0000000000020955
AUTHOR
Thomas Voigtmann
Residual Stresses in Glasses
The history dependence of the glasses formed from flow-melted steady states by a sudden cessation of the shear rate $\dot\gamma$ is studied in colloidal suspensions, by molecular dynamics simulations, and mode-coupling theory. In an ideal glass, stresses relax only partially, leaving behind a finite persistent residual stress. For intermediate times, relaxation curves scale as a function of $\dot\gamma t$, even though no flow is present. The macroscopic stress evolution is connected to a length scale of residual liquefaction displayed by microscopic mean-squared displacements. The theory describes this history dependence of glasses sharing the same thermodynamic state variables, but differi…
Force-induced diffusion in microrheology
We investigate the force-induced diffusive motion of a tracer particle inside a glass-forming suspension when a strong external force is applied to the probe (active nonlinear microrheology). A schematic model of mode-coupling theory introduced recently is extended to describe the transient dynamics of the probe particle, and used to analyze recent molecular-dynamics simulation data. The model describes non-trivial transient displacements of the probe before a steady-state velocity is reached. The external force also induces diffusive motion in the direction perpendicular to its axis. We address the relation between the transverse diffusion coefficient D(perpendicular) and the force-depende…
Structural relaxation in a binary metallic melt: Molecular dynamics computer simulation of undercooledAl80Ni20
Molecular dynamics computer simulations are performed to study structure and structural relaxation in the glassforming metallic alloy ${\text{Al}}_{80}{\text{Ni}}_{20}$. The interactions between the particles are modeled by an effective potential of the embedded atom type. Our model of ${\text{Al}}_{80}{\text{Ni}}_{20}$ exhibits chemical short-range order (CSRO) that is reflected in a broad prepeak around a wave number of $1.8\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$ in the partial static structure factor for the Ni-Ni correlations. The CSRO is due to the preference of Ni atoms to have Al rather than Ni atoms as nearest neighbors. By analyzing incoherent and coherent intermediate scattering…
Slow dynamics in ion-conducting sodium silicate melts: Simulation and mode-coupling theory
A combination of molecular-dynamics (MD) computer simulation and mode-coupling theory (MCT) is used to elucidate the structure-dynamics relation in sodium-silicate melts (NSx) of varying sodium concentration. Using only the partial static structure factors from the MD as an input, MCT reproduces the large separation in relaxation time scales of the sodium and the silicon/oxygen components. This confirms the idea of sodium diffusion channels which are reflected by a prepeak in the static structure factors around 0.95 A^-1, and shows that it is possible to explain the fast sodium-ion dynamics peculiar to these mixtures using a microscopic theory.
From equilibrium to steady state: The transient dynamics of colloidal liquids under shear
We investigate stresses and particle motion during the start up of flow in a colloidal dispersion close to arrest into a glassy state. A combination of molecular dynamics simulation, mode coupling theory and confocal microscopy experiment is used to investigate the origins of the widely observed stress overshoot and (previously not reported) super-diffusive motion in the transient dynamics. A link between the macro-rheological stress versus strain curves and the microscopic particle motion is established. Negative correlations in the transient auto-correlation function of the potential stresses are found responsible for both phenomena, and arise even for homogeneous flows and almost Gaussia…