Interfacial Mechanics and Contact Properties of Model Membranes

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Summary and Intellectual MeritA group led by the PI will develop a highly sensitive adhesion experiment designed to assessthe interactions between well characterized model surfaces in water. The measurement is basedon the contact between a synthetic polymer membrane and a quartz crystal resonator. Themembrane consists of an elastic layer at the air/water interface, and is an excellent model for avariety of materials ranging from synthetic polymer gels to living tissues. The membranesurface and the surface with which it interacts will both be coated with molecules that aredesigned to eliminate non-specific background adhesion between the two surfaces. Specificadhesion molecules will then be added to both surfaces, and the adhesion energy will be relatedto the interaction potential of the individual bonds, and to their surface concentration. Theexperiments are conceptually similar to dewetting experiments, with contact of the membraneand the surface corresponding to a region where water has been largely excluded from betweenthe contacting surfaces. The experimental geometry is designed to give sensitivity to very smallcontact angles, and hence to very small adhesive interactions. Membrane elasticity serves twoimportant functions. The first of these is to extend the range of adhesive interactions that can beprobed so that the molecular origins of mechanical toughness of soft, highly deformablematerials can be thoroughly investigated. The second function of membrane elasticity is toprovide a means for fixing the adhesive molecules in place while isolating the system from thesurrounding air environment so that the surfaces remain hydrated, even while in mechanicalcontact with one another. A detailed mechanical analysis will be developed for cases where theadhesion energy is large enough so that elastic effects must be taken into account. Additionalinformation about the nature of the mechanical contact will be obtained by monitoring thefrequency shift and dissipation of the quartz crystal resonators in the vicinity of a mechanicalresonance of the crystal. The experimental methodology can be applied to a wide variety ofadhesion problems in polymer physics and in biology, but experiments will begin with studiesutilizing adhesive molecules that are very well characterized at the single molecule level.Broader ImpactsThe PI has developed a unique online multimedia text in polymer science that will undergocontinued revision during this funding cycle. Additional contributions to undergraduate andundergraduate education will result directly from the proposed work, with three Ph.D. studentsand 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 ofa 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 ofthis project, and two students from the Johannsmann laboratory will visit the PI's lab atNorthwestern. Existing outreach activities will continue at local high schools and middle schoolsin Evanston and Chicago. The enhancements in mechanical characterization capabilitiesresulting from the proposed work will benefit a variety of research groups at Northwestern, inaddition to scientists and engineers at local companies who utilize the micromechanics labsupervised by the PI. The broader scientific impact of the work will be felt most strongly in thebiomaterials community, by providing a means for quantifying adhesion in practically relevantbiological systems that are amenable to the membrane geometry.