Optical force-based detection of splay and twist viscoelasticity of CCN47 across the Nematic-to-Smectic A transition

Abstract

We measured the splay and twist elastic constant K and viscosity η of CCN47 by coupling optical forces with the nematic director and detecting the resulting deformation. This approach enables to separately induce each of the two distortions and allows to characterize the viscoelastic properties of both the nematic and the smectic A phase. We find that in the nematic phase far from the transition the splay elastic constant K1 is larger than that of twist K2, while its viscosity is smaller, ηsplay < ηtwist. Upon lowering the temperature, K1 and ηsplay continuously increase in the nematic and smectic phase. On the contrary, K2 and ηtwist exhibit an abrupt growth when approaching the N/SmA transition, confirming the expected critical behavior. The twist viscoelastic ratio remains approximately constant in the whole nematic phase, indicating that the critical exponents for K2 and ηtwist are similar.