Dielectric breakdown in liquid crystals

Dielectric breakdown in insulators is a commonly observed phenomenon in gases, fluids and solids at large electric field amplitudes. The resulting discharge patterns generally exhibit a non-Euclidean, fractal dimension, representing breakdown channels in the insulating material. In this study dielectric breakdown is investigated for anisotropic fluids in Hele-Shaw cells, i.e. liquid crystals in a plate capacitor. The observed burn patterns are inscribed into the substrates, suggesting that they are the result of the surface degradation, due to the breakdown current at the electrode surface. They are the same on both substrate plates with an equal fractal dimension and of radial symmetry, even though the insulating material is anisotropic. Investigations were carried out as the electric field was varied from dc to ac conditions, for instant field application versus slow ramping, for materials with positive and negative dielectric anisotropy, for a variation of the alignment coating, in several different liquid crystal phases and as a function of cell gap, applied field amplitude, frequency and temperature. In all cases the fractal dimension of the burn pattern increases as the breakdown voltage increases.