Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD, USA.
Soft Matter. 2017 Dec 20;14(1):83-91. doi: 10.1039/c7sm01681f.
We have investigated the gravity-driven transport of spherical colloids suspended in the nematic liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) within microfluidic arrays of cylindrical obstacles arranged in a square lattice. Homeotropic anchoring at the surfaces of the obstacles created periodic director-field patterns that strongly influenced the motion of the colloids, whose surfaces had planar anchoring. When the gravitational force was oriented parallel to a principal axis of the lattice, the particles moved along channels between columns of obstacles and displayed pronounced modulations in their velocity. Quantitative analysis indicates that this modulation resulted from a combination of a spatially varying effective drag viscosity and elastic interactions engendered by the periodic director field. The interactions differed qualitatively from a sum of pair-wise interactions between the colloids and isolated obstacles, reflecting the distinct nematic environment created by confinement within the array. As the angle α between the gravitational force and principal axis of the lattice was varied, the velocity did not follow the force but instead locked into a discrete set of directions commensurate with the lattice. The transitions between these directions occurred at values of α that were different from those observed when the spheres were in an isotropic liquid, indicating the ability of the liquid crystal forces to tune the lateral displacement behavior in such devices.
我们研究了在微流控阵列中，重力驱动的球形胶体在向列液晶 4-氰基-4'-戊基联苯（5CB）中的输运，该微流控阵列中的圆柱形障碍物呈正方形晶格排列。障碍物表面的垂直各向异性锚定形成了周期性的指向场模式，强烈影响了胶体的运动，胶体的表面具有平面锚定。当重力方向与晶格的主轴平行时，粒子沿障碍物列之间的通道移动，并表现出明显的速度调制。定量分析表明，这种调制是由空间变化的有效阻力粘度和由周期性指向场引起的弹性相互作用的组合造成的。这种相互作用与胶体和孤立障碍物之间的成对相互作用的总和有明显区别，反映了由阵列内的限制产生的独特向列环境。随着重力和晶格主轴之间的夹角α的变化，速度并没有跟随力，而是锁定在与晶格一致的离散方向集。在这些方向之间的转变发生在与球体处于各向同性液体中观察到的不同的α值处，这表明液晶力能够调整此类器件中的横向位移行为。
