Department of Chemical Engineering, University Federico II, Naples, Italy.
Langmuir. 2010 Mar 2;26(5):3016-9. doi: 10.1021/la904775c.
Ordering induced by shear flow can be used to direct the assembly of particles in suspensions. Flow-induced ordering is determined by the balance between a range of forces, such as direct interparticle, Brownian, and hydrodynamic forces. The latter are modified when dealing with viscoelastic rather than Newtonian matrices. In particular, 1D stringlike structures of spherical particles have been observed to form along the flow direction in shear thinning viscoelastic fluids, a phenomenon not observed in Newtonian fluids at similar particle volume fractions. Here we report on the formation of freestanding crystalline patches in planes parallel to the shearing surfaces. The novel microstructure is formed when particles are suspended in viscoelastic, wormlike micellar solutions and only when the applied shear rate exceeds a critical value. In spite of the very low volume fraction (less than 0.01), particles arrange themselves in 2D crystalline patches along the flow direction. This is a bulk phenomenon because 2D crystals form throughout the whole gap between plates, with the gap thickness being much larger than the particle size. Shear flow may hence be an easy method to drive particles into crystalline order in suspensions with viscoelastic properties. The crystalline structure reported here could be used to design new materials with special mechanical, optical, thermal, or electric properties.
剪切流引起的有序排列可用于指导悬浮液中颗粒的组装。流动诱导有序是由一系列力的平衡决定的，如直接颗粒间力、布朗力和流体动力。当处理粘弹性而不是牛顿流体时，后者会发生变化。特别是，在剪切变稀的粘弹性流体中，已经观察到球形颗粒的 1D 线状结构沿流动方向形成，而在类似的颗粒体积分数的牛顿流体中则没有观察到这种现象。在这里，我们报告了在与剪切面平行的平面上形成自由站立的结晶斑块的情况。当颗粒悬浮在粘弹性蠕虫状胶束溶液中，并且只有当施加的剪切速率超过临界值时，才会形成这种新颖的微观结构。尽管体积分数非常低（小于 0.01），但颗粒会沿着流动方向排列成二维结晶斑块。这是一种整体现象，因为二维晶体在整个板之间的整个间隙中形成，间隙厚度远远大于颗粒尺寸。因此，剪切流可能是一种在具有粘弹性的悬浮液中驱动颗粒进入结晶有序的简单方法。这里报道的结晶结构可用于设计具有特殊机械、光学、热或电性能的新材料。
