Induced cooperative motions in a medium driven at the nanoscale: Searching for an optimum excitation period

Abstract

Recent results have shown the appearance of induced cooperative motions called dynamic heterogeneity during the isomerization of diluted azobenzene molecules in a host glass-former. In this paper we raise the issue of the coupling between these "artificial" heterogeneities and the isomerization period. How do these induced heterogeneities differ in the saturation regime and in the linear response regime ? Is there a maximum of the heterogeneous motion versus isomerization rate and why ? Are the heterogeneity evolution with the isomerization rate connected with the diffusion or relaxation time evolution ? We use out of equilibrium molecular dynamics simulations to answer these questions. We find that the heterogeneity increases in the linear response regime for large isomerization periods and small perturbations. In contrast the heterogeneity decreases in the saturation regime, i.e. when the isomerization half-period (τp/2) is smaller than the relaxation time of the material (τα). This result makes possible a test of the effect of cooperative motions on the dynamics using the chromophores as Maxwell demons that destroy or stimulate the cooperative motions. Because the heterogeneities increase in the linear regime and then decrease in the saturation regime, we find a maximum for τp/2 ≈ τα. The induced excitations concentration follows a power law evolution versus the isomerization rate and then saturates. As a consequence the α relaxation time is related to the excitation concentration with a power law, a result in qualitative agreement with recent findings in constrained models. This result supports a common origin for the heterogeneities with constrained models and a similar relation to the excitation concentration. Published in Phys. Rev. E 89, 012303 (2014).

The microscopic structure of cold aqueous methanol mixtures

“…37,38,55 We did not investigate here the dynamical properties related to relaxations, such as dynamical structure factors 27,28 or non-Gaussian parameters, 72,71 for example, which are a modern measure of glassiness. 27,29 We postpone such studies for future investigations.…”

“…62,64,65 This situation happens equally in computer simulations. 29 The computation of the structure factors and Kirkwood-Buff integrals through correlation functions evaluated in computer simulation is a topic we have addressed in previous papers (see, for example, Ref. 7), which we briefly recall here.…”

“…It is now well known that low temperature liquids, in the supercooled phase and beyond in the glass phase, are characterised by dynamical heterogeneity. [24][25][26][27][28][29][30] This type of heterogeneity is characterised by the existence of spatial domains where molecules of high mobility are aggregated. It is therefore interesting to see how the existence of microheterogeneity correlates with the concomitant existence of dynamical heterogeneity, specially in a mixture where species have different mobilities.…”

Stimuli Thresholds for Isomerization-Induced Molecular Motions in Azobenzene-Containing Materials

“…Consequently we find a linear increase of the induced diffusion with the chromophore's concentration. A detailed description of the simulation procedure can be found in previous works 39,40 . The main difference is that, taking into account the universality of the effect 1 , in this paper we simplify the host molecules as much as possible to better understand the effects of the isomerization on the medium.…”

“…We use the Gear algorithm with the quaternion method 32 to solve the equations of motions with a time step ∆t = 10 −15 s. The temperature is controlled using a Berendsen thermostat 43 . We model the isomerization as a uniform closing and opening of the probe molecule shape 16,17,39,40 during a characteristic time t 0 = 1ps. The period of the isomerization cis − trans and then trans − cis is also fixed in the study τ p = 1ns.…”