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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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