Electrohydrodynamic Instabilities in Doped M5 Nematic Liquid Crystals

Abstract

We report the study of the electrodynamic behavior of the nematic mixture M5 doped at several percentages by its reaction precursors 4-(n-Octyloxy)phenol and 4-Heptylbenzoic acid [1]. The influence of the doping impurities on the M5 clearing temperature has been studied by micro differential scanning calorimetry and their conductivities have been determined by impedance analysis as well as their dielectric constants. Finally we studied by polarizing microscopy the influence of the dopants on the voltage threshold for the onset of the orientational instabilities and we determined the cut-off frequency of the low frequency conductive regime.

Dielectric Characterization of Doped M5

“…It follows that, in the limit of small ω, X (ω) ∼ 2λ/(ω ε S ); i.e., it depends only on λ. From the frequency dependence of X (ω), in the limit of ω → 0, we obtain λ, and hence N , assuming for ε the one reported in ref . Increasing the frequency, the resistance decreases and then presents a new plateau, characterized by a resistance R ′ ∼ λ 2 d /( ε D f S ), where D f = ( D p + D m )/2 is the diffusion coefficient for the free diffusion.…”

“…The investigated cells, bought from EHC Ltd. Japan, are in the shape of a slab of thickness d ∼ 25 μm and surface area S ∼ 1 cm 2 . The electrodes of the cells were covered by rubbed polyimide to induce planar alignment . The surface treatment allows to assume that there is not charge injection into the liquid crystal, and that the electrodes can be considered blocking.…”

“…We analyze the impedance spectroscopy of the samples of M5 with several concentration of doping using the models described in refs and , according to which the difference in the diffusion coefficients is responsible for a new plateau in the limit for ω → 0, connected to the ambipolar diffusion, not present in our spectra, where just a tendency to increase for the real part of the impedance has been observed. We assume that there is a pure resistance in parallel to the sample, R p .…”

“…The best fit of the experimental data have been done by means of a Genetic Algorithm (GA), , using as starting values the ones obtained from the limit for f → 0 and from R ′ as discussed above, and for ε and d the ones reported in ref .…”

“…A large part of investigations on the EHD instability have been performed on the liquid crystal known as MBBA (methoxybenzilidenebutylaniline), which possesses values for the dielectric constant and electrical conductivity such to observe, easily, this instability. 5 However, this liquid crystal degrades when it is irradiated with light, or when it is used in measurements relevant to EHD instabilities. Consequently, the experiments on EHD instability in MBBA are not fully reproducible.…”