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Wrinkling of Flexible Solids
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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Update: June 2012
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