Crumpling under an ambient pressure.

A pressure chamber is designed to study the crumpling process under an ambient force. The compression force and its resulting radius for the ball obey a power law with an exponent that is independent of the thickness and initial size of the sheet. However, the exponent is found to be material dependent and less than the universal value, 0.25, claimed by the previous simulations. The power-law behavior disappears at high pressure when the compressibility drops discontinuously, which is suggestive of a locked state.