Session L28: Biofluids: General VI - Fluid Film Flows

Tear Movement through a Contact Lens of Variable Thickness

This work is on a two-dimensional tear film with a movable porous contact lens. The inclusion of a contact lens into a tear film results in three layers: Pre-Lens Tear Film, Contact Lens, and the Post-Lens Tear Film layers. The interfaces between the contact lens and the tear films are modeled as planar interfaces. There is a free surface interface between the tear film and the outside air. The goal is analyze the effects of the spatial variability of thickness on the Post-Lens Tear Film thickness and on the fluid flow through the Contact Lens layer.   

The influence of surfactant on the stability of a liquid bilayer inside a rigid tube

Many airways in the lung are coated with a bilayer consisting of a serous layer adjacent to a more viscous mucus layer which is contiguous with the gas core. An instability due to high surface tension at the interfaces may lead to the formation of a liquid plug that blocks the passage of air. This phenomenon is known as airway closure. Here we investigate the linear stability for the case when the thin liquid bilayer is Newtonian and coated within a rigid tube with the presence of an insoluble surfactant monolayer at the mucus-gas interface. Surfactant affects the surface tension and also induces a surface stress at the interface. A system of equations for the deflections of the interfaces and the surfactant concentration is derived by using lubrication theory. These equations are linearized, and by applying the method of normal modes, a dispersion equation for the growth rate of the disturbances is obtained. Its dependence on the viscosity ratio, the thickness ratio of the two liquid layers, the base state surface tension ratio, and the Marangoni number is investigated, and comparisons with previous single layer models are discussed.   

A Model for Lipid Layer Dynamics on a Moving Domain

The human tear film consists of an aqueous layer and a thinner layer of nonpolar lipids, with polar lipids along the interface between them. Dynamics of the nonpolar lipid layer are not yet well understood. Experimental observations indicate that visible features in the lipid layer can persist through multiple blinks, but how this occurs is a matter of debate. One possibility is a concertina-like folding, corresponding to a serpentine instability in the lipid layer during a blink. Another possibility is due to a varicose instability. We use a two-layer thin film model to understand the dynamics of the lipid layer. We study the dynamics of a model Newtonian extensional layer floating on a less viscous shear layer. The upper layer includes van der Waals terms so that it dewets from the lower layer as expected in the tear film system. We give results for expanding and contracting domains for both varicose and sinuous disturbances.   

A New Model for the Suction Pressure Under the Contact Lens

We study the dynamics of the contact lens to better understand how the design of the lens can be optimized for patient comfort and ocular fit. When a contact lens is inserted on an eye, it is subjected to forces from both the tear film in which it is immersed and the blinking eyelid. In response, the lens bends and stretches. These forces center the lens, and they produce the suction pressure that keeps the lens on the cornea. In this presentation, we couple fluid and solid mechanics to determine the most prominent forces acting on the lens. We present a mathematical model that predicts the suction pressure. We explore the influence of contact lens properties on the suction pressure.   

How flies clean their eyes

Flying insects face a barrage of foreign particles such as dust and pollen, which threaten to coat the insect's eyes and antennae, limiting their sensing abilities. In this study, we elucidate novel aerodynamic and elastic mechanisms by which insects keep these organs clean. The compound eye of many species of insects is covered by an array of short bristles, or setae, evenly spaced between each photoreceptor unit. Among these insect species, setae length is triple their spacing. We conduct numerical simulations and wind tunnel experiments using an insect eye mimic to show this critical setae length reduces shear rate at the eye surface by 80{\%}. Thus, the setae create a stagnant zone in front of the eye, which diverts airflow to reduce deposition of particles. Setae can also act as springboards to catapult accumulated particles.~In high speed videography of insects using their legs to clean themselves, we observe deflected setae hurling micron scale particles at accelerations over 100 times earth's gravity. The dual abilities of setae to divert airflow and catapult particles may motivate bio-inspired designs for dust-controlling lenses, sensors, and solar panels.   

Amyloid fibril formation at a uniformly sheared air/water interface

Amyloid fibril formation is a process by which protein molecules in solution form nuclei and aggregate into fibrils. Amyloid fibrils have long been associated with several common diseases such as Parkinson's disease and Alzheimer's. More recently, fibril protein deposition has been implicated in uncommon disorders leading to the failure of various organs including the kidneys, heart, and liver. Fibrillization can also play a detrimental role in biotherapeutic production. Results from previous studies show that a hydrophobic interface, such air/water, can accelerate fibrillization. Studies also show that agitation accelerates fibrillization. When attempting to elucidate fundamental mechanisms of fibrillization and distinguish the effects of interfaces and flow, it can be helpful to experiment with uniformly sheared interfaces. A new Taylor-Couette device is introduced for in situ, real-time high resolution microscopy. With a sub-millimeter annular gap, surface tension acts as the channel floor, permitting a stable meniscus to be placed arbitrarily close to a microscope to study amyloid fibril formation over long periods.   

Dynamics of the Primary Cilium in an Oscillatory Flow

In this work we investigate the dynamics of a primary cilium under oscillatory flows. The primary cilium is modeled as an elastic slender beam coupled to an elastic shell with a local torque (mimicking the sub-axonemal anchorage) at the beam-shell junction. We examine how a primary cilium responds to oscillatory flows depending on its axonemal stiffness and the initial base angle. In particular we focus on the tension and forces at the cilium base where ion channels are speculated to be ``activated" by fluid flow via cilium bending. We find that a tilted cilium base gives rise to slightly larger magnitude in tension and forces at the base. We further compare the cilium bending dynamics between oscillating and pulsing flows, and investigate the effect of oscillation frequency. From our simulation results we speculate that the reduced ability of periodic pulsing flow to stimulate the primary cilia responses at high frequencies may be due to lack of time for ion channels to respond to the stress at the filament base.   

The influence of nonpolar lipids on tear film dynamics

We will examine the effects of the presence of nonpolar lipids on the evolution of a tear film during a blink. We will track the thickness of the aqueous tear layer, the thickness of the nonpolar lipid layer, and the concentration of the polar lipids that reside between the two. Our model can be reduced in various limits to previous models for tear dynamics studied. We present numerical solutions for the evolution of the tear film and show how the key parameters play a role in determining how the nonpolar lipid spreads.