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We introduce a new theoretical model of simple fluid, whose interactions, defined in terms of the Voronoi cells of the configurations, are local and many-body. The resulting system is studied both theoretically and numerically. We show that the fluid, though sharing the global features of other models of fluids with soft interactions, has several unusual characteristics, which are investigated and discussed., Comment: 19 pages, 9 figures

We revisit the relation between the shear stress relaxation modulus $G(t)$, computed at finite shear strain $0 < \gamma \ll 1$, and the shear stress autocorrelation functions $C(t)|_{\gamma}$ and $C(t)|_{\tau}$ computed, respectively, at imposed strain $\gamma$ and mean stress $\tau$. Focusing on permanent isotropic spring networks it is shown theoretically and computationally that in general $G(t) = C(t)|_{\tau} = C(t)|_{\gamma} + G_{eq}$ for $t > 0$ with $G_{eq}$ being the static equilibrium shear modulus. $G(t)$ and $C(t)|_{\gamma}$ thus must become different for solids and it is impossible to obtain $G_{eq}$ alone from $C(t)|_{\gamma}$ as often assumed. We comment briefly on self-assembled transient networks where $G_{eq}(f)$ must vanish for a finite scission-recombination frequency $f$. We argue that $G(t) = C(t)|_{\tau} = C(t)|_{\gamma}$ should reveal an intermediate plateau set by the shear modulus $G_{eq}(f=0)$ of the quenched network., Comment: 8 pages, 4 figures

We present results of molecular dynamics (MD) simulations for a non-entangled polymer melt confined between two completely smooth and repulsive walls, interacting with inner particles via the potential $U_{\rm wall}\myeq (\sigma/z)^9$, where $z \myeq |z_{\rm particle}-z_{\rm wall}|$ and $\sigma$ is (roughly) the monomer diameter. The influence of this confinement on the dynamic behavior of the melt is studied for various film thicknesses (wall-to-wall separations) $D$, ranging from about 3 to about 14 times the bulk radius of gyration. A comparison of the mean-square displacements in the film and in the bulk shows an acceleration of the dynamics due to the presence of the walls. %Consistent with this result is the observation that, within %the film, regions closer to the walls have higher mobility than those further %away towards the film center. This leads to a reduction of the critical temperature, $T_{\rm{c}}$, of mode-coupling theory with decreasing film thickness. Analyzing the same data by the Vogel-Fulcher-Tammann equation, we also estimate the VFT-temperature $T_{\rm{0}}(D)$. The ratio $T_{\mr{0}}(D)/T^{\mr{bulk}}_{\mr{0}}$ decreases for smaller $D$ similarly to $T_{\mr{c}}(D)/T^{\mr{bulk}}_{\mr{c}}$. These results are in qualitative agreement with that of the glass transition temperature observed in some experiments on supported polymer films.