Engineering Mechanics in Nature
As we continue from last week’s blog that examined the biomimicry seen in gecko’s feet, this week we will examine another animal trait that can be applied to engineering; shark skin. Over the vast time period of the sharks’ existence, nature has perfected the design to address many challenges that faced them as well as offer solutions to many challenges we see today. Recently scientists and many engineers from all fields have been closely examining the design of sharks’ skin and are actively attempting to recreate it for use in a variety of ways.
Contrary to popular belief, while a sharks’ body is generally hydrodynamic, its scales are anything but smooth. Unlike fish, sharks’ skin is made of countless overlapping scales known as dermal denticles, or “little skin teeth”. Unlike regular scales, these denticles have small grooves running down their length aligned with the water flow, which allow for more efficient water travel because they disrupt turbulent swirls of slower water, known as eddies. As water passes over a smooth surface it “trips” over itself because it is moving faster than water that is further away from the object. The grooves accomplish this more efficient movement in four ways:
- The grooves reinforce the flow direction by forcing it into channels.
- They speed up the slower water at the sharks’ surface which reduces the difference in speed of the surface flow and the water beyond.
- They pull faster water towards the shark’s surface, so that it mixes with the slower water and reduces the speed differential.
- They divide up the sheet of water flowing over the shark’s surface so that any turbulence created results in a smaller vortex.
Also, the three main reasons that microbes and bacteria are unable to adhere to the surface are:
- The increased water reduces the contact time for the organisms
- The roughened nano texture of shark skin reduces the surface area that organisms can adhere to
- The dermal scales themselves are constantly in flux which creates a moving target for the microbes to adhere to.[ii]
Application and Simulation
One of the most common applications for sharkskin technology is on the bottom surface of boats which can not only reduce drag over conventional surfaces by about 67%, at 4 or 5 knots, these types of surfaces are self-cleaning of algae and barnacles. The boats treated with shark skin surfacing are in turn better for the environment in two ways. One is that they are much more energy efficient resulting in improved fuel economy. Also they do not require harsh chemicals and cleaners to remove organisms from their hulls.[iii] Similar to boats, many car developers have invested research into the possibility of increasing efficiency on the road. However the challenge is much larger on land than it is on water. Some of the main challenges are that most of the drag in cars comes from the profile design of the car, while for submerged objects; most of the drag comes from the surface texture. Using computational fluid dynamics for simulation, the grooves on a car were not big enough to make a difference because of crosswinds and the ratio of the grooves to the weight of the car. Continuing to work in the field with 3D CAD software and similar technology will hopefully lead to innovations in the future that lead to incredible efficiency both on and off the water. Another application possibility has biomedical engineers and researches attempting to use shark skin technology on medical equipment because of their unmatchable ability to repel microorganisms. Replacing current methods of cleaning would work in two main ways: first they do not kill microbes so the microbes have nothing to adapt to out of resistance, and second, the use of hazardous chemicals is no longer necessary to wash equipment.
These are only a few of the possibilities in which science and engineering are looking at nature to improve technology. The application possibilities in the fields of mechanical engineering alone are too numerous to imagine. As we move forward in this series, we will discuss some of these applications as well as future technological ideas.
[i] Biomimicry Institute “Biomimicking Sharks: Beauty is Skin Deep” 2007
[ii] Miracle, Barbara Florida Trend “Sharkskin Technology Inhibits Germ Growth” October 10, 2008
[iii] Lang, Amy Science Daily “Exploring Energy Conservation Through Shark Research” December 1, 2007