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Name: Tim
Status: other
Grade: 12+
Location: NJ
Country: USA
Date: April 2009



Question:
I am curious as to why materials (flexible solids?) tend to wrinkle. I am vaguely aware of memory metals and plastics, so why can I fold a piece of paper and it stays folded, or a shirt stays wrinkled?


Replies:
Hi Tim, it all depends on the nature of the material. Sometimes the physical size/shape of the material is what allows it to be flexible or to shatter, while other times it is the properties of the atoms themselves. You mention two materials made of fibers (paper and shirts/fabric). Fibers have a lot of strength in one dimension, but are very flexible in the other two. When woven into a fabric, the fibers are somewhat free to move around, and they can be strong when pulled along their length, but flexible when bent. Rather than breaking, the fibers just move around enough so that they can bend/fold without breaking. In the case of, for example, a dinner plate, the ceramic materials do not have that freedom to move around, so they cannot bend, but instead can only fracture. In the case of metals, although they are not made of fibers, they have a unique molecular structure that allows their atoms to re-arrange without fracturing. So metals can bend, or spread out, or pull into wires without breaking.

Hope this helps,
Burr Zimmerman


Tim,

The short answer is that in substances that retain folds or get wrinkled, there is a mechanism to either break bonds and reform them, or to break intermolecular forces and reform them. In materials that do not retain folds or wrinkles, that snap back to a shape similar to their original form, there is a mechanism for the material to deform to a higher energy state that, when the force is removed, makes the material go back to a lower energy state that is similar to the original bulk shape.

The long answer depends on the material itself and variations in the answer depend on the type of intermolecular attractive force or chemical bond. Metals, for example, form structures where electrons are reasonably mobile and, in what is called the "electron sea model", these electrons form bonds between atoms that are as easily broken as reformed. Thus when energy is put into the metal (as when folding it) the metallic bonds are broken but then reform so that the new shape is held in place. This is similar for example to giving hair a perm. Heat breaks some S-S chemical bonds which can then reform (with the aid of oxidants) when the hair is set into a different shape.

In molecules that have "memory", a higher energy state, such as in stretched rubber is achieved when energy is put into the system. This shape can be held for as long as energy is in the system. When the energy is released, the natural lower energy state structure is reacquired. So, again in the case of rubber, when the bulk material is stretched, coiled long-chain molecules uncoil and become more linear. But this is an unstable high energy state. When the rubber is released, the molecules go back to their coiled state.

Greg (Roberto Gregorius)



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