Session X01: Biological Fluid Dynamics: General (10:45am - 11:30am CST)

How to deform an egg yolk The Mechanism of Concussive Brain Injury

In this study, we report a novel experimental and theoretical study to examine the response of a soft capsule bathed in a liquid environment to sudden external impacts. Taking an egg yolk as an example, a translational and a rotational impact system have been developed to investigate the deformation of the egg yolk, in which a transparent container holding both the egg yolk and egg white is used to replace the eggshell. We found that the egg yolk is not sensitive to translational impacts, but is very sensitive to rotational, especially decelerating-rotational impacts. To reveal the fundamental mechanism of the soft matter deformation observed in the experiment, a theoretical model has been developed. In our model, it shows that pressure distribution around the yolk experienced a sever change during the deceleration impact. It is the pressure distribution, the centrifugal force, and the shape of the membrane together play a critical role in causing the deformation of the soft object. The experimental and theoretical study with the egg yolk provides a new perspective for understanding the response of a membrane-bound soft object to sudden external impacts. It helps us advance the understanding of the flow physics related to the head injury.   

Effect of Channel Length on Peristaltic Pumping

Peristalsis occurs when a wave of area contraction/expansion propagates along a flexible channel filled with fluid. This pumping mechanism is common in nature; in perivascular pumping, the wavelength may exceed the channel length. Here we study the effect of channel length on flux produced by an arbitrary, periodic peristaltic wave, with no external pressure gradient. We use a long-wave approximation and present results for time-averaged and instantaneous flux as a function of channel length-to-wavelength ratio in a two-dimensional channel, and validate with finite-element simulations. Our model shows that net flux is significantly reduced when the ratio drops below 1. We conclude that models that consider small domain lengths and do not adjust the boundary conditions accordingly do not adequately represent the physics of longer systems.   

Zebrafish larvae evade by swimming orthogonally to predators

Finding an efficient and effective evasion strategy is an age-old problem for prey survival. Existing theories suggest that it is preferable for prey survival to escape in a direction that either maximizes the minimal distance from a predator or is random and thus unpredictable. However, there is a lack of systematic studies accounting for both sensory processing and biomechanics of the prey during evasion. To this end, we build stocastic models combing both sensory and motor response noises for an array of evasion tactics, and assess their validity based on experimental data collected using larval zebrafish. Our evaluations show that moving orthogonal to the predator heading best explains the experimental observations, as it uses less sensory cues while giving same or better fits than the classic distance-optimal strategy. Moreover, predictions of the orthogonal strategy can be further improved by addressing the physical constraints in larval zebrafish: smaller turns are favorable for faster responses. Our results show that prey tends to adopt strategies that reduce perception complexity and is strongly influenced by its biomechanical capability. We conclude on the implication of our framework for deciphering the neuro-sensory motor mechanisms underlying other animal behaviors.   

A multi-modality approach for enhancing 4D flow MRI in cerebral aneurysms via sparse representation

A multi-modality approach is introduced to enhance the resolution and accuracy of time-resolved, three-directional magnetic resonance imaging (4D flow MRI) of velocity fields in cerebral aneurysms. Using a library of high-resolution velocity-fields constructed from patient-specific computational fluid dynamic (CFD) simulations and in vitro particle tracking velocimetry (PTV) measurements, the sparse representation of the flow-library was obtained to reconstruct the flow field of 4D flow MRI data. The method was evaluated with synthetic 4D flow MRI data in two patient-specific cerebral aneurysm models. Compared to the synthetic MRI data, the reconstruction increased the resolution by 3-4 times along each spatial dimension, reduced the velocity error by up to 75{\%}, and reduced the bias error by more than 50{\%} in flow-derived hemodynamic quantities affecting aneurysm progression, including pressure and wall shear stress (WSS). The method was finally applied to in vivo 4D flow MRI data in a basilar tip and an internal carotid artery aneurysms. The results suggested using the sparse-representation flow-reconstruction provides more reliable hemodynamic analysis in cerebral aneurysms with in vivo 4D flow MRI.   

3D-printing Mask filters inspired by animal nasal cavity

Since the outbreak of COVID-19, wearing a face mask has become essential all over the world. However, unfortunately the N95 respirator, which has an antiviral effect, is suffering from a supply shortage, and cloth face coverings are vulnerable to virus blocking, so the supply of next-generation masks is highly demanded. In this study, we investigated the nasal structure of animals with high olfactory capabilities (dog, pig, rodents, etc.), and proposed the design of new respiratory filters. First, we characterized the geometric properties of animal's nasal structures using CT scanned images. Animal's olfactory chambers have a complicated maze-like structure for air to sharply turn around and bifurcate. The gap width and the radius of curvature are on the order of a millimeter or slightly less. Based on the geometric dimensions in animals, filters with various tortuous air pathways were constructed using a 3D printer, and then the inhalation/exhalation resistance and filtration performance were evaluated under human breathing conditions (5\textasciitilde 100 LPM). We found that these bio-inspired mask cartridges work well to collect micron-sized particles and lower the pressure drop across the cartridges.   

Impact force of human high diving.

Diving from high platforms, and cliffs into water is a popular adventure sport and is expected to become a part of summer Olympics in the near future. High diving, as it is commonly known, involves participants jumping from heights beyond 20m. Naturally, the body orientation is crucial for surviving the large impact forces during such a dive. The rule of thumb among divers is that for heights above 14m, the safest orientation to dive is feet first. In this talk, we will discuss the impact forces that a human body experiences in diving head first, hands first, or feet first. Since most of the diving related injuries happen during the impact phase, we will focus on the early time dynamics of the water entry, where the unsteady liquid forces are dominant and dependent on the shape of the body. We will show that the impulse due to impact, which incorporates the relevant timescale of unsteady forces, varies across the different diving forms and is responsible for muscle/bone injuries. As such, this study presents a fluid mechanics based protocol for safe high diving.   

On the fluid dynamics of tonometry: An experimental and theoretical approach

This work addresses the fluid dynamics involved during an ophthalmologic procedure called tonometry that is used to measure the intraocular pressure of human eye. The mechanisms involved during tonometry were studied experimentally and theoretically in detail for dry and teary eye conditions using real human subjects. High speed imaging was used to capture the transient three-dimensional fluid mechanical processes that are involved. An air puff is ejected from the nozzle which is kept at approximately 10mm from the eye. It has been found that the air puff has a leading vortex and a trailing jet. The leading vortex approaches the eye which increase the air velocity field locally as a result the local pressure reduces causing an initial sheet ejection for wet eye. Wet eye is simulated by using eye drops before undergoing the tonometry technique. While the sheet ejects out of the eye the trailing jet hits the cornea leading to a deflection which causes capillary waves on the surface of the eye. The sheet expands in two phases first due to the leading vortex and then due to the trailing jet. Due to the highly transient and three-dimensional nature of the flow, bag like structures begin to appear which undergo disintegration into droplets by Rayleigh Plateau breakup.   

Activity-enhanced phase separation in a model of interphase chromatin

The large scale organization of chromatin -- the functional form of DNA -- is critical for nuclear processes such as transcription, machinery of which must physically access particular genes within the tightly-packed, micron-scale nucleus. Two major components of chromatin, heterochromatin and euchromatin, are spatially segregated inside the nucleus, with the mostly transcriptionally active euchromatin being loosely packed while predominantly silent genes are condensed into heterochromatin regions. We present a bottom-up approach to model the dynamics of chromatin phase segregation into spatially defined domains in the presence of active events and nucleoplasmic fluid. We develop a coarse-grained model of alternating polymeric blocks of active euchromatin and silent heterochromatin immersed in a viscous solvent and confined to a spheroidal nucleus. In active regions, stochastic force dipoles drive self-organization and long-ranged fluid flows, while inactive regions contract through long-lived cross-links. Using Brownian dynamics simulations based on a boundary integral formulation, along with a kernel-independent fast multipole method, we demonstrate the roles played by hydrodynamic interactions and topological constraints in driving large-scale motion and phase separation.   

Mucosalivary droplet clouds and deposition fields from synthetic coughs and sneezes

Mounting evidence shows that the main transmission mode of the COVID-19 disease caused by the novel coronavirus is through aerosol and droplet generated by expiratory events, such as breathing, talking, coughing, and sneezing. We present a systematic study of mucosalivary droplet dispersal through the air and their deposition distribution on surfaces after expiratory events. We developed a mechanical apparatus that produces synthetic droplets with a controlled size and speed distribution corresponding to the wide range of exhalations. The rheology of the mucosalivary droplets is varied by changing the mucin content to study its effects on the dispersal dynamics. Using laser profilometry, we find that the viscoelastic properties of the medium has significant effects on the distance traveled by the droplet cloud and deposition onto surfaces. Complementary experiments were conducted to test the efficacy of mask fit and material on the spatial temporal evolution of the droplet distributions. This study provides improved guidelines for safe physical distancing practices to mitigate the COVID-19 pandemic and other respiratory diseases.   

Abstract Withdrawn

Changes in the curvature and topology of cell membranes are responsible for numerous biological processes. Many of these changes seem to be driven by interactions with thin filament-like protein structures which form on the surface of membranes. While there are a number of proposed mechanisms, how exactly the filament-membrane interactions produce changes in curvature remains an open question. We investigate these mechanisms from a continuum mechanics perspective, modeling the filament as a Kirchhoff rod which is confined both in position and orientation to the membrane surface. The interplay between the geometries of the the surface and the filament give rise to complex distributions of force and torque which are believed to play a crucial role in reshaping the membrane. We discuss the mechanics of surface-bound filaments and present a collection of analytical and numerical results.   

Effects of Shear Rate, Blood Hematocrit, and Cell Stiffness on Circulating Tumor Cell Transport in a Microfluidic Device

Though circulating tumor cells (CTCs) are rare in comparison to red blood cells (RBCs), microfluidic devices (MFDs) have shown promising results in isolating CTCs from blood samples. MFDs isolate CTCs from blood samples by exploiting adhesion between CTCs and microposts coated with a selected ligand. Computational modeling is employed in this work to understand MFD performance, where cell membranes are modeled using a coarse-grained molecular approach, fluid is modeled using the lattice Boltzmann method, and the immersed boundary method is used to couple the fluid and solid. MFD performance is strongly related to flow in the device, in particular to shear rates near microposts. High shear regions generally prohibits lasting adhesion, whereas low shear regions generally promote lasting adhesion. Further studies are needed to fully understand these relationships over an extended range of shear rates. The effect on performance from cell stiffness for both RBCs and CTCs is not known, as well as the role that blood hematocric levels play in device performance. Thus, further studies of these variables and their effect on CTC transport within the device and overall device performance.   

Effects of coral colony morphology on local turbulent flow dynamics

Coral reefs rely on the local flow field to carry out physiological processes like respiration and nutrient uptake. Despite the importance of corals and the pervasive threats facing them, characterizing the local hydrodynamics between their branches has remained a significant challenge. Here, we investigate the effects of colony branch density and surface roughness on the flow field using three-dimensional immersed boundary, large-eddy simulations for four different colony geometries under unidirectional oncoming flow conditions. We compare loosely and densely branched \textit{Pocillapora} colonies, and \textit{Monitipora} colonies with and without roughness elements called verrucae. In the \textit{Pocillopora} colonies, we found that the mean velocity profiles changed substantially in the center of the dense colony, becoming reduced at middle heights where flow penetration was poor, while the profiles in the loosely branched colony remained similar throughout the colony. In the \textit{Montipora} colonies, counterintuitively, the colony without verrucae produced almost double the maximum Reynolds stress magnitude above the colony compared to the colony with verrucae. This implies that the smooth colony will have comparatively higher mass transport, bed shear stress, and friction velocity values.   

Three-dimensional imaging of tympana provides critical correction to model for hearing in a parasitoid fly

\emph{Ormia ochracea} is a parasitoid fly known for its precise sound localization abilities. The model for hearing in \emph{O. ochracea} may be the only detailed mathematical model of the mechanics of hearing in response to incident acoustic waves in a binaural organism. It accurately predicts the interaural amplitude difference (ITD) between the tympana for all incident sound angles, but fails to predict the interaural time delay (IAD) accurately for high incident sound angles. Here, we determined the 3D morphology of the tympana of two \emph{O. ochracea} specimens using synchrotron radiation microtomography (SR-$\mu$CT). In contrast to previous models that assume 2D-like plates, imaging reveals that the tympanal structures are complex and three-dimensional. We used this new information to improve the existing model for hearing in \emph{O. ochracea} by adding a term that represents the tympana’s elastic material response in the lateral direction and recovers observed IAD for all incident sound angles. This work demonstrates that hearing in \emph{O. ochracea} involves acoustic information in two primary planes, rather than one. The improved model may be useful in the design of further directional microphones and hearing aids based on hearing in \emph{O. ochracea}.   

Urinary Reflux

Urinary reflux refers to urine flow backward in the ureter from the bladder toward the kidneys. Using nonlinear, large deformation finite element simulations, the deformation of the bladder wall during urine storage is modeled in this study. The bladder wall is assumed to be a homogeneous, isotropic, hyperelastic spherical shell with a finite thickness. A straight elliptical cylindrical hole through the bladder wall at the reference configuration before inflation is the extension or the ureter through the wall. A simple fluid analysis of the tunnel flow resistance compares different bladder inner surface stretch ratios. Our model shows that the hole deformation depends on its orientation with respect to the bladder wall radial direction. As the orientation angle increases, its cross section decreases and its length increases during urine storage causing the hole closure and a rise in its flow resistance. The modeling results indicate that this closure could be explained by bladder wall deformation rather than the local differential pressure. Our findings are consistent with the accepted primary anti-reflux mechanism of minimum hole length-to-diameter ratio and consequently its insertion angle.   

Role of Interstitial Flow in Migration of Breast Cancer through Dual-gel Dual-porosity 3D ECM Mimics

Interstitial flow (IF) in the extracellular matrix (ECM) has been postulated to play a key role in regulating behaviors of breast cancer cells. Despite substantial advances made in understanding the effects of IF on cancer cell migration in vitro models, existing artificial environments deserve further improvement in controlling individual components and properties of the realistic tumor microenvironment. For example, the permeability of the ECM, together with IF velocity, governs flow-induced force on residential cells and thus regulate cell migration behaviors; however, the inevitable correlation between permeability and stiffness of traditional single-hydrogel-based 3D cell culture matrices prevents an independent control of these two important properties. To address this challenge, we developed a dual-gel dual-porosity metamaterial integrating with a microfluidic platform for studying the effects of IF on cellular behaviors. This novel in-vitro model allows for independently controlling stiffness, cell-binding sites, permeability, and IF, as well as live imaging of migrating dynamics. We will present our preliminary results showing the effectiveness of our platform by characterizing the migration of breast cancer cells under different flow conditions and ECM properties.   

Unsteady Ground Effects On A Rectangular and Swept Wing During Deceleration.

An unsteady aerodynamic model in ground effect, validated by experiment and numerical simulation, has been developed to understand the aerodynamic characteristics of a rectangular and swept wing in ground effect. Here,we consider gradual deceleration to stop from a steady velocity with decreasing ground height applicable to a landing situation. The wing also undergoes a heaving and pitching motion during deceleration. During the heaving motion, the lift and drag forces increase to an initial peak force from the initial force value; this can be attributed to the change in effective angle of attack caused by changes in relative velocity. The initial peak force is even higher in the heaving and pitching motion case; however, in the later stages of this configuration, the aerodynamic forces drop rapidly. The rapid decrease in the lift force on the heaving and pitching wing planform is correlated to the detachment of the LEV and trailing edge vortex (TEV) from the wing surface. In the steady phase, the swept wing produced a relatively higher value of the forces compared to the rectangular wing planform. However, during the growth phase, the forces overlap between both the planform shapes. The fluid physics are explored and discussed in this study..