Fluid and solute transport in the brain
Mammals' brains contain not only blood vessels to bring nutrients and oxygen, but also a separate set of passageways that evacuate metabolic waste, collectively called the "glymphatic" system. Recently discovered by the team of Maiken Nedergaard, the glymphatic system is the subject of many open questions: What mechanisms pump away waste? Where does it go? How do sleep, exercise, and age affect waste evacuation? In collaboration with Nedergaard, Ali Erturk, John H. Thomas, Jessica Shang, Giuilio Tononi, and Chiara Cirelli, we are using experiments and simulations to attack these questions and more. In one project, we focus on links between glymphatic malfunction and Alzheimer's disease, which is known to correlate with waste buildup. In another, we focus on links between glymphatic function and cognition, to help soldiers stay alert even when sleep-deprived. We are grateful for support for this work through grants from the National Institutes for Health and Army Research Laboratories.
Above: Flow in the glymphatic system of the brain of a live mouse, measured with particle tracking. Left: The measured paths of particles flowing through the brain extend much further than the distances they would travel by diffusion alone (indicated with circles). Center: Each arrow indicates the position and velocity of one tracked particle. Waste flows through the space surrounding an artery. Right: Each dot indicates the speed of one tracked particle (in color).
Transcranial optical imaging reveals a pathway for optimizing the delivery of immunotherapeutics to the brain
Flow of cerebrospinal fluid is driven by arterial pulsations and is reduced in hypertension
PDGF-B is required for development of the glymphatic system
Hydraulic resistance of perivascular spaces in the brain
Mixing in Liquid Metal Batteries
We are studying the effects of fluid flow on the performance of liquid metal batteries, a technology being developed by Ambri, Inc. for large-scale energy storage on Earth's electrical grids. More broadly, we are interested in using fluid dynamics and mass transport to revolutionize energy storage technology, improve energy efficiency, and enable widespread use of renewable energy. We are grateful for support for this work through a CAREER Award from the Fluid Dynamics program of the National Science Foundation.
Above: Fluid flow in a liquid metal electrode, measured with ultrasound. Reds indicate flow away from the probe, and blues indicate flow toward it. Adding electrical current causes a global rearrangement of the convection pattern and makes mixing more efficient.
Mixing in a liquid metal electrode
Ultrasound velocity measurement in a liquid metal electrode
Molten amide-hydroxide-iodide electrolyte for a low-temperature sodium-based liquid metal battery
Convection-diffusion model of Lithium-Bismuth liquid metal batteries
Competing forces in liquid metal electrodes and batteries
Fluid mechanics of liquid metal batteries
More than 90% of U.S. manufactured goods contain cast metal components made with a process involving four steps: melting the metal feedstocks, degassing and controlling chemistry, distributing the melt, and solidifying it in casts. The commercial success and energy efficiency of the entire operation require controlling and predicting product quality while maximizing the rate of casting. Unfortunately, measuring the properties of the melt in-situ is difficult, and to date quality tests occur only after solidification. This project, a collaboration with Antoine Allanore at MIT, aims at providing a new tool to evaluate the melt properties using sound waves at very high frequency (ultrasound). The anticipated results are expected to enable real-time monitoring of casting and lead to new processing methods that will increase the commercial success and energy efficiency of casting. We are grateful for support for this work through a grant from the Materials Engineering and Processing program of the National Science Foundation.
Above: Our apparatus to enable ultrasound in metals casting allows us to apply sophisticated surface treatments to test plates, so that ultrasound passes through them easily and high-precision measurements become possible.
Ultrasound velocity measurement in a liquid metal electrode
Ion mixing in the inner ear
Incredibly sensitive, the mammalian cochlea can detect sounds more than a million times quieter than loud sounds causing pain. The cochlea gets its energy from a "silent current" of ions constantly leaking from one fluid chamber to another, which can work only if the chambers stay well-mixed. In collaboration with our colleague Jong-Hoon Nam, we hypothesize that molecular diffusion alone cannot keep the chambers mixed; instead, the cochlea stirs the chambers mechanically. We will test the hypothesis with laboratory models to complement Prof. Nam's simulations. We are grateful for support for this work through a grant from the Biomechanics and Mechanobiology program of the National Science Foundation.
Above: This experimental device will test our hypothesis that peristaltic motions can keep chambers within the mammalian cochlea well-mixed, in order to enable the silent current of ions that enables the cochlea's exquisite sensitivity.
Advection-Diffusion-Reaction in Earth's oceans
Fluid mixing of dye or other passive materials is complicated, but fairly well-studied. We are pushing further, to study mixing of materials that also react (or grow) at the same time. One important application is ocean flows, where growing plankton are mixed by currents and chemical spills react away as they are swept along.
Above: The Belousov-Zhabotinsky reaction causes a color change that spreads over time, even in stagnant fluid. Here we've used image processing to draw black curves along reaction fronts in a movie of a laboratory experiment, sped up by a factor of 25. We use B-Z to study reaction and growth in fluid flows.
Optimal stretching in advection-reaction-diffusion systems
Front tracking for quantifying advection-reaction-diffusion. Supplementary software: FrontTracking.zip
Optimal stretching in the wake of a bluff body
Front tracking velocimetry in advection-reaction-diffusion systems
Coherent Structures in mixing
To simplify the dizzyingly complex process of fluid mixing, scientists have long dreamed of “coherent structures”, regions or boundaries in the flow that could be tracked over time and would give a simplified—but still powerful—description of the mixing process. We work on this problem by considering Lagrangian Coherent Structures (LCS), clusters of tracer particles (multi-point correlators), and stretching and folding.
Above: In turbulence, Lagrangian Coherent Structures separate regions where energy is transferred to smaller length scales, from regions where energy is transferred to larger length scales. This movie shows LCS (white curves) and energy flux (reds and blues) calculated from a laboratory experiment and played at 1/5 speed.
Lagrangian Coherent Structures separate dynamically distinct regions in fluid flows
Spatiotemporal persistence of spectral fluxes in two-dimensional flow
Mechanisms driving shape distortion in two-dimensional flow
Separating stretching from folding in fluid mixing
Scale-dependent statistical geometry in two-dimensional flow
Three-dimensionality of one- and two-layer electromagnetically driven thin-layer flows
Comparing free surface and interface motion in electromagnetically driven thin-layer flows