Conflicts in Action

Shrinking Plastic

Some plastics shrink when you heat them. If they shrink uniformly then they simply get smaller. But if some regions shrink at different rates we end up with conflicts. Black regions absorb more heat from an infrared light, causing them to shrink faster. With a black ring around the edge, there is a conflict between the outer region that tries to shrink quickly than the interior. The conflict is resolved by the disc deforming into a dome, allowing the edge to shrink more than the centre. A black spot in the centre gives the opposite conflict, with the centre trying to shrink faster than the edge. The conflict is resolved by the disc buckling to form a wavy edge, allowing the plastic to maintain a long perimeter. This is similar to the way thin potato slices shrink more in the centre than the edge when you fry them, giving saddle-shaped crisps with a wavy edge.

These areal conflicts are based on different regions trying to change their area at different rates.

Curling paper

Paper absorbs water and expands when you wet it. If you stick tape along the short edge of printing paper and then wet the strip, it curls. There is a conflict between one side of the paper which tries to expand and the other side that is prevented from expanding by the tape creates a conflict. Curling resolves the conflict by allowing one side of the paper side to expand while the other remains the same. This is an example of a surface conflict. If you stick the tape along the long edge of the paper, the strip rolls into a tube. This is because there paper is made of cell wall fibres that are oriented preferentially along its length, making it expand less in length than width. If you stick the tape diagonally, it forms a helix when you wet it because the fibres are oriented at an angle to the strip. For more technical information on paper expansion see Chung et al 2014.

Curling cellophane

Cellophane expands when exposed to moisture. If you place a piece of cellophane on your hand, it curls. There is a surface conflict between the side next to your hand which is exposed to moisture and tries to expand, while the opposite side is not exposed to moisture and therefore does not expand. As with the curling paper, this conflict between the surfaces is resolved by the cellophane curling, allowing one surface to remain smaller than the other.

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