Scientists at Massachusetts Institute of Technology and Harvard University have identified two optical structures within the blue-rayed limpet’s shell that give its blue-striped appearance. While such details may seem to be largely irrelevant for most of the people, optical features embedded in marine shells may help eventually develop responsive transparent displays.
The limpet in question is a tiny mollusc that lives along the coasts of Norway, Iceland, the United Kingdom, Portugal, and the Canary Islands. It is as small as a fingernail, but its shell is striped with bright blue dotted lines that run in parallel along the length. These lines can flash, if light hits them at a right angle, even in murky water. It is speculated that such patterning may have evolved to protect the limpet, as the blue lines resemble the colour displays on the shells of more poisonous soft-bodied snails. Scientists believe that this curious property of the limpet’s shell can be used as inspiration in developing optic technology.
The newly identified structures of the limpet’s shell are configured to reflect blue light while absorbing all other wavelengths of incoming light. Scientists believe that these natural optical structures may serve as design guides for colour-selective, controllable, transparent displays that require no internal light source and could be incorporated into windows and other glasses. It could potentially be used, for example, to display maps on car windscreens. But such discoveries were not made in one day just by looking at the mollusc.
Researchers performed a detailed structural and optical analysis of the limpet shells. They discovered that blue stripes first appear in juveniles, dashed lines and stripes grow more continuous as a limpet matures. Shade of blue varies from individual to individual. Scanning electron microscopy showed no difference between stripes and surface in between, which made them think that the stripes arose from features embedded deeper in the shell.
Further analysis showed that layers of shells were relatively uniform, with dense stacks of calcium carbonate platelets and thin organic layers, similar to the shell structure of other molluscs. But about 30 microns beneath the shell surface the researchers found that the regular plates of calcium carbonate morphed into two distinct structural features. First one – a multi-layered structure with regular spacing between calcium carbonate layers resembling a zigzag pattern, and beneath that – a layer of randomly dispersed spherical particles.
Results of the research showed that the zigzag pattern acts as a filter, reflecting only blue light and the structure beneath the zigzag structures serves to absorb transmitted light that otherwise could de-saturate the reflected blue colour. Scientists were fascinated by such defence mechanism that formed these complex structures without overly compromising the shell’s mechanical integrity.
It is unclear whether or not engineers are going to search for ways to apply these new findings to create innovative transparent displays. But the researchers believe that limpet’s optical properties may be exploited to engineer advanced optical displays in the future and scientists should continue to look for inspiration in the nature.