Learning from Living Organisms to Create Low Carbon Vessels

We have developed a superhydrophobic material with excellent durability by combining the tough, supple, spiky skin of the Porcupinefish with the hydrophobic structure found on lotus leaves and diving flies (Ephydra hians). We then used this material in ship bottom paint, intending to reduce the hull's water resistance. Highly durable superhydrophobic materials will enable air-water gaps on various hull shapes, and it is expected that this will reduce greenhouse gas emissions by improving fuel efficiency through the reduction of fluid resistance.

The Aim to Reduce GHG Emissions from ships

In Japan, various measures are being considered to achieve net-zero greenhouse gas (GHG) emissions from shipping by 2050, with the aim of achieving carbon neutrality nationwide in the same year. Measures under consideration in the shipping industry include hydrogen and ammonia-fueled ships and wind propulsion, along with air lubrication systems (ALS). ALS reduces frictional resistance between the hull bottom and the water by mixing millimeter-order bubbles into the flow around the hull. In Japan, a new ship equipped with the Mitsubishi Air Lubrication System (MALS) developed by Mitsubishi Heavy Industries, Ltd. was completed in 2010. Reportedly, the system achieved energy savings of at least 10%.¹
Flat-hulled vessels with shallow drafts are considered most suitable for ALS because of the ease of covering their bottoms with air bubbles. To put it another way, ALS technologies that work independently of hull shape will dramatically expand the range of applications of this technology and thus significantly reduce GHG emissions from marine transportation. To overcome these technical challenges, we turned our attention to the paints used on the bottoms of ship hulls. Our thinking was that although the shape of a ship's hull cannot be easily changed, if we could develop a ship bottom paint that could coat air in the water, ALS could be applied to a greater variety of ships without depending on their hull shapes.

Creatures that Dive While Clad in Air

How could we make a film of air underwater? We focused on diving flies (also known as alkali flies), which dive underwater using a suit of air bubbles formed around their bodies (Figure 1, left).² Alkali flies (Ephydra hians), which live in Mono Lake in California, in the United States, are characterized by their body surface, which is covered with hydrophobic hairs (Figure 1, right). When the flies dive into the water, these hydrophobic and dense hairs repel water due to the hydrophobic effect, which covers their bodies with air, creating a suit made of air. Alkali flies have uniquely evolved to use this air suit for respiration, allowing them to catch food and lay eggs while underwater.
This air suit that alkali flies make is one of the most common material designs seen when organisms develop water repellent functionality. It is referred to as the "lotus leaf effect" from the discovery of the mechanism for how lotus leaves repel water. The surface of a lotus leaf has micro concavo-convex structures made of water-repellent wax. These structures produce a cushion of air that supports water droplets, thereby enhancing the water-repellent effect (Figure 2 (a)).

Figure 1 Alkali flies diving underwater (left), scanning electron microscope image (right)

Copyright 2017 National Academy of Sciences, reprinted with permission

Durable Hydrophobic Materials

Since the discovery of the lotus leaf effect, various hydrophobic materials have been developed that mimic this principle. Microfabrication and self-assembly techniques, which make full use of lithography used in the manufacture of integrated circuits, have been used to produce micro concavo-convex structures on surfaces. However, in both cases, their lack of durability has created a barrier to practical applications. To overcome this problem, we focused on the spiky skin of the Porcupinefish (Figure 2). When the Porcupinefish senses danger, it quickly expands to about five times its original size. At the same time, it intimidates its enemies by protruding rigid spines from its skin (Figure 2 (b)). A closer look at Figure 2 (c) and (d) reveals an orderly arrangement of tetrapod-shaped spines on the surface. From this, we believed that we could produce a tough and flexible superhydrophobic material by creating micro concavo-convex structures of lotus leaves (Figure 2(a)) that develop superhydrophobicity through the imitation of a Porcupinefish's spiky skin.
We focused on using tetrapod-shaped zinc oxide (ZnO) whiskers (Figure 3(a)). Tetrapod-shaped zinc oxide is shaped like a regular tetrahedron with spines protruding from the center toward four vertices, like the wave-dissipating blocks laid along harbors and coastlines or the makibishi caltrops used by the ninja to block enemy pursuit. This structure means that no matter what orientation it is placed in, at least one spine inevitably faces upwards. By optimizing the ratio of silicone resin and zinc oxide whiskers used in the base material, we obtained a superhydrophobic material with a contact angle of at least 150° (Figure 3(b)). Another characteristic of this superhydrophobic material is that the tetrapod-shaped zinc oxide spikes produce water repellency on any surface. Therefore, when this material was poured into a mold to make a monolith, new superhydrophobic material appeared on the surface even after it was cut with a blade, a feature not seen in previous superhydrophobic materials (Figure 3(c)).³

Figure 2 (a) Photograph and scanning electron microscope image of a lotus leaf (provided by the author), (b) External image of Porcupinefish, and (d) X-ray CT image (provided by JMC Corporation)

Figure 3 (a) Electron microscope image of tetrapod-shaped zinc oxide, (b) laser microscope image of the composite surface obtained by changing the ratio of tetrapod-shaped zinc oxide and silicone resin and the water drop contact angle, (c) Water repellent material that provides a water repellent surface wherever it is cut.

Quick Realization of Water Repellent Coatings that Provide Air-Coating in the Water

Figure 4 shows what happens when a superhydrophobic material that imitates the Porcupinefish is applied to a substrate to simulate the bottom of a ship, submerged in water, and supplied with air bubbles on its surface. The air film formed on the surface of this water-repellent material was found to be stable for more than one week. It was also found that applying this to the bottom of a small model ship reduced fluid resistance by about 10%, indicating that GHG reductions would be possible by combining this superhydrophobic material and ALS (see the website of the National Institute for Materials Science (NIMS)).
In conclusion, this article briefly describes our efforts to reduce shipping-related GHG emissions, by using superhydrophobic materials that cover the bottom of a ship with an air film. By combining the water repellency of lotus leaves with the tough, supple, yet spiky skin of the Porcupinefish, we have succeeded in developing a superhydrophobic material with excellent durability. Another advantage of this newly developed material is that it can be applied using ordinary coating methods such as spraying. Although many issues need to be resolved before it can be put to practical use as ship bottom paint, we are accelerating our research with the aim of early-stage commercialization (End)

Figure 4: Superhydrophobic material mimicking a Porcupine fish, forming an air film in water.