Take a piece of plastic wrap that you use to cover food and stretch out a piece; or push together the skin on your hand or arm. You will see the formation of ripples or wrinkles. These are basically surface instabilities that form due to the force that you are applying and, like with the plastic wrap or your skin, the direction of the wrinkles will depend on the direction that you are pushing or pulling. The mechanical properties of the plastic wrap or your skin, and the amount of pulling or pushing that you are doing, will determine the frequency of the wrinkles that are formed. Here, the wrinkles are formed by a soft polymer. The direction of the wrinkles will be directly related to the direction in which you are pushing or pulling. Near the outer edge, the wrinkles go out radically, as you would expect. But in the middle, the wrinkles are every which way, since here the force is in all directions.
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