Mechanochemical reactions occur by an applied force modifying and ideally accelerating the rate of reaction of a mechanically active species, a mechanophore. Thermal reactions are described by the steepest descent pathway (SDP) of the potential energy surface (PES) from the transition state to the reactant state, which are stationary points on the SDP. The activation energy is calculated from the energy difference between these two points. The PES is modified by an imposed force to yield a reaction pathway given by the force-displaced stationary points (FDSPs), which depend on the magnitude and direction of the force and shape of the PES. However, the SDP has zero force in a direction perpendicular to it so that the PES can be visualized as a “harmonic valley” that forms a potential energy trough about the SDP. If the walls of the potential trough are sufficiently steep, a mechanochemical reaction should be constrained to occur along the SDP, and the influence of an applied force should depend only on the component of the force along it. If this is the case, it should be possible to use just the shape of the PES around the initial and transition states to calculate the effect of an imposed force or stress on mechanochemical reaction rates. The postulate is tested for the mechanically induced decomposition of an adsorbed methyl thiolate species on a Cu(100) single-crystal surface by measuring the azimuthal angular dependence of the mechanochemical methyl thiolate decomposition rate by varying the sliding direction of a sharp atomic force microscope tip over the surface in ultrahigh vacuum. The concept is also illustrated using a model 4-fold potential using a Remoissenet–Peyrard function to mimic the potential of a Cu(100) surface. This yields an angular dependence that agrees well with the prediction from the above postulate. This simplification will facilitate the analysis of mechanochemical rates of both surface and bulk reactions.

This paper reports the stochastic resonance (SR) phenomenon with memory effects for a Brownian particle in a potential whose shape is subjected to deformation. We model the deformation in the system by the Remoissenet–Peyrard potential and the memory effects by the time-delayed feedback. The question of the possible influence of time-delayed feedback on the occurrence of SR is then of our interest. We examine numerically the effect of feedback strength as well as time delay on SR phenomenon in terms of hysteresis loop area. It is found that time-delayed feedback has a significant effect on SR and can induce double resonances in the system. We show that the properties of SR are varying, depending on interdependence between feedback strength, time delay and shape parameter. This article is part of the theme issue ‘Vibrational and stochastic resonance in driven nonlinear systems (part 1)’.

Abstract:Computer simulations of friction between polymer brushes are usually simplified compared to real systems in terms of solvents and geometry. In most simulations, the solvent is only implicit with infinite compressibility and zero inertia. In addition, the model geometries are parallel walls rather than curved or rough as in reality. In this work, we study the effects of these approximations and more generally the relevance of solvation on dissipation in polymer-brush systems by comparing simulations based on different solvation schemes. We find that the rate dependence of the energy loss during the collision of brush-bearing asperities can be different for explicit and implicit solvent. Moreover, the non-Newtonian rate dependences differ noticeably between normal and transverse motion, i.e., between head-on and off-center asperity collisions. Lastly, when the two opposing brushes are made immiscible, the friction is dramatically reduced compared to an undersaturated miscible polymer-brush system, irrespective of the sliding direction.

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