WHY WOODEN CORK FOR WINE BOTTLE?

Take a piece of rubber. Stretch it. It will elongate lengthwise but will contract breadth wise. Now compress it. It gets shortened length wise. But stretches breadth-wise. Rubber is said to have 'positive poisson's ratio'.
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Suppose, we use rubber cork with wine bottle. When we press it into the mouth of the bottle, it will expand sideways. It may not go inside. It may even break the neck of the bottle. The wooden cork has 'zero poison's ratio'. That means, forces causes no effect in it. Hence, we can easily close and open the bottle with it.

There are some materials with negative poison's ratio. They are called "auxetic material". They have special structures as shown in the figure. Hence, their behavior is exactly opposite to that of the rubber. When stretched, they elongate on both sides. When compressed, they are shortened on both sides (length, breadth). Hence, they withstand strong force and pressure. They cannot be easily fractured.

' Auxetics' are used in the sports helmets (bike races).
Also used in body armor, packing material, knee and elbow pads.
Tendons in the body are natural auxetics.
In sport shoes, running track etc.

POISON'S RATIO -SIMPLIFIED

Strain = change in length/original length
Poison's ratio = breadth wise strain/lengthwise strain

It is normally in the range -1 to 0.5. Mostly poison's ratio is positive. That is, if the material is enlarged in one dimension, it will be shrieked in other two dimensions.. The materials of negative poison's ratio' is very rare. These are meta-materials belonging to the future.

Poison's ratio is mainly used by engineers to find"how much pressure a material can withstand before it collapses".
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