Ironing out the causes of wrinkles
Physicists find clues to the origin and evolution of wrinkles in thin sheets.
As a sign of aging or in a suit, wrinkles are almost never welcome, but two papers in the current issue of Physical Review Letters offer some perspective on what determines their size and shape in soft materials.
Jiangshui Huang and colleagues at the University of Massachusetts, Amherst explore how wrinkles in a sheet adapt to an edge that prefers to be flat. They float a thin, rectangular film of common plastic (polystyrene) on water and compress the sheet along one direction to make folds. In the middle of the film, competition between gravity (which prefers shallow, frequent ripples) and the energy cost of bending the film (which favors longer, higher folds) determine the height and frequency of the folds. Near the edge, however, surface tension forces the film to lie flat. Huang et al. show the film interpolates between these two limits by smoothly tapering from larger, undulating folds in the center to higher frequency ripples at the edge.
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