Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, Stockholm, Sweden.
Phys Chem Chem Phys. 2013 Mar 14;15(10):3442-53. doi: 10.1039/c2cp42582c.
The lack of a centre of inversion in a cholesteric liquid crystal allows linear cross couplings between thermodynamic forces and fluxes that are polar vectors and pseudovectors, respectively. This makes it possible for a temperature gradient parallel to the cholesteric axis to induce a torque that rotates the director, a phenomenon known as the Lehmann effect or thermomechanical coupling. The converse is also possible: a torque applied parallel to the cholesteric axis rotates the director and drives a heat flow. In order to study this phenomenon, nonequilibrium molecular dynamics simulation algorithms and Green-Kubo relations evaluated by equilibrium molecular dynamics simulation have been used to calculate the Leslie coefficient, i.e. the cross coupling coefficient between the temperature gradient and the director angular velocity, for a model system composed of soft prolate ellipsoids of revolution interacting via the Gay-Berne potential augmented by a chiral interaction potential causing the formation of a cholesteric phase. It is found that the Leslie coefficient is two orders of magnitudes smaller than other transport coefficients such as the heat conductivity and the twist viscosity, so that very long simulations are required to evaluate it. The Leslie coefficient decreases with the pitch but it has not been possible to determine the exact functional dependence of this coefficient on the pitch. Since very long simulations have been performed to evaluate the Leslie coefficient, very accurate values have been obtained for the twist viscosity and the heat conductivity as a by-product and it is found that they are very similar to the values of the corresponding quantities in the achiral nematic phase that arises when the pitch goes to infinity.
胆甾相液晶中没有倒转轴，使得热力学力和通量之间的线性交叉耦合分别为极向量和赝向量。这使得平行于胆甾相轴的温度梯度能够产生一个力矩，从而使指向矢旋转，这种现象称为 Lehmann 效应或热机械耦合。反之亦然：平行于胆甾相轴施加的力矩会使指向矢旋转并驱动热流。为了研究这种现象，已经使用非平衡分子动力学模拟算法和通过平衡分子动力学模拟评估的 Green-Kubo 关系，来计算由通过 Gay-Berne 势增强的手性相互作用势相互作用的软长椭球体模型系统中的 Leslie 系数，即温度梯度和指向矢角速度之间的交叉耦合系数，从而形成胆甾相。结果发现，Leslie 系数比其他输运系数（如热导率和扭曲粘度）小两个数量级，因此需要进行非常长的模拟才能评估它。Leslie 系数随螺距而减小，但无法确定该系数与螺距的确切函数关系。由于为了评估 Leslie 系数进行了非常长的模拟，因此作为副产品，获得了扭曲粘度和热导率的非常精确的值，并且发现它们与当螺距趋于无穷大时出现的非手性向列相中的相应量的值非常相似。
