Sarman Sten, Laaksonen Aatto
Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, 106 91 Stockholm, Sweden.
Phys Chem Chem Phys. 2014 Jul 28;16(28):14741-9. doi: 10.1039/c4cp01451k.
The director alignment relative to the temperature gradient in nematic liquid crystal model systems consisting of soft oblate or prolate ellipsoids of revolution has been studied by molecular dynamics simulation. The temperature gradient is maintained by thermostating different parts of the system at different temperatures by using a Gaussian thermostat. It is found that the director of the prolate ellipsoids aligns perpendicularly to the temperature gradient whereas the director of the oblate ellipsoids aligns parallel to this gradient. When the director is oriented in between the parallel and perpendicular orientations a torque is exerted forcing the director to the parallel or perpendicular orientation. Because of symmetry restrictions there is no linear dependence of the torque being a pseudovector on the temperature gradient being a polar vector in an axially symmetric system such as a nematic liquid crystal. The lowest possible order of this dependence is quadratic. Thus the torque is very weak when the temperature gradient is small, which may explain why this orientation phenomenon is hard to observe experimentally. In both cases the director attains the orientation that minimises the irreversible entropy production.
通过分子动力学模拟研究了由软扁长或长扁旋转椭球体组成的向列相液晶模型系统中指向矢相对于温度梯度的排列情况。利用高斯恒温器在不同温度下对系统的不同部分进行恒温，从而维持温度梯度。研究发现，长扁椭球体的指向矢垂直于温度梯度排列，而扁长椭球体的指向矢平行于该梯度排列。当指向矢在平行和垂直取向之间取向时，会施加一个扭矩，迫使指向矢达到平行或垂直取向。由于对称性限制，在诸如向列相液晶这样的轴对称系统中，作为赝矢量的扭矩与作为极矢量的温度梯度之间不存在线性依赖关系。这种依赖关系的最低可能阶数是二次的。因此，当温度梯度很小时，扭矩非常弱，这可能解释了为什么这种取向现象在实验中很难观察到。在这两种情况下，指向矢都达到了使不可逆熵产生最小化的取向。
