Physicists discover mechanisms of wrinkle and crumple formation – China Electric Motor Magnets

Posted: Jun 8th, 2012 Physicists discover mechanisms of wrinkle and crumple formation ( Nanowerk News ) Smooth wrinkles and sharply crumpled regions are familiar motifsin biological and synthetic sheets, such as plant leaves andcrushed foils, say physicists Benny Davidovitch, Narayanan Menonand colleagues at the University of Massachusetts Amherst, but howa featureless sheet develops a complex shape has long remainedelusive. Now, in a cover story of the journal, Proceedings of the National Academy of Sciences ( “Elastic sheet on a liquid drop reveals wrinkling andcrumpling as distinct symmetry-breaking instabilities” ), the physicists report that they have identified a fundamentalmechanism by which such complex patterns emerge spontaneously. In recent experiments, Davidovitch and colleagues confirmed theirearlier theoretical predictions using an ultra-thin film, just tensof nanometers thick. They say the wrinkle-to-crumple transitionreflects a dramatic change called “symmetry breaking” in thedistribution of stresses in the sheet. Davidovitch says they were inspired and moved toward a solution bythinking about how a familiar birthday balloon, made of twocircular mylar foils, wrinkles and crumples (two separateprocesses). Neodymium Ring Magnets

The two foils start flat, but when glued togetheraround their edges and injected with helium gas to createhigher-than-atmospheric pressure inside, they spontaneously changesshape to accommodate the gas. “This simple process leads to a fascinating pattern of wrinkles andcrumples that emerge spontaneously near the perimeter of eachfoil,” Davidovitch points out. “What we discovered is an unusualsequence of transitions that underlie this and possibly other typesof morphological complexity.” In the laboratory, rather than balloons, the researchers includingdoctoral student Hunter King, who conducted many of theexperiments, and postdoctoral researcher Robert Schroll, whocarried out theoretical calculations, used microscopically thinfilms and a drop of water to model the effects they wished tostudy. They cut a circle of ultra-thin film from a sheet 10,000times thinner than a piece of paper, only tens of nanometers thick,and place it flat on the water drop nestled in a circular collar,where surface tension holds it in place. “We then very, very gently inject more and more water into thebubble, very gradually, so it becomes more and more curved withoutspilling over,” says Davidovitch. China Electric Motor Magnets

“When the radius of the drop getssmall enough, the thin film starts to develop fine radial wrinklesnear its outer perimeter as the water pressure increases If youkeep adding pressure, decreasing the radius further, a secondtransition takes place and the film starts to crumple and to lookmore like a table cloth, draping with sharp creases over the edgeof a flattened top,” he adds. Watching this process through incremental steps, the researcherswere able to observe and describe through mathematical formulas howthe drop imposes confinement on circles of latitude of the sheet.”The degree of this confinement increases as the drop’s radiusdecreases, and an unusual sequence of transitions can then beobserved,” says Davidovitch. With this work the investigators, who had earlier proposedquantitative predictions of wrinkle patterns in ultra-thin sheetsby following the principle that such sheets must be free ofcompression, confirm their theoretical predictions. The currentexperiments also suggest that the wrinkle-to-crumple transitionreflects a dramatic change called “symmetry breaking” in thedistribution of stresses in the sheet, rather than just a furtherdisruption of its symmetric shape, Davidovitch points out. The researchers are now working on new puzzles regarding theformation of crumpled features posed by the experiment. Neodymium Ring Magnets Manufacturer

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