Flow-induced delayed Freedericksz transition.

N J Mottram,I C Sage,C Tsakonas,C T Russell,C V Brown,G Mckay

doi:10.1103/physreve.93.030701

N J Mottram, I C Sage + Show 4 more

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We demonstrate that a compact manometer experiment allows direct observation of a delay to the classical electric-field-induced Freedericksz transition produced by flow in a highly dispersive nematic liquid crystal layer. The Ericksen-Leslie equations are used to show that a flow aligning torque generated in the nematic layer under Poiseuille flow competes with the orthogonal electric-field reorientation torque. This model fully reproduces the experimental results using only self-consistently determined viscosity values, and predicts a more generally applicable expression for the dependence of the delay E(c)∝sqrt[ζ/Δχ(e)] on the shear rate ζ and on the electric susceptibility anisotropy Δχ(e).

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We demonstrate that a compact manometer experiment allows direct observation of a delay to the classical electric-field-induced Freedericksz transition produced by flow in a highly dispersive nematic liquid crystal layer
There are three important features of these effects that we investigate here: first, the director-shear stress interaction tends to orient the director towards a preferred “flow alignment” angle relative to the flow velocity direction [9]; secondly, an applied electric field tends to reorient the director, an effect known as the Freedericksz transition, and may compete with the flow alignment effect; thirdly, due to the director–electric-field interaction and the asymmetric fluid stress tensor, flowing nematic liquid crystals tend to exhibit an effective viscosity that depends on the magnitude of an applied electric field [11,12]
The dynamics of the director angle and flow speed can be modeled as being governed by the Ericksen-Leslie equations [9,19] with the electric potential U determined through the coupling of Maxwell’s equations [20] and the height difference determined by a flux condition. We tackle this rather complicated model analytically by integrating the Ericksen-Leslie linear momentum equation along the length of the liquid plug and by considering a situation where the nematic director is uniform throughout the U-tube, aligned through a competition between the electric field and flow, and where the velocity inertial time scale is much shorter than the evolution of the height

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We demonstrate that a compact manometer experiment allows direct observation of a delay to the classical electric-field-induced Freedericksz transition produced by flow in a highly dispersive nematic liquid crystal layer. Nematic liquid crystals are important examples of such materials, where the orientation of the average molecular direction, the director, is affected by, and affects, both the flow properties and electric-field effects [1,2].

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