Giant electroviscous effects in a ferroelectric nematic liquid crystal

TL;DR

This study experimentally investigates the electroviscous effects in a ferroelectric nematic liquid crystal, revealing unprecedentedly large viscoelectric coefficients and phase transition behaviors that could impact future applications.

Contribution

It reports the synthesis of a ferroelectric nematic liquid crystal with a novel method and characterizes its electric field-dependent viscosity, highlighting the largest viscoelectric coefficient measured to date.

Findings

Viscosity increases proportional to E^2 at low fields.

Viscoelectric coefficient is two orders of magnitude larger than in apolar nematics.

Viscosity diverges near the phase transition and drops sharply below it.

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 was synthesised accomplishing a new synthetic route which provides higher yield than conventional one. We measure electric field-dependent viscosity under a steady shear at different temperatures. In the low field range, the increase in viscosity ($\Delta\eta=\eta(E)-\eta_0$) is proportional to $E^2$ and the corresponding viscoelectric coefficient ($f\approx10^{-9}$m\textsuperscript{2}/V\textsuperscript{2}) of the ferroelectric nematic is 2 orders of magnitude larger than the apolar nematic liquid crystals and largest ever measured for a fluid. The apparent viscosity measured under a high electric field shows a power-law divergence $\eta\sim(T-T_c)^{-0.7\pm0.05}$, followed by nearly an order of magnitude drop below the N-N\textsubscript{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\textsubscript{F} phase transition is approached. The gigantic electroviscous effects demonstrated here are important for emerging applications and understanding striking electrohydromechanical effects in ferroelectric nematic liquid crystals.