Cryptographie et Signatures Eléctoniques

Article for general public (2006)

Crystal growth from a supersaturated melt: Relaxation of the solid-liquid dynamic stiffness

in Journal of Chemical Physics (2014), 141

We discuss the growth process of a crystalline phase out of a metastable over-compressed liquid that is brought into contact with a crystalline substrate. The process is modeled by means of molecular ... [more ▼]

We discuss the growth process of a crystalline phase out of a metastable over-compressed liquid that is brought into contact with a crystalline substrate. The process is modeled by means of molecular dynamics. The particles interact via the Lennard-Jones potential and their motion is locally thermalized by Langevin dynamics. We characterize the relaxation process of the solid-liquid interface, showing that the growth speed is maximal for liquid densities above the solid coexistence density, and that the structural properties of the interface rapidly converge to equilibrium-like properties. In particular, we show that the off-equilibrium dynamic stiffness can be extracted using capillary wave theory arguments, even if the growth front moves fast compared to the typical diffusion time of the compressed liquid, and that the dynamic stiffness converges to the equilibrium stiffness in times much shorter than the diffusion time. [less ▲]

Crystal nucleation mechanism in melts of short polymer chains under quiescent conditions and under shear flow

in Journal of Chemical Physics (2014), (141), 124910

We present a molecular dynamics simulation study of crystal nucleation from undercooled melts of n-alkanes, and we identify the molecular mechanism of homogeneous crystal nucleation under quiescent ... [more ▼]

We present a molecular dynamics simulation study of crystal nucleation from undercooled melts of n-alkanes, and we identify the molecular mechanism of homogeneous crystal nucleation under quiescent conditions and under shear flow. We compare results for n-eicosane (C20) and npentacontahectane (C150), i.e., one system below the entanglement length and one above, at 20%– 30% undercooling. Under quiescent conditions, we observe that entanglement does not have an effect on the nucleation mechanism. For both chain lengths, the chains first align and then straighten locally, then the local density increases and finally positional ordering sets in. At low shear rates the nucleation mechanism is the same as under quiescent conditions, while at high shear rates the chains align and straighten at the same time. We report on the effects of shear rate and temperature on the nucleation rates and estimate the critical shear rates, beyond which the nucleation rates increase with the shear rate. In agreement with previous experimental observation and theoretical work, we find that the critical shear rate corresponds to a Weissenberg number of order 1. Finally, we show that the viscosity of the system is not affected by the crystalline nuclei. [less ▲]

Crystallization Close to the Glass Transition: Dynamic heterogeneities do not precede crystallization

Presentation (2013, July)

We address the question whether a crystallization event can be predicted based on observations of the mobility distribution in a supersaturated melt. We have carried out computer simulations of ... [more ▼]

We address the question whether a crystallization event can be predicted based on observations of the mobility distribution in a supersaturated melt. We have carried out computer simulations of overcompressed suspensions of hard monodisperse ellipsoids and observed their crystallization dynamics. The system was compressed very rapidly in order to reach the regime of slow, glass-like dynamics. We nd that, although particle dynamics become sub-diffusive and the intermediate scattering function clearly develops a shoulder, crystallization proceeds via the usual scenario: nucleation and growth for small supersaturations, spinodal decomposition for large supersaturations. In particular, we compared the mobility of the particles in those regions where crystallization set in with the mobility in the rest of the system. We did not find any signature in the dynamics that pointed towards the imminent crystallization event. [less ▲]

Crystallization in colloidal suspensions

Scientific Conference (2013)

Crystallization in colloidal suspensions

Presentation (2014, November)

Crystallization in colloidal suspensions

Scientific Conference (2014, December)

Crystallization in colloidal suspensions

Presentation (2014, November)

Crystallization in glassy suspensions of hard ellipsoids

in Journal of Chemical Physics (2013), 139(12),

We have carried out computer simulations of overcompressed suspensions of hard monodisperse ellipsoids and observed their crystallization dynamics. The system was compressed very rapidly in order to reach ... [more ▼]

We have carried out computer simulations of overcompressed suspensions of hard monodisperse ellipsoids and observed their crystallization dynamics. The system was compressed very rapidly in order to reach the regime of slow, glass-like dynamics. We find that, although particle dynamics become sub-diffusive and the intermediate scattering function clearly develops a shoulder, crystallization proceeds via the usual scenario: nucleation and growth for small supersaturations, spinodal decomposition for large supersaturations. In particular, we compared the mobility of the particles in the regions where crystallization set in with the mobility in the rest of the system. We did not find any signature in the dynamics of the melt that pointed towards the imminent crystallization events. [less ▲]

Crystallization in hard spheres

Presentation (2011)