Summary and Intellectual Merit A group led by the PI will develop a highly sensitive adhesion experiment designed to assess the interactions between well characterized model surfaces in water. The measurement is based on the contact between a synthetic polymer membrane and a quartz crystal resonator. The membrane consists of an elastic layer at the air/water interface, and is an excellent model for a variety of materials ranging from synthetic polymer gels to living tissues. The membrane surface and the surface with which it interacts will both be coated with molecules that are designed to eliminate non-specific background adhesion between the two surfaces. Specific adhesion molecules will then be added to both surfaces, and the adhesion energy will be related to the interaction potential of the individual bonds, and to their surface concentration. The experiments are conceptually similar to dewetting experiments, with contact of the membrane and the surface corresponding to a region where water has been largely excluded from between the contacting surfaces. The experimental geometry is designed to give sensitivity to very small contact angles, and hence to very small adhesive interactions. Membrane elasticity serves two important functions. The first of these is to extend the range of adhesive interactions that can be probed so that the molecular origins of mechanical toughness of soft, highly deformable materials can be thoroughly investigated. The second function of membrane elasticity is to provide a means for fixing the adhesive molecules in place while isolating the system from the surrounding air environment so that the surfaces remain hydrated, even while in mechanical contact with one another. A detailed mechanical analysis will be developed for cases where the adhesion energy is large enough so that elastic effects must be taken into account. Additional information about the nature of the mechanical contact will be obtained by monitoring the frequency shift and dissipation of the quartz crystal resonators in the vicinity of a mechanical resonance of the crystal. The experimental methodology can be applied to a wide variety of adhesion problems in polymer physics and in biology, but experiments will begin with studies utilizing adhesive molecules that are very well characterized at the single molecule level. Broader Impacts The PI has developed a unique online multimedia text in polymer science that will undergo continued revision during this funding cycle. Additional contributions to undergraduate and undergraduate education will result directly from the proposed work, with three Ph.D. students and between 6 and 8 undergraduate students being trained in various aspects of mechanics, polymer physics and polymer synthesis. International experience will also be obtained as part of a collaboration with Prof. Diethelm Johannsmann at the University of Clausthal in Germany. Two Ph.D. students will conduct research in the Johannsmann laboratory during the course of this project, and two students from the Johannsmann laboratory will visit the PI's lab at Northwestern. Existing outreach activities will continue at local high schools and middle schools in Evanston and Chicago. The enhancements in mechanical characterization capabilities resulting from the proposed work will benefit a variety of research groups at Northwestern, in addition to scientists and engineers at local companies who utilize the micromechanics lab supervised by the PI. The broader scientific impact of the work will be felt most strongly in the biomaterials community, by providing a means for quantifying adhesion in practically relevant biological systems that are amenable to the membrane geometry.
