Role of Structure and Dynamics of Molecules at the Interface in Controlling Friction, Adhesion and Adhesion Hysteresis

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We propose to combine surface sensitive infrared-visible sum frequency generationspectroscopy (SFG) with insitu friction and adhesion measurements. The experimentsare motivated by results of Chaudhury and Brown using poly (dimethylsiloxane) (PDMS)lenses and Israelachvili using surface force apparatus; where they find a relationshipbetween friction, adhesion hysteresis and mobility of the molecules at the interface.However, there is no information on how different relaxation processes are atpolymer/solid or hidden interfaces in comparison to that in bulk. Recent experiments ofglass transition temperature of polymer thin films indicate that the relaxation at the airinterface can be several orders of magnitude faster than in the bulk. The relaxationtimes of polymer chains are expected to be altered in contact with surfaces of differentsurface energies. Furthermore, there is no direct information of the change in thestructure of molecules at moving or sliding interfaces. Understanding these properties iscrucial to understand friction. Recently, we demonstrated that SFG can be applied tostudy the contact interface between an elastomeric PDMS lens and a polymer or solidsubstrate. Here, we take advantage of this design to measure simultaneously thechanges in structure between two sliding surfaces, friction and adhesion. In addition, wepropose to measure the relaxation dynamics at interfaces using SFG to understand therelationship between mobility and friction coefficient. As a Director of District 5, I havebeen actively involved in organizing District 5 Science Day for students from K5-K12 forthe past five years. In addition to this outreach activity, this proposal offers researchopportunities to two high school students from a local high school to work duringsummer in my laboratory under the supervision of graduate students. The research ofthese high school projects can be submitted as a part of their science project forcompetition in the Intel Science and Engineering Fair. This effort will be integrated withthe NSF-REU undergraduate program already existing at The University of Akron. Broader Impact: Friction between two surfaces is of utmost importance in a variety oftechnological and biological applications. The understanding and control of thesephenomena will have a tremendous impact in conserving and using precious energyresources. The advances in this field are limited by the lack of understanding of thestructure and properties of molecules at the interface. This proposal provides a newexperimental method to probe molecular motion or deformation taking place at theinterface and studies its relationship to friction and adhesion. This will broadly impactour design of surfaces, lubricants and micro or nano-actuators. The education andoutreach activities target local schools in District 5 to promote an interest in science byencouraging students to undertake science projects and compete at district, state andinternational level.