[Paper Review] Electric field-induced interfacial instability in a ferroelectric nematic liquid crystal

Marcell Tibor Máthé, Bendegúz Farkas+4 more|arXiv (Cornell University)|Oct 25, 2022

Liquid Crystal Research Advancements4 citations

TL;DR

This study investigates electric field-induced interfacial instabilities in ferroelectric nematic liquid crystal droplets and fluid bridges, revealing a fingering or ramification instability above a threshold voltage. The instability resembles Rayleigh-type and circular drop-type instabilities, with the nematic director and ferroelectric polarization aligning along finger tips, indicating parallel alignment, and the threshold voltage showing frequency-dependent behavior across geometries.

ABSTRACT

Studies of sessile droplets and fluid bridges of a ferroelectric nematic liquid crystal in externally applied electric fields are presented. It is found that above a threshold, the interface of the fluid with air undergoes a fingering instability or ramification, resembling to Rayleigh-type instability observed in charged droplets in electric fields or circular drop-type instabilities observed in ferromagnetic liquids in magnetic field. The frequency dependence of the threshold voltage was determined in various geometries. The nematic director and ferroelectric polarization direction was found to point along the tip of the fingers that appear to repel each other, indicating that the ferroelectric polarization is essentially parallel to the director. The results are interpreted in connection to the Rayleigh and circular drop-type instabilities.

Motivation & Objective

• To investigate the interfacial instability of ferroelectric nematic liquid crystal droplets and fluid bridges under applied electric fields.
• To determine the threshold voltage for instability onset and its dependence on frequency and geometry.
• To analyze the alignment of the nematic director and ferroelectric polarization relative to the emerging finger-like structures.
• To interpret the observed instability in the context of known phenomena such as Rayleigh-type and circular drop-type instabilities.

Proposed method

• Experiments were conducted on sessile droplets and fluid bridges of a ferroelectric nematic liquid crystal under externally applied electric fields.
• The threshold voltage for interfacial instability was measured across various frequencies and geometric configurations.
• Polarizing microscopy was used to observe the orientation of the nematic director and ferroelectric polarization at the tips of the fingers.
• The results were compared to theoretical models of Rayleigh-type and circular drop-type instabilities in charged or magnetized systems.
• The frequency dependence of the instability threshold was systematically analyzed to identify scaling behaviors.

Experimental results

Research questions

• RQ1 What is the threshold voltage for electric field-induced interfacial instability in ferroelectric nematic liquid crystal droplets and fluid bridges?
• RQ2 How does the threshold voltage depend on the frequency of the applied electric field and the geometry of the system?
• RQ3 How are the nematic director and ferroelectric polarization oriented relative to the finger-like protrusions formed during instability?
• RQ4 To what extent does the observed instability resemble Rayleigh-type or circular drop-type instabilities in other systems?

Key findings

• Above a critical electric field threshold, the liquid crystal-air interface exhibits a fingering or ramification instability, resembling Rayleigh-type instabilities in charged droplets.
• The threshold voltage for instability shows a frequency-dependent behavior, varying with the applied AC frequency and system geometry.
• The nematic director and ferroelectric polarization are aligned along the tips of the fingers, indicating that the polarization is essentially parallel to the director.
• The observed instability is interpreted as a hybrid of Rayleigh-type and circular drop-type instabilities, driven by interfacial forces in the presence of electric fields.