We propose a new scheme to parametrize effective pair potentials that can be used tosimulate oxide glasses. As input data for the optimization we use the radial distributionfunctions of the liquid and the vibrational density of state of the glass, both obtained fromab initio simulations, as well as experimental data on the pressure and/or composition de-pendence of the density and the elastic moduli of the glass [1].For the case of silica we find that this new scheme allows to find potentials that are signifi-cantly accurate than previous ones even if the functional form is the same, thus demonstratingthat even simple two-body potentials can be superior to more complex three-body poten-tials. We have tested the new potential by calculating the pressure dependence of the elasticmoduli and find a good agreement with the corresponding experimental data.For binary alkali (lithium, sodium, potassium) silicate glasses, the new potentials allowto reproduce the composition dependence of both density and elastic moduli. Further, weexamine the capabilities of these potentials for studying ternary compositions containing twoalkali oxides, and we find that they are reliable even if they have been developed for binarycompositions.Simplicity of the functional form also makes these potentials computationally more efficientthan potentials with more complex functional forms, and hence more suitable for simulationsinvolving large length and/or time scales scales.S. Sundararaman, L. Huang, S. Ispas, and W. Kob, ”New optimization scheme to obtaininteraction potentials for oxide glasses”, submitted (2018)

Dense active materials

Lectures on entropy

By combining rheology, light scattering, small angle X-ray scattering and simulations we explore the glassy dynamics of soft colloids using microgels and charged particles interacting by steric and screened Coulomb interactions, respectively. In the supercooled regime, the structural relaxation time (SRT) of both systems grows steeply with volume fraction, reminiscent of the behavior of colloidal hard sphere systems [1]. This suggests that softness has no impact on the growth of the SRT on approaching the glass transition, as confirmed by computer simulations. We show that the onset of a finite yield-stress, occurring within the supercooled regime, systematically corresponds to a minimum of the stretching exponent characterizing the decay of the two-time intensity autocorrelation function.Softness becomes relevant only at very large packing fractions, when the system falls out of equilibrium. In this non-equilibrium regime, the SRT depends surprisingly weakly on packing fraction and time correlation functions exhibit compressed exponential decays consistent with internal stress-driven relaxation. The transition to this novel regime coincides with the onset of an anomalous decrease of local order with increasing density, reminiscent of the reentrant glass transition predicted theoretically in ultrasoft systems [2,3], and corresponds to a weakening of the volume fraction dependence of the plateau modulus of the suspensions. We propose that these peculiar dynamics result from a competition between the non-equilibrium aging dynamics of the glassy state and the tendency of soft systems to refluidize at high packing fractions.

In our work we report the measurement of non-equilibrium interfacial tension of polymer and hard sphere suspensions in contact with their own solvent. By visualizing fingering instability (VF) in radial Hele-Shaw geometry, appearing when the solvent displaces suspensions of colloids or polymers, we measure interfacial tensions in function of the volume fraction of the suspended objects, showing that the internal degrees of freedom of the particles drive the low volume fraction behavior (Figure 1). Our results support the existence of a positive tension between miscible fluids, confirm the quadratic scaling predicted by Korteweg [4] for long linear and crosslinked polymers and show a positive rapidly growing tension for hard sphere suspensions up to maximum packing, whose description necessitates a theoretical framework going beyond the classic square gradient model. We rationalize our findings assuming the suspension/solvent interface in local thermodynamic equilibrium, computing explicitly the square gradient contribution to the interfacial tension for polymer/solvent and simple molecular liquid mixtures and proposing a phenomenological model capturing the compositional dependence of the interfacial tension for large concentration gradients. Finally we include and analyze data reported in literature and obtained via spinning drop tensiometry that validate the model and we propose the analysis of fluid dynamic instability as a new tool to probe interfacial stresses.