Open questions about polymer friction

‘‘Friction’’ is so much part of everyday experience that our language generalizes the idea to the relations between individuals and nations. To manage and optimize this fact of life has always been an engineering problem, but the present is an age when scientists often choose problems for their complexity and relevance—and what could be more relevant than friction? Careful economic studies estimate that an annual sum of money equivalent to 1% of the GNP (gross national product) could be saved if use were made of known methods to reduce friction and wear. With advances in understanding one should do better than this. The relevance is not just to machinery, though machinery is the classical justification to study friction. Can we justify with honest scientific answers why we lubricate our eyes with tears—the artificial tears containing hydrosoluble polymers? Why joggers’ knees? Why artificial hips and other biomedical implants? Neither our machinery nor our bodies could operate unless polymers—synthetic in some of these examples, biological in others—mediated what happens. These problems of ‘‘low-pressure’’ friction become especially urgent not only in biological contexts but also in the burgeoning fields of MEMS and microfluidics. The point of this essay is to highlight the related opportunities to do fundamental and interesting science. In fact, objects in sliding contact present among the best model systems to study nonequilibrium polymer physics of which one can conceive. It is the system that matters. Numerous conjoint variables come into simultaneous play: polymer, shear rate, and solid surface, to which polymers adhere or not as they flow. The thermodynamic variables (temperature, pressure) are inhomogeneous—in time and in spatial position within the sample. Pressure is inherently spatially inhomogeneous when surfaces are squeezed together. Temperature is inherently inhomogeneous because the local viscous dissipation, and the local sliding-induced activation of potential chemical reactions, depends on the local pressure at each given point within the sliding system. Thus, a sliding contact presents a convenient microcosm in which, to study systems driven far from equilibrium. The friction problem is ‘‘driven physics’’—a problem distinctly different from the extensive mainstream focus on equilibrium properties in polymer science, those properties susceptible to analysis by statistical thermodynamics. The small thickness and very high shear rate when solids are in sliding contact make this problem attractively amenable to molecular dynamics simulation. Polymers are used ubiquitously to control friction and wear but with insufficient recognition, on the engineering side, that their physical responses differ from those of small molecules. In the lubrication of magnetic disks, the lubricant is a polymer—but the literature on this subject is unclear why perfluoropolyethers are more effective than competing polymers. In the regulation of the viscosity of engine oils, polymers are used too. Classically, when dealing with metal-based machinery, the maximum pressure (load divided by area of actual contact between adjoining rough surfaces) is on the order of GPa, yet in softer contexts, involving polymers, it is 2–3 orders of magnitude less. Correspondence to: S. Granick (E-mail: sgranick@uiuc. edu)