Session M07: Biological Fluid Dynamics: General I

Cavitation Causes Brain Injury

In this study, an artificial transparent head surrogate with high-speed photography discovers the formation and collapse of cavitation bubbles near the countercoup regions, as the head is exposed to a sudden translational impact. The cavitation damages the brain surface and produces a shock wave through the brain matter. Based on a novel experimental design, this new finding uncovers the mystery of the motion and deformation of the soft brain matter, which is not visible otherwise. It suggests that current brain injury criteria may underestimate the risk of head collision.   

On the study of brain injury due to rotational impact

Impacts on the head are commonly identified as the cause of concussive brain injury, which has imposed huge negative health and economic impacts on the world. While brain injury has been recognized as a real issue, researchers are still trying to fully understand how the brain works and how it is injured. Especially, the interactions among the rigid skull, the cerebrospinal fluid (CSF), and the soft brain matter as the head is exposed to a rotational impact, remain unclear. In this study, a lifelike head surrogate, including a soft, artificial brain matter, a transparent, rigid skull, and the CSF in between, has been developed. The slippery motion of the CSF and the motion/deformation of the artificial brain matter were captured using a high-speed camera, as rotational impacts in the sagittal, horizontal, or corona plane were imposed on the head surrogate. The results indicate that the slippery motion of the CSF causes three types of brain injuries: the contusion at the direct impacting region, the diffuse injury widely distributed on the brain surface, and the laceration injury inside the brain. As the first study of its kind, this paper significantly improves the understanding of the fundamental mechanisms involved in the incidence and mitigation of brain injury and provides critical insights into head protection mechanisms.   

Hydrodynamics of DNA loop extrusion

The compaction of chromosomal DNA during various stages of the cellular life cycle has been shown to involve a class of enzymatic molecular motors known as structural maintenance of chromosomes (SMC) complexes, of which condensin and cohesin are two important examples. These complexes are long ring shaped motors and have been observed to translocate along chromatin in a force-dependent directed manner to form loops via a process known as "loop extrusion" using the energy obtained from the hydrolysis of ATP. This loop formation has been studied in recent in vitro single-molecule experiments on lambda phage DNA under imposed shear flow, where the two ends of the DNA were tethered to a wall and loops were shown to nucleate and grow in the presence of active SMC complexes. Motivated by these experiments, we present a microscopic model for the dynamics of active loop extrusion that accounts for the structural features and dynamics of the motor proteins. Combining modeling and Brownian dynamics simulations, we demonstrate the roles played by hydrodynamic interactions, the active response of the motors and chain mechanics in assisting loop extrusion under different parameter regimes.   

Bacterial chromatin as a phase-separating nematic elastomer

Upon stress, bacteria compact their chromosomes into condensates to protect their genome from damage. These condensates are organized by proteins (Dps) that cooperatively bind with the DNA. The resulting DNA-Dps structure is a polymer network characterized by local nematic order and viscoelastic properties. Towards understanding the rheology and phase separation dynamics, we examine a minimal continuum model that couples Cahn-Hilliard phase separation with the Landau-de Gennes theory of liquid crystals and an elastic strain model. While the ground state of the material is ordered, the strain of the polymer network that emerges during the dynamics of phase separation can create long-lasting disorder in the material that impacts its mechanical properties.    

Three-Dimensional Particle Image Velocimetry on Thermally Induced Fluid Mixing in the Eye

Age-related macular degeneration (AMD) is the leading cause of central vision loss in the developed world. Wet AMD can usually be managed through intravitreal injections of anti-vascular endothelial growth factor (anti-VEGF) agents. However, the treatment's effectiveness is varied. The half-life of anti-VEGF agents is limited, and the drug delivery mechanism in the eye is not well understood. This study investigates how thermally induced fluid mixing accelerates drug transport in a model eye and demonstrates the efficacy of this added circulation for retinal drug delivery. This research extends previous work that utilized two-dimensional planar particle image velocimetry (PIV) to three-dimensional PIV. The setup captures circulation across the entire eye model and enables a deeper understanding of heat addition's potential to improve drug delivery and treatment effectiveness.   

The effect of turbulent strain rate on sturgeon larvae in fishways

Lake sturgeon (Acipenser fulvescens) have recently been a target for conservation in the Laurentian Great Lakes. While improving spawning success has been a major goal of these efforts, an often-overlooked component is the survival of the larvae after hatching, during the period of downstream drift. In a dammed river system, during this phase, they may need to drift past dam infrastructure. This journey past dams often results in an increase in larval mortality for a variety of reasons, including exposure to highly turbulent flow. Quantifying the aspects of turbulence related to larval mortality will inform retrofitting or future design efforts of fishways to improve larval viability. This project uses dimensional arguments to characterize the flow conditions influencing larvae viability through fishways. Following Kolmogorov’s theory (1941), ??=ηRe^-3/4, where ?? is the smallest eddy diameter and η is the smallest fishway pool dimension. The strain present in the fishway at the pertinent scale for sturgeon larvae, S, can be estimated using the water’s kinematic viscosity, v, the fishway’s Reynolds number, Re, and the smallest pool dimension as S=vRe^3/2/η². This analysis is then illustrated in the case of the Legendre-Vianney Fishway in Quebec.   

Deep learning model inspired by lateral-line system for underwater object detection

This study develops a deep learning-based object identification model that can identify objects through flow information measured from a moving sensor array, inspired by the lateral-line systems of various aquatic organisms capable of hydraulic imaging with ambient flow information. A hydrofoil navigates around four stationary cylinders in uniform flow, and two types of sensory data, flow velocity and pressure, are obtained numerically from an array of sensors located on the surface of the hydrofoil with potential flow assumption. Several neural network models based on the flow velocity and pressure are built to identify the positions of the foil and surrounding objects. The LSTM-based model, which is a type of recurrent neural network capable of learning order dependence in sequence prediction problems, outperforms the other network models. The optimization of the number of sensors is then performed, using feature selection techniques, LASSO and Elastic Net. Through sensor optimization, a new object identification model shows impressive accuracy in predicting the locations of the foil and objects with only 40% of the sensors used in the original model.   

Phase separation during the spreading of human blood pool

The study of the spreading and evaporation of blood films is of crucial importance for biomedical and forensic applications. For the second application, we are interested in film (pool) spreading because it represents one of the most important evidence at a crime scene after a bloody event. By their interpretation, the investigator can make an opinion about what took place and at what time.

We will present our published results on phase separation, i.e., when the serum spreads outside the main blood pool. This latter occurs during the spreading of blood pools on a horizontal substrate, which was explained by the competition between coagulation and evaporation. 

To investigate the phase separation, blood pools with constant initial volumes on wooden floors that were either varnished or not were created at ambient temperatures of 21 °C, 29 °C, and 37 °C with a relative humidity varying from 20 to 90%. The same whole blood from the same donor was used for all experiments, and the pools were created directly. As a result, an increase in relative humidity was found to result in an increase in the final pool area. In addition, at the three different experimental temperatures, the serum spread outside the main pool at relative humidity levels above 50%.