Nonstandard electroconvection

In nematics with ea < 0 and sa < 0 or with ea > 0 and sa > 0 the Carr-Helfrich mechanism cannot provide a positive feedback, thus the standard model of EC excludes the occurrence of EC.

Experimentally, however, EC patterns could be observed even in such compounds. Those instabilities have therefore been coined nonstandard EC.


Nematics with ea < 0 and sa < 0:

Parallel rolls and oblique rolls with large obliqueness have been observed in planar samples depending on the frequency.

It has been proved recently that extension of the standard model of EC by including flexoelectricity can explain this pattern formation (the charge separation is due to the flexoelectric polarization instead of sa).

        Oblique rolls              Parallel rolls       


Nematics with ea > 0 and sa > 0:

A direct transition to cellular patterns has been observed in homeotropic 5CB followed by another transition at higher voltages to a roll pattern.

The same patterns are observable also in planar samples; however, they are preceded by a Freedericksz transition.

A precise theoretical explanation of this pattern formation is still in demand.

      Cellular pattern    Roll pattern



Prewavy pattern:

The prewavy pattern is a large wavelength stripe pattern characterized with azimuthal director modulation and the stripes running normal to the director. It has been observed as a high frequency pattern in calamitic nematics with ea < 0 and sa > 0 as well as in bent-core nematics with ea < 0 and sa < 0. Its occurrence cannot be explained within the standard model of EC.

Prewavy pattern in MBBA

Prewavy2 and Prewavy1 patterns in a bent-core compound