*Forma,* Vol. 7 (No. 3), pp. 189-227, 1992

*Review*

## Stability Diagram, New Patterns and Turbulence of Electroconvection in Nematics

Shoichi Kai

Department of Electrical Engineering, Kyushu Institute of Technology, Kitakyushu 804, Japan

(Received December 13, 1990; Accepted January 25, 1993)

**Keywords: **
Electrohydrodynamics, Stability Diagram, Periodic Defect Structures, Pattern Formation, Turbulence-Turbulence Transition, Co-Dimension Two Bifurcation

**Abstract. **
The recent development of research on the pattern formation in electrohydrodynamic instability in nematic liquid crystals is discussed. One of current topics is a defect turbulence which is defined as temporally nonperiodic motion of defect whereas the whole convective structure is *almost* spatially periodic. Statistical properties of the defect turbulence, for example power spectra and the frequency distribution of defects, are experimentally described. The phase diagram of the electroconvection is given in the vicinity of the convective threshold, which is well agreed with a recent theoretical result in part. New patterns made from defects, called defect mediated periodic structures, are shown. The defect lattice pattern can be observed above a roll convection state in which defects are regularly oriented in a periodic roll structure. The phase waves of an oscillatory grid pattern (GP), which is a three-dimensional cellular convection, show macroscopic spiral and turget patterns resembling them observed in the Belousov-Zhabotinsky chemical reaction system. This is due to the cooperative phase propagation. Several other new patterns are also introduced. Further increasing the electric voltage, turbulence-turbulence transition occurs even in fully developed turbulence. Physical properties for this transition is discussed. In higher frequency regime, a co-dimension-two-bifurcation is observed and the experimental results in the vicinity of the bifurcation point are discussed. The model for the chevron pattern, a kind of periodic defect patterns, is proposed.