We study the structural and dynamical characteristics of the sodium atoms inside and outside the "diffusion channels" in glassy Na2O-4SiO2 (NS4) using classical molecular dynamics. We show that on average neither energetic arguments nor local environment considerations can explain the increased density of sodium atoms inside the subspace made of the channels. Nevertheless we show that at low temperature the mean square displacement of the sodium atoms inside this subspace is significantly larger than the one of the atoms outside the channels.PACS numbers: 61.20. Ja, 61.43.Fs, 66.30.Hs The diffusion of alkali atoms in silicate glasses is an important matter under investigation since several years. In particular the properties of sodium atoms inside the amorphous tetrahedral network of silica have been the topic of both experimental work [1] and moleculardynamics simulations (MD) [2] since a simple glass like Na 2 O-4SiO 2 (NS4) can be used as a prototype for more complicated glasses. The question of how the alkali atoms diffuse inside the glassy network is still a matter of debate and a generally accepted theory of ion transport in glasses is still missing [3,4]. In a previous MD study of NS4 we have shown that the ions follow preferential pathways ("channels") inside the glassy matrix [5]. Nevertheless contrarily to the popular idea proposed by Greaves [6] and developed in further studies [7], these channels are neither static nor due to a microsegregation of the sodium atoms but have to be seen dynamically in the sense that the channels are those regions of space in which a great number of sodiums have passed during a given simulation time. The existence of these channels gives rise to a pre-peak in the structure factor at around q = 0.95Å −1 seen both in experiments [8] and classical MD simulations [9]. In this last paper Horbach et al. show that the slow dynamics of the sodium atoms is closely related to the one of the underlying silica matrix which is coherent with the fact that there exists a strong correlation between the channels and the location of the non-bridging oxygen atoms, as shown in previous MD studies [10,11]. Once the existence of the channels is established, the next step is naturally to analyze their characteristics and to determine why the sodium atoms take these preferential pathways. This is the aim of the present study. In that direction we analyze the potential energy and the local structure of the sodium atoms whether they are INside the channels (Na In ) or OUTside the channels (Na Out ). It is indeed generally believed that the diffusion of the ions occurs via "hopping motions between well-defined potential minima" [4] which we should be able to detect in our simulations. We analyze also the time evolution of the sodium densities inside and outside the channels which permits us to detect the dynamics of the channels as a function of temperature and to quantify the differences between the sodium densities, differences that so far have only been suggested [9]. To elucidate the diffu...

We study the structure and dynamics of a transient network composed of droplets of microemulsion connected by telechelic polymers. The polymer induces a bridging attraction between droplets without changing their shape. A viscoelastic behaviour is induced in the initially liquid solution, characterised in the linear regime by a stretched exponential stress relaxation. We analyse this relaxation in the light of classical theories of transient networks. The role of the elastic reorganisations in the deformed network is emphasized. In the non linear regime, a fast relaxation dynamics is followed by a second one having the same rate as in the linear regime. This behaviour, under step strain experiments, should induce a non monotonic behaviour in the elastic component of the stress under constant shear rate. However, we obtain in this case a singularity in the flow curve very different from the one observed in other systems, that we interpret in terms of fracture behaviour.

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To find the origin of the diffusion channels observed in sodium-silicate glasses, we have performed classical molecular dynamics simulations of Na2O-4SiO2 during which the mass of the Si and O atoms has been multiplied by a tuning coefficient. We observe that the channels disappear and that the diffusive motion of the sodium atoms vanishes if this coefficient is larger than a threshold value. Above this threshold the vibrational states of the matrix are not compatible with those of the sodium ions. We interpret hence the decrease of the diffusion by the absence of resonance conditions. PACS numbers: 61.43. Fs, 61.43.Bn, 66.30.Hs, 63.50.+x The mechanism of ionic transport in amorphous materials [1,2] at the atomic scale is not completely elucidated so far and therefore it is the subject of numerous experimental [3,4] and numerical [5][6][7][8][9] studies. Among the most studied systems are the sodium-silicate glasses since they contain the essential ingredients, namely the amorphous matrix (silica) and the mobile ions (sodium), as a first step in the simulation of more complex glasses of higher practical interest.In previous studies [10-12], we have shown by means of classical molecular dynamics simulations of Na 2 O-4SiO 2 (NS4), that the sodium atoms diffuse through a well connected network of pockets (which represents only a limited fraction of the entire available space) that we have called "channels" to be coherent with the literature. The existence of the channels, which are not due to microsegregation effects [6,7], has been confirmed by the existence of a pre-peak in the partial Na-Na structure factor at a wave-vector q = 0.95Å −1 [13]. This pre-peak has also been observed experimentally [14] and numerically [15] in another study. We have also shown that the location of the channels is strongly correlated to the positions of the non-bridging oxygens [13] and Horbach et al. have shown that the sodium dynamics should be related to that of the underlying silica network [15]. This suggests that the origin of the channels could be related to the dynamical properties of the matrix. To check this idea we present in this letter classical molecular dynamics simulations on a series of "toy" systems in which the atomic masses of both the oxygen and silicon atoms have been systematically changed after artificially multiplying their experimental values by a common factor µ varying from 0.5 to infinity, the usual NS4 system being recovered for µ = 1.By studying both the mean square displacement of the sodium atoms and the characteristics of the channels, we find a change in the sodium diffusion properties when the parameter µ is increased above a value of about 30. Above this threshold, the sodium diffusion decreases and the channels can no more be clearly defined. Guided by the concomitant change in the short time characteristics of the velocity autocorrelation function, we have calculated the vibrational density of states (VDOS) for both the sodium atoms and the atoms of the matrix for various values of µ. We obse...