On the smectic cybotactic groups and pretransition effects in the Miesowicz viscosity coefficient η₂

Abstract

A new method of investigation of smectic cybotactic groups is proposed. Cybotactic nematic is considered to be a heterogeneous mixture of smectic cybotactic groups in ‘nematic’ solvent. In the first approximation, the cybotactic groups were treated as hard spheres and their volume fraction φ _cyb was calculated using the formula for the viscosity of a hard sphere suspension out of the Miesowicz viscosity coefficient η₂. The reduced temperature (T_cyb – T_NA)/T_NA at which the cybotactic groups start to influence the behaviour of η₂ was found to be constant for all the liquid crystals under investigation (the results of our measurements for octyloxycyanobiphenyl were compared with other data for smectogenic liquid crystals). The temperature dependence of φ _cyb is discussed. The number of cybotactic groups was estimated. This number increased with decreasing temperature, and diminished after reaching a maximum. This behaviour seems to be universal, with a maximum at similar reduced temperatures for the liquid crystals investigated.

Keywords:

• cybotactic groups
• viscosity

Acknowledgements

Financial support from the Polish State Committee for Scientific Research (KBN) under Project No. 2 P03B 086 23 is acknowledged. I would like to thank Professor Józef K. Mościcki for drawing my attention to this interesting subject and for discussions.

Notes

1. The critical coefficient ν for the same data set of the rotational viscosity coefficient γ₁ for 8OCB was equal to 0.51 and 0.35 [5] depending on the choice of the regular part. Note, however, that for the viscosity coefficient γ₁ the regular part is much more important than for η₂ [7].