On Modeling Tear Breakup Dynamics with a Nematic Lipid Layer

TL;DR

This paper models tear film lipid layer as a nematic liquid crystal to better understand its role in tear film stability and breakup, revealing new effects due to molecular orientation.

Contribution

It introduces a novel model of the tear film lipid layer using nematic liquid crystal dynamics, accounting for internal molecular structure and its impact on tear film behavior.

Findings

Incorporating liquid crystal structure affects evaporation rates.

The model predicts new tear film breakup behaviors.

Internal molecular orientation influences tear film stability.

Abstract

One of the main roles of the lipid layer (LL) of the tear film (TF) is to help prevent evaporation of the aqueous layer (AL). The LL thickness, composition, and structure all contribute to its barrier function. It is believed that the lipid layer is primarily nonpolar with a layer of polar lipids at the LL/AL interface. There is evidence that the nonpolar region of the LL may have liquid crystalline characteristics. We investigate the structure and function of the LL via a model of the tear film with two layers, using extensional flow of a nematic liquid crystal for the LL and shear-dominated flow of a Newtonian AL. Evaporation is taken into account, and is affected by the LL thickness, internal arrangement of its rod-like molecules, and external conditions. We conduct a detailed parameter study with a focus on the evaporative resistance parameter, the Marangoni number, and primary liquid crystal parameters including the Leslie viscosities and director angle. This new model responds similarly to previous Newtonian models in some respects; however, incorporating internal structure via the orientation of the liquid crystal molecules affects both evaporation and flow. As a result, we see new effects on TF dynamics and breakup.