Electrorheological study of low molecular weight nematic liquid crystals: experimental and theoretical results
Cidade, M.T.1, Boto, A. 1, Pinto, L.1, Leal, C.R.1,2 and Patrício, P.2,3
1 Materials Science Department and CENIMAT/I3N, Universidade Nova de Lisboa, Portugal; 2 ISEL, Portugal; 3 Centro de Física Teórica e Computacional da Universidade de Lisboa, Portugal
The experimental and theoretical study of the electrorheological effect (ER) observed in the nematic phase of low molecular weight cyanobiphenyls is the aim of this work.
We will present the flow curves of our samples for different temperatures and under the influence of an external electric field, ranging from 0 to 3kV/mm, applied perpendicular to the flow direction. We will also present the viscosity as a function of the temperature, for the same values of electric field, obtained for different shear rates.
Nematic liquid crystals with positive dielectric anisotropy, which is the case of our samples, show a significant increase of the apparent viscosity upon application of the external electric field, for small shear rate values, which is due to the molecular alignment in the direction perpendicular to the flow field, in consequence of the application of the electric field. For higher shear rates a progressive decrease of the viscosity is observed, reflecting the director alignment balance between the electric field and the flow direction. For sufficiently high shear rates, the flow field completely dominates and the viscosities of the different flow curves converge into the curve obtained without electric field applied.
Theoretical interpretation of the observed behaviours is proposed in the framework of the continuum theory of Leslie-Ericksen for low molecular weight nematic liquid crystals. In this description the director alignment angle is a function of the electric field and the flow field - boundary conditions are neglected. Some Leslie viscosity coefficients and Miesowicz viscosities and the dielectric anisotropy εa are estimated for each temperature, by fitting the theoretical model to the experimental data.