A 2D Front-Tracking Lagrangian Model for the Modeling of Anisotropic Grain Growth

Grain growth is a well-known and complex phenomenon occurring during annealing of all polycrystalline materials. Its numerical modeling is a complex task when anisotropy sources such as grain orientation and grain boundary inclination have to be taken into account. This article presents the application of a front-tracking methodology to the context of anisotropic grain boundary motion at the mesoscopic scale. The new formulation of boundary migration can take into account any source of anisotropy both at grain boundaries as well as at multiple junctions (MJs) (intersection point of three or more grain boundaries). Special attention is given to the decomposition of high-order MJs for which an algorithm is proposed based on local grain boundary energy minimisation. Numerical tests are provided using highly heterogeneous configurations, and comparisons with a recently developed Finite-Element Level-Set (FE-LS) approach are given. Finally, the computational performance of the model will be studied comparing the CPU-times obtained with the same model but in an isotropic context.

Biaxial shear of confined colloidal hard spheres: the structure and rheology of the vorticity-aligned string phase

Using a novel biaxial confocal rheoscope, we investigate the flow of the shear induced vorticity aligned string phase [X. Cheng et al., Proc. Natl. Acad. Sci. U. S. A., 2011, 109, 63], which has a highly anisotropic microstructure. Using biaxial shear protocols we show that we have excellent control of the string phase anisotropic morphology. We choose a shear protocol that drives the system into the string phase. Subsequently, a biaxial force measurement device is used to determine the suspension rheology along both the flow and vorticity directions. We find no measurable dependence of the suspension stress response along the shear and vorticity directions due to the hydrodynamically induced string morphology. In particular, we find that the suspension's high frequency stress response is nearly identical along the two orthogonal directions. While we do observe an anisotropic stress response at lower shear frequencies associated with shear thinning, we show that this anisotropy is independent of the shear induced string structure. These results suggest that for the range of flows explored, Brownian and hydrodynamic contributions to the stress arising from the anisotropic suspension microstructure are sufficiently weak that they do not significantly contribute to the rheology. Collectively, this study presents a general and powerful approach for using biaxial confocal rheometry to elucidate the relationship between microstructure and rheology in complex fluids driven far-from-equilibrium.