Overview

Unraveling the ingenious engineering embedded in nature has attracted the interest of researchers worldwide. Many engineering ideas draw inspiration from the complex designs found in living creatures, from animals to plants. When revealed, these designs possess the key to survival secrets, which are applied in engineering solutions and known as biomimetics. For tribologists, the natural world serves as a vast repository of advice, teaching valuable lessons on addressing frictional forces, wear resistance, and special lubrication abilities for specific applications.

This opening segment of ‘Nature’s Ingenious Tribology’ spotlights how engineers have recently found inspiration in the realm of the sea’s rulers — the sharks. Exploring the secrets behind their ability to swim faster and survive in the harsh underwater environment, we highlight how these insights are not just observed but also actively imitated in scientific ways. The goal is to develop surface designs that hold promise for enhancing the efficiency performance of, for example, water and air transport vehicles in the real world. Join us as we dive into the depths of nature’s brilliance to uncover the tribological wonders behind shark skins and their potential applications in engineering.

Drag in Fluid Friction

Let’s start by going over the basics at the surface before diving deeper. Unlike solids, fluids lack a definite shape, constantly changing under external forces or pressure. Fluids usually are categorized into liquids and gases, and they interact with other matters (e.g. solid, liquid, or gas materials), creating a force known as fluid friction. Imagine fishes wobbling as they swim through facing water resistance, or leaves falling slowly due to air resistance from a tree.

Fluid friction is the force that resists motion within the fluid itself or of another medium moving through it. The nature of the fluid is crucial, and different fluids like water, coffee, oil, air, and smoke have varying thicknesses or viscosities – a measure of a fluid’s resistance to flow because of internal friction. Viscosity plays a big role; thicker fluids resist motion more, making it harder to move underwater compared to walking on a dry street. Object speed matters too; faster objects experience more fluid friction. Also, the shape of the object is another factor, like comparing a big lorry to a streamlined racer car with an aerodynamic body.

The force that fluids exert, known as drag, is a significant concern for engineers designing vehicles like cars, aircraft, and boat hulls. They aim to minimize drag to achieve fuel efficiency and longer transport distances. Drag is not only about vehicles; in sports physics, it explains the motion of balls, arrows, javelins, and the suits worn by swimmers and runners. Tribologists, specifically those working on drag friction, actively seek ways to modify surface textures or profiles to reduce drag force or fluid friction in various applications.

Ingenious Design of Shark Skin

In the vast ocean, sharks are known as ‘the king of the sea’, with amazing adaptations that ensure their survival. They possess intelligent senses like acute smell and electroreception for prey detection, and their streamlined bodies and powerful tails enable swift propulsion and precise maneuvering. Their ability to regulate buoyancy through adjustments in liver oil content allows for efficient depth control [1]. Another remarkable feature contributing to their dominance is their textured skin.

Imagine running your hand along a shark’s body; it feels smooth as suede from head to tail but becomes rough like sandpaper in the opposite direction. Under a microscope, their skin reveals ribbed scales known as “dermal denticles,” made from dentin and enamel, arranged in a layered pattern. The brilliance of shark skin lies in its seamless guidance of water across the body, reducing friction and drag [2]. This adaptation empowers sharks to navigate effortlessly, with some species reaching speeds of almost 50 kilometers per hour.

The ingenious design of shark riblets adds another layer to their exceptional hydrodynamics. These riblets feature textured with directionally grooved, wedge-shaped scales on the epidermis contributing to their excellent drag reduction properties [3]. In engineering terms, this riblet structure enhances the hydrodynamic design of sharks as they swim in the sea, affecting factors like wall shear, velocity profile, and turbulence kinetic energy [4]. Fast sharks, like the Mako shark, operate at high Reynolds numbers (Re ≈ 106–107), and their skins exhibit a microstructure with small scales and ridges aligned in the flow direction of roughly 1/20 mm [5]. Researchers developed ribbed surfaces based on fluid-dynamic reasoning, while parallel work in other groups was motivated by observations of shark skin structure. Engineers, inspired by the efficiency of shark skin, actively incorporate these insights into various biomimetic engineering applications, marking a significant leap in innovative design.

Shark Skin-Inspired Engineering

Reducing drag in moving objects, whether animals or vehicles, involves minimizing wall shear stress and, in some cases, controlling separation inspired by the shark skin patterns. This concept is being applied to various technologies by engineers, such as long-range ships, submarines, aircraft, wind turbine blades, and even sports suits. The focus here is on the progress in these applications, considering parameters like traveling speed and energy consumption crucial for the operational performance of navigation objects.

Researchers have searched into marine drag reduction technologies, specifically examining the performance of riblets [6]. They have explored different aspects, including riblet geometry, continuous and segmented configurations, fluid velocity (both laminar and turbulent flow), fluid viscosity (water, oil, and gas), and wettability. Notably, the study highlights the significance of the area of riblet surfaces in understanding their impact on frictional force. Think of riblets as an anti-friction coating, promoting slip over the surfaces, especially when the riblet valleys are filled with still water. Interestingly, transverse hydrophobic riblet surfaces maintaining gases can achieve a drag reduction efficiency exceeding 13%. This is attributed to the coexistence of gas and vortex in the riblet valley, leading to slippage at the liquid-gas interface and reducing the velocity gradient of the boundary layer and the solid-liquid interface area when water flows over the surface. The applications discussed revolve around vital ocean navigation objects like ships, submarines, and torpedoes, critical for the development and defense of the ocean economy.

Moving into the aircraft application, a group of researchers has attempted to develop designs inspired by shark skin [7]. Experimental and simulation-based investigations were conducted, focusing on denticle-inspired designs along the suction side of an aerofoil. Parametric modeling identified structures that simultaneously reduce drag and generate lift on the aerofoil. These structures outperformed traditional low-profile vortex generators, boasting a remarkable 323% improvement in the lift-to-drag ratio, especially at low angles of attack. Two key mechanisms contribute to this success: a separation bubble in the denticle wake, altering flow pressure distribution, and streamwise vortices filling momentum loss in the boundary layer due to skin friction. Despite continuous research on riblets, challenges persist in determining the optimal size and shape for maximum drag reduction and addressing antifouling issues [8].

Closure

In the scope of biomimetics, the study of shark skins’ ingenious tribology has uncovered groundbreaking insights for engineering. From understanding fluid friction basics to mimicking the remarkable adaptations of sharks, this exploration has stimulated innovations in drag reduction technologies. Inspired by the textured skin of sharks, riblets are now influencing diverse applications, from marine vessels to aircraft. Challenges including optimizing size and shape for maximum drag reduction still remain. See you all in the next chapter of Nature’s Ingenious Tribology!

Acknowledgment

The artwork was illustrated by S. Almagfirah ([email protected]) based on an idea by M. Taufiqurrakhman. The microscopic image of dermal denticles in the artwork was sourced from Wikimedia Commons.

References

1. Bottaro, M., Sixth sense in the deep-sea: the electrosensory system in ghost shark Chimaera monstrosa. Sci Rep, 2022. 12(1): p. 9848.

2. Fan, S., et al., Shark Skin—An Inspiration for the Development of a Novel and Simple Biomimetic Turbulent Drag Reduction Topology. Sustainability, 2022. 14(24): p. 16662.

3. Liu, Z., et al., A glimpse of superb tribological designs in nature. Biotribology, 2015. 1-2: p. 11-23.

4. Ibrahim, M.D., et al., The Study of Drag Reduction on Ships Inspired by Simplified Shark Skin Imitation. Applied Bionics and Biomechanics, 2018. 2018: p. 7854321.

5. Bechert, D. and W. Hage, Drag reduction with riblets in nature and engineering. 2006.

6. Fu, Y.F., C.Q. Yuan, and X.Q. Bai, Marine drag reduction of shark skin inspired riblet surfaces. Biosurface and Biotribology, 2017. 3(1): p. 11-24.

7. Domel, A.G., et al., Shark skin-inspired designs that improve aerodynamic performance. Journal of The Royal Society Interface, 2018. 15(139): p. 20170828.

8. Walsh, M., Turbulent Boundary Layer Drag Reduction using Riblets, Paper# AIAA-1982-0169, presented at AIAA 20th Aerospace Sciences Meeting, Orlando FL, AIAA, New York. WALSH, MJ & ANDERS, JB JR 1989 Riblet/LEBU research at NASA Langley. Appl. Sci. Res, 1982.