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The nitty gritty of the itty bitty

POSTED 31 JANUARY 2008

A knack for nacre: Seeking wisdom in the mother-of-pearl
Have you ever ogled the iridescence of a soap bubble, a compact disk, or an abalone shell? Then you're looking at interference: an optical effect caused by the addition or subtraction of light waves that causes the color to change along with your viewing angle.Perfectly round, iridescent soap bubble floats gently through the air

The iridescence in this soap bubble comes from interference in the light waves it reflects. Photo: Wikipedia

But that iridescent lining of an abalone's shell is not simply nice to look at. What caught the eye of University of Wisconsin-Madison biophysicist Pupa Gilbert is the material's fantastic resistance to fracture. Mother-of-pearl, she says, "is the most beautiful material on the planet, and also the most interesting, from the point of view that it outperforms the sum of its parts like no man-made composite material."

She says, "outperforms the sum of its parts," but maybe "two plus two = googol" is more accurate.

The material in mother-of-pearl, AKA nacre ("NAY-ker"), is more than 95 percent aragonite, a crystal form of calcium carbonate, but nacre is 3,000 times harder to fracture than aragonite. "There is no other material that is more conductive than its most conductive part, or more elastic than its more elastic component," says Gilbert. "For any thermal or physical aspect you can define, a synthetic material is never better than its best components."

 Rock-like creature with tiny spikes on floor of ocean is covered with marine life
The white abalone from Southern California (Haliotis sorenseni) is an endangered species. Based on its humble housing, would you suspect it contains that gorgeous, iridescent mother-of-pearl? Photo: NOAA

Itty bitty crystals
In a recent study, Gilbert explored the nitty-gritty of nacre's itty bitty with a new instrument, and found a jig-saw full of stacked crystals. "When we looked at a single layer, the crystals are there in several orientations. But across different layers, we found stacks of co-oriented crystals that go on for a little bit and then stop abruptly."

Circular image shows grain structure described above
The structure of mother-of-pearl (seen from the side) shows parallel horizontal layers made of calcium carbonate, which generate the iridescence. Each layer comprises tablets with different gray levels, which indicates the orientation of the crystals. The stacks of similarly oriented tablets, slightly offset, abruptly start and end from one layer to the next. The trace white lines show different layers of crystals. Courtesy Pupa Gilbert, adapted from Phys. Rev. Lett. 98, 268102, 2007.

And why is nacre so resistant to fracture? One factor is the shock-absorbing organic "mortar" that the abalone inserts between the "bricks" of aragonite. But Gilbert notes that a recent paper listed nine other possible explanations for nacre's excellent fracture resistance, such as the interlocking, offset nature of the crystals, and says "We don't know for sure. It could be that all 10 factors coalesce to give the superior mechanical performance. Certainly the interlocking in the differently oriented stacks of tablets we observed could contribute to toughness. In particular it could help disperse a crack into myriad nano-cracks, thus dispersing the energy"

At any rate, it's humbling to think that life has produced such a perfect material, but biology does rather well in the nanoscale design department: Spider venom and botulinum toxin (made by bacteria) are both paramount biological weapons. Spider silk is super-strong and light. The lens in the eye is an ultra-transparent crystal of proteins, and of course, north of the average pair of shoulders you can (often) find an ultra-brainy thinking machine.

Iridescent face of shell is shown
The mother-of-pearl surface inside an abalone is iridescent. But why is it 3,000 times more resistant to fracture than the material inside it? Photo: Jeff Miller, University of Wisconsin-Madison.

Making mother-of-pearl
As we strive to understand why mother-of-pearl is so tough, Gilbert says, "Biology has a half-billion-year head start on us on how to make a mollusk shell."

And a need for strength, not for beauty, explains why abalones produce nacre, Gilberts says. "If a loggerhead turtle or a great white shark wants to eat it, the abalone is very well protected." And she adds that protection is one possible role for synthetic nacre-inspired materials. "I can envision a day when we'll have a material like this saving lives, as shielding. It could protect cars and Humvees when there is a bomb blast. It could finally help protect people who are clearing mine fields."

Here's what will happen to you: You will click into our "nitty-gritty on the itty-bitty" bibliography.

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