On the critical behaviour of the twist viscosity γ

We present a new analysis of a number of quoted experimental data on the critical behaviour of the twist viscosity γ1(T) above the second-order N — A transition, for cyano-benzylidene-octyloxyaniline (CBOOA), octyloxy-cyano-biphenyl (80CB), and hexyl-azoxybenzene (HAB). We show that all data are compatible with a single value ν/2 = 0.33 of the critical exponent in expression γ1(T) = [math](Ύ) + [math](Τ/Τc–ν/2) thus supporting the helium-analogy proposed by de Gennes. This uniform result is obtained here for the first time thanks to the use of the theoretical expression [math](T) = cS2 exp (εS/kT) to describe the regular part of γ1(T). We also present new evaluations of the elastic constants [math] and [math], the rigidity modulus BN (T), the viscosity γ3 (T) and the relaxation time τψ (Τ) of the fluctuations of the smectic order parameter above Tc