Revisiting the slow dynamics of a silica melt using Monte Carlo simulations

We implement a standard Monte Carlo algorithm to study the slow, equilibrium dynamics of a silica melt in a wide temperature regime, from 6100 K down to 2750 K. We find that the average dynamical behaviour of the system is in quantitative agreement with results obtained from molecular dynamics simulations, at least in the long-time regime corresponding to the alpha-relaxation. By contrast, the strong thermal vibrations related to the Boson peak present at short times in molecular dynamics are efficiently suppressed by the Monte Carlo algorithm. This allows us to reconsider silica dynamics in the context of mode-coupling theory, because several shortcomings of the theory were previously attributed to thermal vibrations. A mode-coupling theory analysis of our data is qualitatively correct, but quantitative tests of the theory fail, raising doubts about the very existence of an avoided singularity in this system. We discuss the emergence of dynamic heterogeneity and report detailed measurements of a decoupling between translational diffusion and structural relaxation, and of a growing four-point dynamic susceptibility. Dynamic heterogeneity appears to be less pronounced than in more fragile glass-forming models, but not of a qualitatively different nature.Comment: 13 pages, 10 figures; to be published in Phys. Rev.

Isomerization of azobenzene and the enhancement of dynamic heterogeneities in molecular glass formers

Prompted by recent findings [Teboul, Saiddine, and Nunzi, Phys. Rev. Lett. 103, 265701 (2009); Orsi et al., Phys. Rev. E 82, 031804 (2010)] that the isomerization of few azobenzene molecules dispersed in a glass former greatly enhances the dynamic heterogeneity (DH) of the medium, we raise the issue as to whether the isomerization process gives rise to additional DHs or whether instead it stimulates the mechanisms at the origin of the thermal DHs, accelerating them in time. To this end, molecular dynamics simulations are made to study the much insightful four-point susceptibility, dynamic facilitation, and Van Hove correlation functions both when the isomerization is activated and when it is artificially switched off. Our results do not rule out any of the two scenarios as a possible cause for the enhancement of DHs upon switching on the isomerization process, but clearly show that the second one is by far the dominant mechanism in the dynamics of the supercooled liquid

Isomerization-induced surface relief gratings formation: A comparison between the probe and the matrix dynamics

We report molecular dynamics simulations of the effect of the photoisomerization of probe molecules on the nonequilibrium dynamics of a bulk amorphous matrix. Is it the matrix or the probe that drives the dynamics in SRG formation? In the first picture, the probe isomerization induces the motion of the probe inside the matrix. The motion of the probe then induces molecular motions inside the matrix. In the second picture, the probe isomerization induces a modification of the matrix diffusion mechanism. The diffusion of the matrix then induces the motion of the embedded probe. To answer this question, we compare the motion of the probe molecules and the motion of the matrix molecules in various thermodynamic conditions. We show that when the isomerization is switched on, the matrix molecules surrounding the probe move faster than the probe. Around the probe, the structural relaxation time of the matrix molecules is shorter than the probe relaxation time and the diffusion of the matrix molecules is larger than the probe diffusion. These results show that the matrix motions drive the dynamics

Coarse grain modeling of liquid methyl methacrylate

We implement a coarse graining procedure in order to construct a simple intermolecular potential model for liquid (poly)methyl methacrylate (P)MMA. The procedure and the potential model obtained thereby aim at building an effective input towards accelerated molecular-dynamics calculations as well as a simpler statistics for oncoming simulations of MMA and PMMA melts. As a result, atomistic description of the molecule is substantially simplified while preserving as many properties of the original substance as possible. The hard core of the approach consists in optimizing iteratively a typical Lennard-Jones (6-12) potential until the radial distribution function generated from the coarse grained model becomes consistent with the atomistic target function. The new model allows one to make an economy by one order of magnitude in the CPU time

Simulation of isomerization-induced motions in a blend of different PMMA melts doped with DR1 chromophores

Date du colloque&nbsp;: 01/2010</p

The microscopic structure of cold aqueous methanol mixtures

The evolution of the micro-segregated structure of aqueous methanol mixtures, in the temperature range 300 K-120 K, is studied with computer simulations, from the static structural point of view. The structural heterogeneity of water is reinforced at lower temperatures, as witnessed by a pre-peak in the oxygen-oxygen structure factor. Water tends to form predominantly chain-like clusters at lower temperatures and smaller concentrations. Methanol domains have essentially the same chain-like cluster structure as the pure liquid at high concentrations and becomes mono- meric at smaller ones. Concentration uctuations decrease with temperature, leading to quasi-ideal Kirkwood-Bu integrals, despite the enhanced molecular interactions, which we interpret as the signature of non-interacting segregated water and methanol clusters. This study throws a new light on the nature of the micro-heterogeneous structure of this mixture: the domain segregation is essentially based on the appearance of linear water clusters, unlike other alcohol aqueous mixtures, such as with propanol or butanol, where the water domains are more bulky.