Abstract
In the passive Fréedericksz transition, liquid crystal molecules initially aligned along cell boundaries reorient when an external electric or magnetic field surpasses a certain threshold, aligning with the field direction. The impact of the passive Fréedericksz transition was transformative for the field of liquid crystals. It enabled the control of electro-optic switching that led to modern LCD technology.
The active Fréedericksz transition, analogous to its passive counterpart, serves as a pivotal focus in understanding the intricate dynamics in the confined 3D active nematics. In contrast to the passive variation dependent on external fields, the active transition is governed by an internally generated active stress, driving a transformation from uniform alignment to non-uniform patterns accompanied by inherent flow. This phenomenon depends on the interplay between nematic elasticity, active stress, and the confining geometry.
Our approach involves a strategic design employing composite materials, by dispersing active Microtubules in a dense 3D colloidal fd virus in nematic liquid crystal phase, decoupling active and passive stresses. This unique framework provides an avenue for delving into the active Fréedericksz transition's depths. Through NADH/ATPase assays, we not only affirm that active stress remains unaffected by fd virus concentration or nematic elasticity, but we also demonstrate that the stable states indeed exhibit activity, as motors actively consume ATP on a microscopic scale and are collectively generating active stresses at the mesoscopic scale.
Further, we introduce a related variant of the active Fréedericksz transition that holds promise for measuring active stress. This involves stabilizing the active nematic within a channel geometry using a strong external magnetic field. Additionally, we explore an alternative active liquid crystal configuration, utilizing longer salmonella flagella ($3-6\mu m$) compared to fd virus (800nM). This approach enhances nematic alignment and elasticity, facilitating the observation of flow transitions at a larger, more manageable scale.
This alternative setup yields distinct behaviors: under conditions of high active stress and large confinement, we observe homogenous turbulent states. Conversely, with lower active stress or smaller confinement settings, we witness the intriguing phase separation of active microtubule bundles within the uniform flagella background. This phase separation is not understood and requires further investigation.