Application of microdielectrometry to probe near‐wall structure evolution in flowing liquid crystals

We demonstrate how microdielectrometry (MD) sensors can be employed to follow the evolution of near‐surface [O(10 μm)] structure in flowing liquid crystals. Theoretical arguments are presented to illustrate how the distribution of the electric field on the sensor can be accounted for in interpreting MD data. We consider methoxy‐benzylidene‐butyl‐anyline, a low molecular weight nontumbling nematic, as a case study. Dielectric measurements on the quiescent liquid crystal indicated a surface‐topography‐induced ordering of the molecules parallel to the electrodes on the sensor. Dielectric‐thermal analysis under both quiescent and flowing conditions and polarized‐microscope observations confirmed the establishment of long‐range order with the director parallel to the metal lines on the sensor surface. The dielectric permittivity of the sheared samples showed strong sensitivity to both shear rate and relative orientation of the electric and velocity fields, confirming that flow‐induced anisotropy can indeed be captured dielectrically. Inception of flow transients at high shear rates showed an unusual strain scaling. This behavior was found to be consistent with the microstructural predictions of the Leslie–Ericksen–Parodi (LEP) theory. The LEP theory generated quantitative predictions for the permittivity relaxation transients observed upon cessation of flow.