The Molecular Origins of Mechanical Processes: From Reactions to Glass Formation
DIPC Seminars
- Speaker
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Wilfred Tysoe
University of Wisconsin-Milwaukee - When
-
2025/07/22
12:00 - Place
- DIPC Josebe Olarra Seminar Room
- Host
- Bo Chen
- Add to calendar
-
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A common thread connecting disparate stress-induced processes, from friction, to fluid viscosity to mechanochemistry is that the activation barrier for the transition from one state of the system to another to another is modified by the external force [1]. Analyses of these mechanically induced processes relies on understanding how applied stresses modify the potential energy surface (PES) to influence the transit rate from one local energy minimum to another over an activation energy barrier. This rate can be conveniently calculated for activated (chemical) processes by using transition-state theory, which makes the simplification that the transition state and initial state are in thermal equilibrium. The unstable transition-state structure decomposes at some characteristic vibrational frequency to form the reaction products to yield a reaction rate constant that is proportional to exp(-Eact/RT), where Eact, is the activation energy, R is the gas constant, and T is the absolute temperature and thus follows the Arrhenius law. Thus, mechanically modified processes operate by the applied normal and shear stresses modifying the shape of the PES, and can therefore, in principle, be combined with other methods for modifying chemical reactivity such as light, electric or by adding a catalyst. Evans and Polanyi demonstrated that transition-state theory could be modified to calculate how the rate of a process can be modified by some stress, σ, by a factor exp((σ∙∆V‡)/RT), where ∆V‡ is known as an activation volume [2]. The underlying theory is tested using model systems comprising surface adsorbates in ultrahigh vacuum where the mechanochemical reaction is induced by an atomic force microscope tip, and for hydrostatic-pressure-induced Diels-Alder reactions in solution. Finally, the method is extended to analysing processes occurring under combined normal and shear stresses for sliding friction, chemical reactions[3, 4], and viscosity[5].
References:
[1] Spikes, H., Tysoe, W. On the Commonality Between Theoretical Models for Fluid and Solid Friction, Wear and Tribochemistry. Tribology Letters 59:1-14 (2015).
[2] Evans, M.G., Polanyi, M. Some applications of the transition state method to the calculation of reaction velocities, especially in solution. Transactions of the
Faraday Society 31:875-894 (1935).
[3] Hopper, N., Sidoroff, F., Cayer-Barrioz, J., Mazuyer, D., Tysoe, W.T. A Molecular-Scale Analysis of Pressure-Dependent Sliding Shear Stresses. Tribology Letters 71:121 (2023).
[4] Rana, R., Hopper, N., Sidoroff, F., Cayer-Barrioz, J., Mazuyer, D., Tysoe, W.T. An Analysis of Shear-Dependent Mechanochemical Reaction Kinetics. Tribology Letters 72:76 (2024).
[5] Hopper, N., Espinosa-Marzal, R.M., Tysoe, W. On the pressure dependence of viscosity, especially for fluids that have a tendency to form glasses. The Journal of Chemical Physics 161:214502 (2024).