Measurements of the dielectric and viscoelastic constants in mixtures of 4,$4^{\prime }$-$n$-octyl-cyanobiphenyl and biphenyl

We performed measurements of the dielectric constants, splay elastic constant, and rotational viscosity of the nematic phase of mixtures of 4,4${}^{\prime }$-$n$-octyl-cyanobiphenyl (8CB) and biphenyl (BP). In contrast with previous results of DasGupta et al. [Phys. Rev. E 63, 041703 (2001); Phys. Lett. A 288, 323 (2001)], we do not find any anomaly of these constants when the smectic-A phase is approached at all concentrations of BP. These results are compatible with recent calorimetric measurements of Denolf et al. [Phys. Rev. Lett. 97, 107801 (2006); Phys. Rev. E 76, 051702 (2007)] and the absence of a tricritical point in the phase diagram. The origin of the anomalies observed by DasGupta et al. at large concentration of BP is also briefly discussed and a likely explanation in terms of biphenyl evaporation is proposed.

Figure 1

Dielectric anisotropy as a function of temperature.

Figure 2

Magnetic anisotropy as a function of temperature.

Figure 3

Rotational viscosity as a function of temperature.

Figure 4

Splay elastic constant as a function of temperature.

Figure 5

Capacitance as a function of the applied voltage. The sample is 7.5 $\mu$m thick and is not sealed on the sides with an UV glue. Curves were obtained by measuring the capacitance every 0.1 Vrms. The waiting time between two measurements was 4 s. All the curves were measured at the same temperature, $\delta T=0.3{}^{\circ }$C, by taking for $T_{NA}$ the value measured immediately after filling.

Figure 6

Weight as a function of time of a small amount of BP powder deposited on the pan of a high precision balance and its best fit with a line. The weighting chamber of the balance was left slightly open in order that the atmosphere inside does not saturate in BP during the measurement. The bottom curve shows a recording at zero load, showing that the electronics does not drift significantly during the experiment.

Figure 7

Transition temperatures measured under the microscope in a free drop deposited on a glass plate as a function of time (in hours). The BP concentration was 3% by weight at the beginning of the experiment. The solid squares have been calculated from the measured $T_{NI}$ and the phase diagram given in Ref. [13]. The solid lines are only guides for the eye.

Figure 8

BP concentration in a free drop as a function of time calculated from the measurements shown in Fig. 7 and the phase diagram given in Ref. [13]. The solid line is only a guide for the eye.