Study of Liquid Crystals in Flow: I. Conventional Viscometry and Density Measurements

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Abstract
Using the technique of conventional viscometry (Haake-Hoeffler Falling Ball and Ostwald-Poiseuille viscometers), we have measured the temperature dependence of the apparent viscosity of three liquid crystals: methoxybenzylidene butylaniline (MBBA), hexyloxybenzylidene amino benzonitrile (HBAB) and cyanobenzylidene octyloxyaniline (CBOOA). The measurements with the falling-ball show a small discontinuity in the apparent viscosity of (HBAB) at ∼ 91°C, where the flow properties of this compound have been found to change from one flow regime in which the “classical” molecular alignment with the flow can be observed (T > 91°C) to another regime in which this alignment cannot be observed (T < 91°C). However, in spite of this small discontinuity, the apparent falling-ball viscosity which we measured for (HBAB) is still very close to the Miesowicz viscosity, ηB, obtained by constraining the director n to remain parallel to the velocity vector during the measurements. Measurements performed with the Ostwald-Poiseuille viscometer show: (i) that the apparent viscosity and the viscous heat of the nematic and smectic-A mesophases depend upon the radius of the capillary; (ii) that the apparent viscosity, in both the flow-alignment and the non-flow-alignment regimes, is very close to ηB and (iii) that there is no convincing evidence of a discontinuity in the apparent viscosity of (HBAB) at 91°C. Our measurements of the temperature dependence of the density of the mesophases show that the isobaric coefficients of thermal expansion for (HBAB) and (CBOOA) are different in the flow alignment and in the non-flow-alignment regimes. This is consistent with the hypothesis that there is a greater degree of molecular association in the non-flow-alignment regime.