. Currently, he is an Assistant Professor of the Department of Materials Science and Engineering.

His research interests are in ceramic processing and is primarily concerned with the preparation of dense and porous ceramics and encompasses the characteristics of materials. A particular field of interest is the development of the ceramics having negative thermal expansion, ceramic membranes for gas separation, absorption and adsorption materials, and oxide-ion and proton conducting electrolyte materials. In 2012 Toshihiro was awarded CerSJ awards for advancements in ceramic science and technology from the ceramic society of Japan.

About the author

Dr. Sachiko Matsushita received her B.S. degree (1996) and her Ph.D. (2000) in Applied Chemistry from the University of Tokyo under the guidance of Prof. Akira Fujishima in the field of self-assembled nanoparticle structure fabrication and application. In 2001, she became a post-doctoral researcher in Prof. Toyoki Kunitake’s group at RIKEN and widened her research to photonic crystals. After her experience as a lecturer and associate professor at Nihon University, she joined the department of Metallurgy & Ceramics Science, Tokyo Institute of Technology, as an associate professor (2010). Currently, she is an associate professor of the Department of Materials Science and Engineering.

About the author

Prof. Akira Nakajima earned his B.E. and M.E. degrees in Inorganic Materials Science from Tokyo Institute of Technology in 1985 and 1987, respectively. He then joined Japan Energy Corp. as a research scientist. In 1997, he earned his Ph.D. from The Pennsylvania State University. He moved to RCAST of The University of Tokyo in 1998 and began research on hydrophobic coatings. He established a venture company (ASTI: Advanced Systems of Technology Incubation) and became the company’s CTO in 2001. In March 2003, he joined the faculty staff of the Tokyo Institute of Technology. Currently, he is a Professor of the Department of Materials Science and Engineering. He was the leader of the “Super Hydrophilic/phobic Surface Project” at the Kanagawa Academy of Science and Technology (KAST) during 2004-2007.

References

Kota Yokoyama1, Munetoshi Sakai2, Toshihiro Isobe1, Sachiko Matsushita1, and Akira Nakajima1. Droplet viscosity effects on dynamic hydrophobicity of a solid–liquid bulk composite prepared from porous glass. J Material Science, volume 52 (2017), pages 595–604.

Show Affiliations
  1. Department of Materials Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, Tokyo, Japan
  2. Strategic Coordination of Research and Intellectual Properties, Meiji University, Kawasaki, Japan

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