Standard electroconvection

The classical example of standard EC is observable in planar cells of a nematic with ea < 0 and sa > 0. The instability occurs due to the Carr-Helfrich feedback mechanism. In the presence of a spatially periodic director tilt (e.g. a fluctuation) the electric current has a component normal to the electric field due to the anisotropic electrical conductivity, which leads to a separation of space charges. The force acting on these charges in electric field induces a flow forming vortices which exert a viscous torque on the director closing the feedback loop. If the voltage exceeds a frequency dependent threshold value, the feedback becomes positive and a macroscopic pattern develops; otherwise the fluctuations decay.

The precise theoretical description of this mechanism, known as the standard model of EC, integrates the equations of nematohydrodynamics with Maxwell’s equations assuming an Ohmic electrical conductivity. The equations have two independent solutions corresponding to the conductive and dielectric regimes, which are characterized by different time symmetries and wave vectors of the patterns.


 A sketch of the the geometry of standard EC.


Morphological phase diagram of standard EC.


Examples for standard EC patterns in a planar nematic:


Oblique rolls


Normal rolls


Dielectric rolls


Dielectric chevrons


Standard EC is observable as a primary instability in planar nematics with ea < 0 and sa > 0 as well as in homeotropic nematics with ea > 0 and sa < 0.

In homeotropic nematics with ea < 0 and sa > 0 as well as in planar nematics with ea > 0 and sa < 0 the primary instability is a homogeneous deformation (Freedericksz transition) resulting in a quasi-planar state. Standard EC may then occur at higher voltages as a secondary bifurcation.


Examples for standard EC patterns in homeotropic nematics:

Disordered oblique rolls

Soft squares

Abnormal rolls

CRAZY rolls