Shearing Forces Increase Molecular Alignment

an oblong fuzzy white cocoon sits on a green leaf

Biological Strategy

Silkworm

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Modify Material Characteristics

The materials found in living systems are variable, yet often made from the same basic building blocks. For example, all insect exoskeletons consist of a material called chitin. Because material resources are limited, each material within or used by a given living system must frequently serve multiple purposes. Therefore, living systems have strategies to modify materials’ softness, flexibility, and other characteristics. To ensure survival, the benefits of these modifications must outweigh the living system’s energy and material expenditure to generate them. For example, spiders store the liquid components of spider silk in a gland, converting them into silk thread when needed. Some threads have different characteristics, such as elasticity and UV reflectance, than others.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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The silk-spinning organ of silkworms enhances crystallization and coagulation of the liquid silk product via shearing forces that help align the molecules.

“RAMSDEN (1938) observed that shearing of the viscous contents of silk glands between glass slides would cause it to coagulate. IIZUKA (1966) studied the effect of shear rate on the coagulation of fibroin solutions and also analyzed the dimensions of the spinneret of the silk worm Bombyx mori with regard to the shearing forces arising in this tube. He concluded that the shearing forces in the spinneret align molecules thus enhancing crystallization and also align the randomly oriented crystals thus promoting coagulation. The greater the shearing forces, the higher the degree of crystallinity of the product.” (Wainwright 1976:77)

Last Updated August 18, 2016

References

Book

Mechanical Design in Organisms

01/07/1982 | Stephen A. Wainwright

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