(245f) Molecular Rotational and Translational Modes - Primary Sources for Energy Dissipation In Frictional Sliding Involving Organic Systems | AIChE

(245f) Molecular Rotational and Translational Modes - Primary Sources for Energy Dissipation In Frictional Sliding Involving Organic Systems



It is commonly assumed for solids that “wearless” frictional dissipation is directly linked to the vibrational modes giving rise to an entropy increase that is noticeable in form of heat. This atomistic or molecular perception is challenged in this presentation. It is argued that for organic systems, possessing other forms of thermally activated modes, i.e., rotational and/or translations modes, the primary source for energy dissipation originates from an entropy decrease not increase, imposed during sliding in the contacted area of the material system. While this phenomenon is known for specific systems that show shear alignment, it has not been recognized as the fundamental mechanism of frictional energy dissipation for organic systems in general. Substantiated by recent studies, we argue that frictional energy dissipation is a mode-coupling process between the one-dimensional sliding process and the thermally active modes in the material.[1-6] Thereby, friction initiated heat generation is considered a secondary mode coupling process, in organic systems that possess thermally activated modes other than vibrations. Consequently, the energy dissipation process in organic systems (in any condensed form) has to be considered a multistep mode-coupling process. If the objective is to lower the frictional energy dissipation then rotational and translational modes have to be considered in the molecular design. If the objective is to lower heat generation, then in addition, coupling between thermal modes have to be addressed. With the inception of the “intrinsic friction analysis” (IFA) methodology,[7] there is a tool that provides the necessary design parameters for molecular engineering, i.e., the enthalpic and entropic energies involved during frictional sliding, to achieve low energy dissipative interfaces.

[1] D.B. Knorr, P. Widjaja, O. Acton, R.M. Overney, Molecular Friction Dissipation and Mode Coupling in Organic Monolayers and Polymer Films, J. Chem. Phys., 134, 104502 (2011).

[2] S. Sills, R.M. Overney, Molecular  Mobility,  Interfacial  Dynamics,  and  Motion  in  Polymeric  Systems, J. Adh. Sci. Techn., 24 2641-2667 (2010).

[3] D.B. Knorr, Jr, X. Zhou, Z. Shi, J. Luo, S.-H. Jang, A. K.-Y. Jen, R.M. Overney, Molecular Mobility in Self-Assembled Dendritic Chromophore Glasses, J. Phys. Chem. B, 113, 14180-14188 (2009).

[4] D.B. Knorr, R.M. Overney, Cooperative and Submolecular Dissipation Mechanisms of Sliding Friction in Complex Organic System, J. Chem. Phys., 129, 074504 (2008).

[5] S. Sills, K. Vorvolakos, M.K. Chaudhury, R.M. Overney, Molecular Origin of Elastomeric Friction in “Nanotribology: Friction and Wear on the Atomic Scale”, E. Gnecco and E. Meyer (eds), Springer-Verlag, Heidelberg, Germany, 659-676 (2007).

[6] R.M. Overney, G. Tindall, J. Frommer, Kinetics and Energetics in Nanolubrication in "Handbook of Nanotechnology", 2nd edition, B. Bhushan (ed.), Springer-Verlag, Heidelberg, Germany, 1439-1453 (2007).

[7] S.E. Sills, R.M. Overney, Creeping friction dynamics and molecular dissipation mechanisms in glassy polymers, Phys. Rev. Lett. 91(9), 095501 (1-4), (2003).

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