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February 2020

Nature-Inspired Surface Engineering
An understanding of and an appreciation for the multifunctional surfaces found in nature have blossomed over the past two decades. Increasingly, scientists and engineers are developing novel surfaces that mimic those found in nature while solving some real-world problems.

In this article, we will focus on five distinct bio-inspired surfaces that are earnestly being studied by engineers and researchers.

1. Superhydrophilic surfaces can be produced by mimicking fish scales which, like the Lotus leaf, possess hierarchical structures at both the micro and nano scale.1 Water contact angles below 5° can be achieved. The surface is also superoleophilic with oil contact angles also below 5°. Underwater, however, the surface becomes superoleophobic or oil-repellant. This can be a real boon to fish who live in polluted waters. In addition to the surface structures, fish have a thin layer of mucus on their scales which is produced when foreign matter is detected. If you've ever gone fishing and had to hold onto a fish as you removed the hook, you know what I'm talking about here. We call it slimy. It's this mucus that permits near 180° oil contact angles in underwater conditions. Surfaces are being created that emulate the both the topography as well as the mucus of fish in an effort to create superoleophobic coatings for ship hulls which in turn would reduce drag, increase fuel efficiency, and prevent bio-fouling.

2. Springtails are creepy little anthropods that typically are under a quarter inch (6mm) long and live under ground. To farmers, they are a pest. Some propose that the US government used springtails during the Korean War to carry fatal diseases to the enemy. Some say that's a conspiracy theory. What's true and amazing about the springtail, however, are their cuticles which exhibit high static repellency and high pressure resistance. A hierarchical structure which consists of T-shaped nanostructures and wrinkles at the macro scale produce a superomniphobic condition. Surfaces made to mimic these structures have yielded contact angles in excess of 150°.2 What's more, these surfaces exhibit the highest pressure resistance on record.

3. Slippery liquid-infused porous surfaces (SLIPS) inspired by the meat-eating pitcher plant (Nepenthes) overcome some of the shortfalls of Lotus-inspired surfaces. For example, while the Lotus-inspired surfaces may be superhydrophobic, they are not always oleophobic. Moreover, they are susceptible to stress-induced failure and do not exhibit self-healing capabilities. SLIPS and LIS (lubricant-impregnated surfaces) overcome these shortcomings thanks to a lubricating fluid that mimics the surface inside a pitcher plant. These surfaces are amazingly robust, lubricious, durable, self-cleaning, self-healing, and repel just about any foreign matter. Commercial SLIPS coating products are being developed that can be applied to plastics, metals, and many other materials.3 Pipelines and ketchup bottles seem to be attracted to this technology.

4. While fish are covered with scales (see item 1 above), sharks are covered with small denticles - tooth-like protrusions with a boney structure covered with an enameloid. The shape and complexity of these denticles can vary from one part of the shark to another as well as from one specie to another. Studies show that shark skin surfaces greatly reduce drag due to its hierarchical structure and exhibit an antifouling property as well. While engineers and biologist have been interested in shark skin and its surface properties for some time, only recently have researchers begun to attempt to create a manmade shark skin that replicates the behavior and properties of the real thing using 3D printing technology based on micro-computer tomography.4 Successful production of a bio-inspired shark skin surface could lead to better ship hull designs, swimsuits, pipelines, and much more.

5. Most surface scientists will agree that the Lotus leaf is the gold standard for superhydrophobic surfaces. The highest contact angle that can be obtained from a solid smooth surface is about 120°. However, by adding asperities, the contact angle can be increased. By adding a hierarchical structure like that of the Lotus leaf, the highest possible contact angles can be achieved. Researchers are exploring these superhydrophobic surfaces for a wide variety of applications.

In addition to the nature-inspired surfaces detailed above, we have also detailed in past newsletters, the rose petal, Gecko feet, the water strider, the Namid desert beetle and moth eyes - to name a few.5

Nature has had billions of years to develop through natural selection a wide spectrum of highly engineered surfaces designed to address a wide variety of specific survival needs. By understanding and emulating these surfaces, we can engineer and design surfaces, coatings, and products that make the world a better place.

Whether you are developing nature-inspired surfaces or studying surfaces on more mundane bulk material, turn to the gold-standard in contact angle measurement, ramé-hart instrument company. We have a tool for every budget and every application. In fact, three out of four US-based universities that have a desire to study wettability turn to ramé-hart for their contact-angle analysis requirements.

1 See this article.
2 See this article.
3 For more detail, see https://adaptivesurface.tech/.
4 See this article and this one.
5 See our December 2019 Newsletter for an overview of these additional nature-inspired surfaces.

NISE 2020
The Second International Conference on Nature-Inspired Surface Engineering (NISE 2020) will be held 10 to 12-Aug-2020 in Seoul, South Korea. For more information, please visit their website.

Carl Clegg
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Phone 973-448-0305
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