Seyednejad Seyed Reza, Mozaffari Mohammad Reza, Ejtehadi Mohammad Reza
Department of Physics, Sharif University of Technology, P.O. Box 11155-9161, Tehran, Iran.
Phys Rev E Stat Nonlin Soft Matter Phys. 2013 Jul;88(1):012508. doi: 10.1103/PhysRevE.88.012508. Epub 2013 Jul 30.
Nematic shells of liquid crystals have been provided in microscales. Defect structures in the shells are very essential in the electro-optical applications of such colloidal objects. We have numerically minimized the free energy of symmetric and asymmetric spherical shells of the nematic liquid crystal. Considering degenerate planar anchoring on the surfaces and isotropic nematic elasticity, a variety of defect structures are observed by controlling or varying the thicknesses of the shell and its degree of asymmetry. In symmetric shells, our calculations show that boojums (bipolar) defects appear in thick shells and tetrahedral (baseball) defects in thin shells. In asymmetric shells, while we are in the bipolar regime, the boojums defects transform to trigonal configurations. Free energy landscape shows that in this regime the inner droplet is not stable in the center and it is trapped in an off-center minimum energy position. For the case of thin shells, there are two degenerate director textures with similar tetrahedral configuration of the disclination lines. The levels are split in asymmetric shells. The stability of the inner droplet in the center position depends on director texture. It is stable for one texture and unstable for the other one. For an unstable pattern there is no minimum energy position for the inner droplet and it moves until it touches the outer boundary.
已制备出微尺度的向列型液晶壳。壳中的缺陷结构在这类胶体对象的电光应用中非常重要。我们已通过数值方法使向列型液晶对称和不对称球壳的自由能最小化。考虑到表面上的简并平面锚定和各向同性向列弹性，通过控制或改变壳的厚度及其不对称程度，可以观察到各种缺陷结构。在对称壳中，我们的计算表明，厚壳中出现布居姆（双极）缺陷，薄壳中出现四面体（棒球）缺陷。在不对称壳中，当处于双极状态时，布居姆缺陷转变为三角构型。自由能景观表明，在这种状态下，内部液滴在中心不稳定，被困在偏离中心的最低能量位置。对于薄壳的情况，有两种具有相似位错线四面体构型的简并指向矢织构。在不对称壳中这些能级会分裂。内部液滴在中心位置的稳定性取决于指向矢织构。对于一种织构它是稳定的，而对于另一种则是不稳定的。对于不稳定模式，内部液滴没有最低能量位置，它会一直移动直到接触到外边界。
