This appeared in yesterday’s WallStreet journal on page C5
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Everyday Physics
By Helen Czerski
It was a bright sunny day on the shore of Lake Windermere in the north of England and the local science festival was in full swing.
A craftsman wearing thick gloves and hard-worn trousers was coiling sturdy strips of wood around a slender post almost as if they were putty.
I think of wood as robust and tough, springy but definitely likely to snap if you bend it too far. And yet here were 2-yard-long strips of wood, a half-inch thick, being wrapped into a corkscrew shape. It was a vivid lesson in how clever the microstructure of wood is.
Trees grow by adding new outer layers made of long, thin cells aligned with the trunk, which form pipes to transport both water and sugars up and down the tree. Apart from transport, a major function of the cells is to provide the structural strength to hold up the tree, resisting high winds and gravity in order to keep the leaves up in the air. This strength is provided by a clever matrix in each cell wall. About half of it is made of long strands of cellulose with good tensile strength called microfibrils; these are surrounded by a filler of the organic polymers hemicellulose and lignin, which resists compression. The matrix is tough, slightly flexible and very strong. But that doesn’t mean it can resist all change, and the effective agents of change are heat and water.
The craftsman had a long insulated box filled with hot steamy air, a miniature steam room in which he had placed his straight wooden strips for a few hours at high temperature. This was the key to the transformation. The water molecules had been able to creep between the cell wall components and replace some of the bonds holding them together. At the same time, the increased temperature softened the matrix filler, allowing its tangled molecules to move around. So now all the components were a bit more mobile, and the next stage was to reposition them. Wood is much stronger in compression than tension. If wood in this steamy state is squashed, the cell walls will buckle, but the wood will remain intact. However, if it’s stretched, cracks will form, which can cause the wood to break. So as each wood strip came out of the steam box, the craftsman clamped it to a strip of metal, which ran along the side that would become the outside of the bend. As he wrapped the wood around the post, the wood on the outside could not stretch because of the metal band, but the cell walls on the inside could buckle, and the cellulose and filler could rearrange themselves as they were squashed.
Once clamped in place, the filler molecules continued to shuffle around to release the stresses until the wood was cool enough to freeze them permanently into their new positions. When completely cool, the wood would hold its new curvy shape.
This technique of bending wood with steam has been used for centuries to make musical instruments, barrels, furniture and boats. It works because wood is fundamentally different from metals, plastics and ceramics— it’s a complex matrix of biological materials that are tough and resilient but still malleable in the right circumstances.
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