Annihilation of a wedge disclination pair in a hybrid aligned nematic cell

We have investigated the dynamics of the process of annihilation of a wedge disclination pair generated in a nematic cell with hybrid alignment, by quenching it from its isotropic phase. The time evolution of the separation between the defect pair can be classified into two stages: the early stage, with a stringlike pattern of schlieren texture immediately after a defect pair is generated; and the late stage, with a round pattern before the defect pair is annihilated. The experimental results show that the separation between the defect pair decreases linearly with time in the early stage, and then diminishes to zero in proportion to the square root of the time to coalesce the defect pair in the late stage; these are easily explained with a two-dimensional phenomenological model by including an additional field with an ordering effect. By numerical simulation, we have also studied the annihilation dynamics of a three-dimensional equation of motion based on the Frank elastic energy without a phenomenological ordering field. The simulation results are in good agreement with the experimental ones, which indicates that the defect pair behavior in the early stage is attributed to the hybrid alignment, with a coupling energy between the tilt angle and the azimuthal angle. From a comparison between the phenomenological model with the ordering field and the numerical simulation, we can clarify the molecular origin of the ordering field, which has been introduced to explain the unusual annihilation process in the early stage.