Electrohydrodynamic instabilities observed in a nematic phase under oblique boundary conditions

摘要

Electrohydrodynamic instabilities in the nematic phase of Merck rsquo;rsquo;Phase Vrsquo;rsquo; with oblique boundary conditions were optically observed with a polarizing microscope in 25ndash;100 mgr;m rsquo;rsquo;sandwichrsquo;rsquo; cells. Oblique anchoring of the nematic was achieved by oblique evaporation of SiO on the plates. Two types of cells were used having the respective inhyphen;plane projection of the direction of evaporation on the two plates either parallel (phyphen;type cells), or antiparallel (ahyphen;type cells). The low voltage dc instability observed for thephyphen;type cells forms in an almost regular hexagonal pattern. By gradually increasing the voltage, the dc instability observed for theahyphen;type cells forms at first as flows which originate at order disturbances created at imperfections in the SiO coating. Voltage increase causes these flows to detach themselves from the places of the imperfections and move solitarily. The moving flows are associated with what appears to be moving tilt inversion deformations (of splayhyphen;bend type) extending from the central part of the flow to some distance from it. When the voltage is further increased, a repeated process of replication of the flows, occurring on the associated tilt inversion deformations, leads to the creation of a periodic grid of moving flows. Other observed types of static and dynamic patterns under ac and dc excitation are reported, in particular: different types of cross rolls (ac conduction regime); variations of a pattern of what appears to be walls associated with flows, exhibiting an approximate wave number dependence on the electric fieldksim;E; a striped pattern associated with what appears to be twist walls and the propagating interference patterns associated with their oscillations; a toroidal flow (sometimes associated with closed inversion walls) which creates and caries along closed nematic threads (dc regime); a polygonal grid of turbulent flows (dc regime); a flow pattern correlated with the movement of the moving chevron pattern; a cellular fast turnhyphen;off pattern related to the chevron pattern. This cellular pattern appears at first as moving snakelike regions in the chevron pattern which are bordered by disclination lines. Some features of dark, spotlike figures appearing on the chevron pattern are described. Preliminary interpretations of some of the observations are offered.