Nonstandard electroconvection

In nematics with e_a < 0 and s_a < 0 or with e_a > 0 and s_a > 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 e_a < 0 and s_a < 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 e_a > 0 and s_a > 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 e_a < 0 and s_a > 0 as well as in bent-core nematics with e_a < 0 and s_a < 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