Electroconvection in nematic mixtures of bent-core and calamitic molecules

The onset of electroconvection in binary mixtures of a bent-core and a rodlike nematic has been characterized by measuring the threshold voltage $U_c$ and the critical wave number of the pattern in a wide range of frequencies $f$. In the mixtures rich in bent-core molecules, a “conductive-prewavy2-patternless-prewavy1” morphological sequence has been detected with an unusual negative slope of $U_c\left(f\right)$ at high frequencies. This latter scenario seems to be related to the bent-core component, as it disappears with increasing the concentration of rodlike molecules. In addition, one of the parameters most relevant for electroconvection, the electrical conductivity, has also been varied by ionic salt doping. It has been found that the above effect of the banana-shaped molecules on the electroconvection scenarios can be suppressed by the conductivity.

Figure 1

Chemical structures of the bent-core ClPbis10BB, the rodlike 6OO8 molecules, and the ionic salt TBABE used in the mixtures.

Figure 2

(Color online) The threshold voltage $U_c\left(f\right)$ measured with crossed polarizers in (a) the mixture 7B3R at $87°\text{C}$ $\left(T_{NI}-1°\text{C}\right)$, $82°\text{C}$ $\left(T_{NI}-6°\text{C}\right)$, and $72°\text{C}$ $\left(T_{NI}-16°\text{C}\right)$ in a $20\text{−}\mu \text{m}$-thick planar cell; (b) in the pure ClPbis10BB at $75°\text{C}$ $\left(T_{NI}-3°\text{C}\right)$ (from [5]). PW1 and PW2 denote the high $f$ and the low $f$ prewavy regimes, respectively.

Figure 3

(Color online) Snapshots of typical electroconvection patterns under crossed polarizers in the nematic phase of the mixture 7B3R ($1°\text{C}$ below the $N\text{−}I$ transition). The rubbing direction of the $20\text{−}\mu \text{m}$-thick planar cell made an angle of $20°$ with the polarizer. (a) Oblique rolls in the conductive regime at 4 V, 10 Hz; (b) prewavy pattern in the PW2 regime at 50 V, 200 Hz $\left(U_c=31.1\text{V}\right)$; (c) prewavy pattern in the PW1 regime at 40 V, 200 kHz $\left(U_c=25.4\text{V}\right)$.

Figure 4

(Color online) Frequency dependence of the dimensionless wave number $q_c^{\ast }$ of the EC patterns in a $20\text{−}\mu \text{m}$-thick planar cell of the mixture 7B3R.

Figure 5

(Color online) Frequency dependence of the EC threshold voltage $U_c\left(f\right)$ and typical EC patterns in the middle of the nematic phase $\left(T=T_{NI}-6°\text{C}\right)$ (a) in the mixture 5B5R featuring conductive rolls, prewavy pattern, and a dynamic aperiodical convection; (b) in the mixture 3B7R featuring conductive rolls and prewavy pattern but no EC at high frequencies; (c) in the pure calamitic 6OO8 featuring conductive rolls and dielectric rolls but no EC at high frequencies. Snapshot photomicrographs of typical EC patterns at onset taken at crossed polarizers; (d) a dynamic aperiodical convection at 55 V, 500 kHz in 5B5R; (e) the prewavy pattern at 60 V, 250 Hz in 3B7R; (f) dielectric rolls at 42 V, 500 Hz in 6OO8. Please note the changes in the magnification as marked by the scale bars.

Figure 6

(Color online) Photomicrographs of the dielectric chevrons in a $13\text{−}\mu \text{m}$-thick planar cell of the pure rodlike nematic 6OO8 at $80°\text{C}$ taken under cross polarizers at applied voltages of (a) 46 V, 500 Hz and (b) 49 V, 500 Hz $\left(U_c=36.2\text{V}\right)$. (a′) and (b′) are digitally zoomed sections of (a) and (b), respectively, in order to demonstrate the presence of the initial fine dielectric rolls.

Figure 7

(Color online) Frequency dependence of the threshold voltage $U_c\left(f\right)$ for the (a) undoped 7B3R at $82°\text{C}$, (b) 7B3R doped with $0.01\text{wt}\%$ of TBABE at $80°\text{C}$, (c) 7B3R doped with $0.1\text{wt}\%$ of TBABE at $80°\text{C}$, and (d) 7B3R doped with $1\text{wt}\%$ of TBABE at $80°\text{C}$.

Figure 8

(Color online) Temperature dependence of the threshold voltage $U_c\left(T\right)$ at two frequencies selected from the lower and the higher frequency parts of the prewavy regime, respectively, for (a) the undoped mixture 7B3R and (b) the mixture 7B3R doped with $0.1\text{wt}\%$ TBABE.