Mechanism of Shear-Induced Alignment in Bilayer Thin Films of Spherical Particles

The phenomenon of shear-induced alignment in polycrystalline bilayer systems is analyzed using Brownian dynamics simulations. We show that the alignment process can be explained in terms of a local grain boundary migration mechanism. Both microscopic and continuum models are constructed that predict the boundary migration velocity as a function of local structure and shear stress. In the continuum picture, shear-induced potential energy gradients act as the driving force for migration, and the grain boundary mobility is sensitive to the degree of misorientation between the grains.

Figure 1

Snapshots of a confined bilayer (only top layer shown) composed of 20 000 particles (a) before and (b) after the application of simple shear. In white regions, one triangular lattice vector is aligned with the shear (horizontal) direction, red (grey) regions are misaligned, and dislocations are indicated by particle pairs that have 5 (green/white) or 7 (blue/black) neighbors, respectively. The initial configuration extends through both layers.

Figure 2

The order parameter $⟨\cos (6\theta )⟩$ (dashed red curve) and the fraction $f_{\mathrm{ord}}$ of the lattice that is aligned (solid black curve) with the shear direction as a function of time. Also shown is a fit to the Avrami growth law (long-dashed blue curve). Inset: $⟨\cos (6\theta )⟩$ (◊) and $f_{\mathrm{ord}}$ (▵) as a function of dislocation density (solid line, linear fit).

Figure 3

Evolution of a two-grain system (20 000 particles) at 4 different times (only part shown). Colors as in Fig. 1.

Figure 4

(a) Mean grain boundary velocity (normalized by $\alpha$) plotted against $v_x$ for four different misorientation angles [30° (□, red), 25° (◊, purple), 20° (▵, green), and 15° (☆, blue)] and different flow velocities. (b) Mean grain boundary velocity as a function of the potential energy jump between the two grains for the same systems as in (a). (c) Linear slopes (mobilities) from the data in (a) (□) and (b) (◊) as a function of $\alpha$.