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Molecular Structure and Friction
Name: Rebecca
Status: student
Age: 15
Location: N/A
Country: N/A
Date: 2000-2001
Question:
How does the molecular structure of a material affect its
smoothness/roughness? Or basically, the coefficient of friction. In
particular, for vinyl, ceramics, and oak.
Replies:
Rebecca,
Smoothness/roughness depends much more on how the material is formed than on
molecular structure.
Some materials are formed in a very random pattern, while others come
togetherin a very well defined fashion. Smoothness/roughness is often on a
scale much larger than individual molecules.
The wood of a tree is rough because the cells of the tree are formed around
paths that transport water and sap through the tree. A tree is made in an
almost random fashion, one ring formed around another, one cell at a time.
Where the next cell forms can depend on many environmental factors, as well
as factors as simple as where a rock hits the bark.
Ceramics start as a fairly liquid substance, a kind of clay. As it bakes,
thr surface gets hot, starts to melt, then cooks solid. Smoothness of a
ceramic material depends mostly on air bubbles and surface paints. A piece
of clay with many air bubbles will have these bubbles expand during baking.
The surface will have tiny popped bubbles. The surface will be rough. A
ceramic material with air bubbles worked out near the surface will not have
many bubbles. The correct kinds of paint will fill in any remaining
bubbles. The heat smooths the paint as it cooks. Using a good paint
results in a "polished" surface.
Plastics are formed in molds. Assuming no air bubbles exist in the liquid
plastic, smoothness of the plastic depends on smoothness of the mold.
Another factor is shrinking during cooling. If the object shrinks evenly,
things work fine. If it is cooled too fast, the surface shrinks faster than
the inside. The surface pulls apart along cracks. As the middle shrinks,
these cracks are brought back together, but now the surface is rough.
As we can see, smoothness and roughness as we see and feel it depend on
things much larger than individual molecules. Friction depends a great deal
on this roughness. It also depends on which molecules of the first surface
are touching which molecules of the second surface. Some molecules hold
together very well, while some do not. This is why some smooth surfaces
slide along each other while other smooth surfaces are very difficult to
slide. But in most cases, it is the roughness that matters most.
Dr. Ken Mellendorf
Illinois Central College
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Update: June 2012
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