The radially symmetric pattern emanating from the contact line of a droplet was characterized with the number and the length of the wrinkles formed. By combining the scaling relations developed for the number of the wrinkles with those for the length of the wrinkles, we constructed a trivial metrology for measuring the elasticity and thickness of ultrathin polymer films. The role of bending mechanism on wrinkling wavelength was understood via inextensibility arguments. However, the stress state dictating the size of the wrinkled region so far has remained as a mystery. Here, we focus on the effective boundary conditions of the same experiment to improve our understanding on this elasto-capillary phenomenon. First, we use air bubbles placed beneath the films inverting the droplet experiment. The resultant wrinkle patterns dispel any uncertainties on assumptions related to the contact angle between the droplet and the floating PS film. Furthermore, fluorescent confocal micrographs show that a spherical cap formed under the droplet affording information to calculate the line tension effective at the contact line. These data provide critical information to test the theories that predict the length of wrinkles in near threshold (NT) and far from threshold (FFT) regime. Hence, our understanding on the relationship between the capillary wrinkling mechanisms and the underpinning boundary conditions is markedly improved.
Thomas P. Russell, the Silvio O. Conte Distinguished Professor of Polymer Science and Engineering, received his PhD in 1979 in Polymer Science and Engineering from the University of Massachusetts Amherst. He was a Research Staff Member at the IBM Almaden Research Center in San Jose, CA (1981-96) and became a Professor of Polymer Science and Engineering at the University of Massachusetts Amherst (1997). His research interests include the surface and interfacial properties of polymers, phase transitions in polymers, directed self-assembly processes, the use of polymers as scaffolds and templates for the generation of nanoscopic structures, the interfacial assembly of nanoparticles, dynamics in polymer thin films, and wrinkling and crumpling behavior of thin polymer films. He currently has over 500 publications and 12 patents dealing with is research. He is the Director of the Energy Frontier Research Center on Polymer-Based Materials for Harvesting Solar Energy, and an Associate Editor of Macromolecules. He is a fellow of the American Physical Society, the American Association for the Advancement of Science, the Materials Research Society, the Polymer Materials Science and Engineering Division of the ACS and the Neutron Scattering Society of America. He is also a member of the National Academy of Engineering.