Colloquium: Dr. Andrew CrollDr. Andrew Croll Department of Physics North Dakota State University
The Role of Adhesion in Crumpled Polymer Films
Once passive barrier coatings, industry now expects much more of thin films, demanding they form thin electronics for devices or conformal antennae, or demanding they become the structural elements they formerly protected through origami and kirigami based design. Despite the simplicity, there are still a significant number of unknowns in the basic physics of thin elastic sheets. For example, it is a challenge to confine a thin sheet in a volume of three dimensional space smaller than the sheets original size. Bending a film is the easiest way to imagine confinement taking place; a sheet of size L can be bent along its axis reducing the overall volume occupied. However, if a second dimension is simultaneously reduced, the sheet cannot smoothly accommodate the boundaries. It is forced to localize stress in a sharp point known as a developable cone (easily seen in bending a sheet along two orthogonal axis). If compression continues, many localized structures form and the sheet is forced into a random structure known as a crumple ball. In this talk we will use confocal microscopy to examine the crumpled ball in an attempt to understand the stiffness that arises from this unique structure. Using polymer films created in house allows us to control many variables and create truly elastic sheets, plastic sheets and sheets in which adhesion can be manipulated. Confocal microscopy allows us to image the structure during the compression. Ultimately, we show a semi-empirical model is superior to current scaling theories and observe adhesion to play a significant role, increasing the ‘effective’ modulus of the structure by an order of magnitude.