Orientational transition in nematic liquid crystals under oscillatory Poiseuille flow

TL;DR

This study explores how oscillatory Poiseuille flow influences the orientation of nematic liquid crystals, revealing a supercritical transition to out-of-plane director motion that aligns with theoretical predictions.

Contribution

It reports the first observation of a supercritical orientational transition in nematic liquid crystals under oscillatory flow, with experimental results matching recent theoretical models.

Findings

Low amplitude oscillations keep the director in-plane.

High amplitude induces out-of-plane director motion.

Transition frequency dependence agrees with theory.

Abstract

We investigate the orientational behaviour of a homeotropically aligned nematic liquid crystal subjected to an oscillatory plane Poiseuille flow produced by an alternating pressure gradient. For small pressure amplitudes the director oscillates within the flow plane around the initial homeotropic position, whereas for higher amplitudes a spatially homogeneous transition to out-of-plane director motion was observed for the first time. The orientational transition was found to be supercritical and the measured frequency dependence of the critical pressure amplitude in the range between 2 and 20 Hz was in quantitative agreement with a recent theory.