Solomon Tesfu, Solomon Michael J
Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA.
J Chem Phys. 2006 Apr 7;124(13):134905. doi: 10.1063/1.2178784.
We report measurements of the spatial distribution of stacking faults in colloidal crystals formed by means of an oscillatory shear field at a particle volume fraction of 52% in a system where the pair potential interactions are mildly repulsive. Stacking faults are directly visualized via confocal laser scanning microscopy. Consistent with previous scattering studies, shear orders the initially amorphous colloids into close-packed planes parallel to the shearing surface. Upon increasing the strain amplitude, the close-packed direction of the (111) crystal plane shifts from an orientation parallel to the vorticity direction to parallel the flow direction. The quality of the layer ordering, as characterized by the mean stacking parameter, decreases with strain amplitude. In addition, we directly observe the three-dimensional structure of stacking faults in sheared crystals. We observe and quantify spatial heterogeneity in the stacking fault arrangement in both the flow-vorticity plane and the gradient direction, particularly at high strain amplitudes (gamma> or =3). At these conditions, layer ordering persists in the flow-vorticity plane only over scales of approximately 5-10 particle diameters. This heterogeneity is one component of the random layer ordering deduced from previous scattering studies. In addition, in the gradient direction, the stacking registry shows that crystals with intermediate global mean stacking probability are comprised of short sequences of face-centered cubic and hexagonal close-packed layers with a stacking that includes a component that is nonrandom and alternating in character.
我们报告了在一个粒子体积分数为52%、对势相互作用为轻度排斥的系统中，通过振荡剪切场形成的胶体晶体中堆垛层错空间分布的测量结果。堆垛层错通过共聚焦激光扫描显微镜直接可视化。与之前的散射研究一致，剪切将最初无定形的胶体排列成平行于剪切表面的密堆积平面。随着应变幅度的增加，（111）晶面的密堆积方向从平行于涡度方向转变为平行于流动方向。以平均堆垛参数表征的层序质量随应变幅度降低。此外，我们直接观察了剪切晶体中堆垛层错的三维结构。我们观察并量化了流动-涡度平面和梯度方向上堆垛层错排列的空间异质性，特别是在高应变幅度（γ≥3）时。在这些条件下，层序仅在流动-涡度平面上在约5-10个粒子直径的尺度上持续存在。这种异质性是先前散射研究中推断出的随机层序的一个组成部分。此外，在梯度方向上，堆垛配准显示，具有中等全局平均堆垛概率的晶体由面心立方和六方密堆积层的短序列组成，其堆垛包括一个非随机且交替的成分。
