We‘ve all crumpled up a piece of paper before tossing it into the trash at one time or another. Whether it is a piece of notebook paper, a receipt from the grocery store, or even a paper bag, chances are that you have noticed that you can only squeeze the ball of paper so much before it seems resistant to any more force. Go ahead, grab a piece of scrap paper and try it.
Although the volume of a ball of paper is very low – it is 90% air – it seems to become stronger the more it is squeezed. Physicists from the University of Massachusetts have been wondering why this is the case.
The Inner Picture
To look for the answer, the scientists have been using an X-ray microtomography machine to take pictures of crumpled balls of paper. This machine works much as a CT scan does. The machine takes pictures of slices of an object, slices that can then be put back together to give scientists a cross-sectional view of the object.
In this case, pictures taken of slices of crumpled paper allow the physicists to study the patterns, or lack thereof, within the paper ball. Since the forces that crumple paper are usually very random, there does not appear to be many predictable patterns, except for one, and this pattern may explain the strength paper gains when crumpled.
Stacking Up Strength
The one pattern that appears in every crumpled ball of paper is that the flat parts of the paper ball always touch – that is, they stack up, one flat edge against another. The more a ball of paper is compressed, the greater the layering. The greater the layering, the harder it is to further compress the ball! For images of the layering that takes place within a crumpled paper ball, visit the link below.