Giant Electroviscous Effects in a Ferroelectric Nematic Liquid Crystal
M. Praveen Kumar, Jakub Karcz, Przemysław Kula, Smarajit Karmakar, Surajit Dhara
Abstract
The electroviscous effect deals with the change in the viscosity of fluids due to an external electric field. Here, we report experimental studies on the electroviscous effects in a ferroelectric nematic liquid crystal. It is synthesized by accomplishing an optimized synthetic route, which provides higher yield than the conventional one. We measure electric-field-dependent viscosity under a steady shear at different temperatures. In the low-field range, the increase in viscosity [$\mathrm{\ensuremath{\Delta}}\ensuremath{\eta}=\ensuremath{\eta}(E)\ensuremath{-}{\ensuremath{\eta}}_{0}$] is proportional to ${E}^{2}$ and the corresponding viscoelectric coefficient [$f\ensuremath{\approx}{10}^{\ensuremath{-}9}\phantom{\rule{0.2em}{0ex}}{\mathrm{m}}^{2}/{\mathrm{V}}^{2}$] of the ferroelectric nematic is 2 orders of magnitude larger than the apolar nematic liquid crystals and the largest ever measured for a fluid. The apparent viscosity measured under a high electric field shows a power-law divergence $\ensuremath{\eta}\ensuremath{\sim}(T\ensuremath{-}{T}_{c}{)}^{\ensuremath{-}0.7\ifmmode\pm\else\textpm\fi{}0.05}$, followed by nearly an order of magnitude drop below the $N$-${N}_{F}$ phase transition. Experimental results within the dynamical scaling approximation demonstrate rapid growth of polar domains under a strong electric field as the $N$-${N}_{F}$ phase transition is approached. The gigantic electroviscous effects demonstrated here are useful for emerging applications and understanding striking electrohydromechanical effects in ferroelectric nematic liquid crystals.