Molecular Dynamics Calculation Of Rotational Viscosity Of Nematic Liquid Crystal Materials

Nematic liquid crystal materials are widely used in many fields,so people have done a lot of researches on the theory and application of nematic liquid crystal materials.The molecular simulation of liquid crystal materials by molecular dynamics method is a relatively hot field,which directly shows the shape and motion state of liquid crystal molecules in the microscopic system.The physical parameters of liquid crystal materials affect the microflow properties of liquid crystals,so it is necessary to study the measurement methods of liquid crystal physical parameters.The liquid crystal parameter measurement instruments are used to measure liquid crystal parameters,but there were some problems such as expensive instrument,inaccurate measurement results,demanding measurement and so on.Therefore,in this study,preliminary results have been obtained in the calculation of macroscopic rotational viscosity of liquid crystal materials by microscopic molecular dynamics method.The calculated results of this study were in good agreement with the experimental results,which lays a solid foundation for the follow-up research in this direction.The main work of this dissertations are as follows:(1)Molecular dynamics calculation: based on the spherical molecular simulation of Lennard-Jones potential energy(LJ),this study was applied to Gay-Berne potential(GB)suitable for the ellipsoidal structure of the liquid crystal molecule in the nematic phase.The force and torque of the liquid crystal molecule were calculated by GB potential energy,and the motion state of both translational motion and rotating liquid crystal molecules was simulated.The key tasks include the initialization of the contained liquid crystal molecular position and direction,the initialization of velocity and angular velocity,the calculation of molecular force and torque,Verlet algorithm,etc.,which can be realized in different ensembles of liquid crystal molecular dynamics simulation.(2)The verification part of the liquid crystal molecular simulation experiment: In the molecular dynamics calculation,the setting of different boundary conditions corresponds to different macroscopic liquid crystaline experiments.Taking DFP-PBC liquid crystal as an example,the periodic boundary conditions are applied to simulate the phase change process of liquid crystal at different temperatures,and the semi-cycle boundary simulation of the liquid crystal cell was carried out in combination with the idea of fixed molecular boundary,and the influence of different liquid crystal moleculars orientation as boundaries on the movement of liquid crystal molecules was studied.Finally,the effect of the electric field on molecular direction in the rotation of liquid crystal molecules was studied by combining molecular dynamics methods to simulate the application of electric fields.The above molecular dynamics simulation experiment was compared with the corresponding macro experiment,and the correctness of parameter setting in molecular dynamics simulation was verified.(3)Liquid crystal rotation viscosity parameter measurement experiment and molecular dynamics solution calculation: First of all,the DFP-PBC liquid crystal material DSC experiment to determine its temperature range in the liquid crystal state used the liquid crystal parameter measurement instrument at different temperatures measured the liquid crystal rotation viscosity parameters.The DFP-PBC liquid crystal material was simulated by molecular dynamics method and calculated the rotation viscosity parameters at different temperatures.The experimental results are identical with the results of molecular simulation.In this study,a large number of molecular dynamics simulations of the nematic liquid crystal DFP-PBC were carried out by molecular dynamics method to determine various important dynamic parameters,and experimental verification was implemented;.Further improvement of molecular dynamics method will provide a new way for the measurement of macro parameters of materials to be performed in a new way that is both economical and convenient.