Abstract: | Abstract— Experimental study and numerical simulations of optical phase modulators based on nematic liquid crystals characterized by frequency sign inversion of the dielectric anisotropy has been performed. The received results point out an extreme role of back‐flow effects and electro‐hydrodynamic instability, which make restrictions for creating high‐speed phase modulators with large switchable values of phase retardation. It is shown by numerical simulations that the hydrodynamic destabilization can be suppressed by decreasing the ratio of Leslie coefficients |α2/α4,5|. |