Scientists at MIT, Harvard University believe that the tiny mollusk the blue-rayed limet has natural optical structures. This could allow for clear displays on windows, such as car windshields, using transparent screens. Blue-rayed limpet, 15 0. 59 an inch), when fully grown, is found in kelp beds on the coasts between the United Kingdom and Portugal. The limpet is an unusual creature, but it has a distinctive feature that will be noticed: the brightly colored dotted lines running along its translucent shells. Even in murky waters, the limpet shell flashes brilliantly. A limpet shell optical image showing light reflection from its shell exterior. Scale bar, 2 mm. Close-up image in image “B” below. Image courtesy of Nature Communications. Scientists now know that there are two structures in the shell of the mollusk which give it its blue-striped appearance. These structures reflect only blue light, and absorb all other wavelengths. Mathias Kolle (MIT assistant professor of mechanical Engineering) and his colleagues think that this patterning evolved from survival. The blue lines look similar to the colours found in poisonous soft-bodied snails. These findings were published in Nature Communicaitons, an academic journal. Citation below. The brilliant colours of butterflies, beetles, and birds are also displayed by organic structures such as plates, scales and feathers. Limpets produce blue stripes by using a combination of organic, mineral structures that are configured to only reflect blue light. These natural optical structures, according to scientists, could be used for colour-selective transparent displays without the need of an external light source. These could then be integrated into glass and windows. A single-stripe reflection optical micrograph. Scale bar, 100 mm. Scale bar, 100 mm. Image by Nature Communications. Prof. Kolle stated that he became fascinated in the limpets’ optical characteristics when Stefan Kolle, his brother and a Harvard marine biologist brought him some of these creatures in small containers. Stefan Kolle became fascinated by the brilliant patterns of the limpet and requested that his brother and other scientists investigate the optical properties of the mollusk shell. The research team, which included Joanna Aizenberg at Harvard and James Weaver from MIT, as well as Ling Li at MIT and Christine Ortiz, conducted a thorough structural and optical analysis on the limpet shells. When the limpets (juveniles) were still young, they would show blue lines resembling dotted lines. The lines became more consistent as the creatures grew older, with different shades varying from deep to turquoise. They used scanning electron microscopy to scan the shell of a limpet and found no difference in the structure of areas without and with stripes. The scientists wondered if the stripes could have been caused by features deeper in the shell. They dug deeper into the shell to discover the 3D nanoarchitecture. The top and bottom shell layers in the areas with blue stripes were uniformly thin with dense stacks calcium carbonate plateslets. This is similar to what you would see in other mollusks. However, 30 microns (. 03mm) below the surface of the shell revealed a striking difference. These regions saw the calcium carbonate plates change into two distinctive structural features. 1. A multilayered structure that has regular spacing between layers of calcium carbonate in a zigzag design. 2. and, further down, another layer of random dispersed, small particles. The zigzagging plates were found to be more spaced apart than the uniformly spaced plates that ran through the unstriped shell sections. The team then attempted to figure out the optical functions of the multilayer zigzagging and spherical particle structures. Professor Kolle’s team applied spectroscopy and optical microscopy as well as diffracting microscopy in order to determine the light-reflection characteristics of the blue line. They measured angles of the zigzagging structure and the angle to the shell and discovered that the structure was optimized for refracting blue and green lights. Scientists believe the scattered spherical particles underneath the zigzagging structures absorb transmitted light, which would otherwise de-saturate the blue light. The scientists concluded that the zigzag patterns act as filters, reflecting only blue light. Because the light is blocked by the underlying spherical particle, the rest of it gets absorbed and makes the shell stripes even more vividly blue. Why are there blue stripes? The blue stripes are used to hide kelp plants’ base from predators. The ones at the bottom of the plants are almost completely white and have a thicker shell. Blue-striped counterparts, however, live higher up on the plant. Because limpets can’t see well, the researchers don’t believe that blue stripes are used as communication tools. Professor Kolle believes the blue stripes may be used as a defense mechanism. There are two options when you find yourself on top of plants that have been exposed to predators: look unappetizing, or be invisible. These blue stripes are very similar to patterns found in poisonous sea snails that also live in similar kelp bed habitats. Professor Kolle believes the findings of this study could be used to create advanced transparent optical displays. The limpet’s shell has a microstructure that allows it to fulfill an optical purpose, without compromising its mechanical integrity. He explains that engineers and materials scientists could learn from the natural balance act. Professor Kolle said: “It is all about multifunctional material in nature. Every organism interacts with the environment in many ways and materials which interface with it frequently must fulfill multiple functions simultaneously.” “[Engineers] have a tendency to not optimize just one property of a device or material, such as a higher resolution screen or pixel density. They are aiming to satisfy several design criteria and performance criteria. Peter Vukusic is an associate professor of Physics at University of Exeter, England. He says that nature can provide inspiration and insights.
