Acoustic/Elastic Sensing with Optical Breakdown Shocks
When managing ships or other ocean infrastructure, it is important to monitor the health of structures and to identify unknown objects on the surface. These types of inspection tasks currently require significant time and money to carry out, largely because the relevant sensors must make contact with the inspection target. In order to enable faster and more thorough inspections, I developed an acoustic sensing technique that uses a high-power laser to create sound at controllable points in the water, through a process called optical breakdown. By listening to how the short optical breakdown pulse scatters off of underwater objects, information about the material's composition and geometry can be calculated. This technique provides a new approach for underwater vehicles to remotely map structural and material properties along an underwater object.
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Vortex Interactions for Underwater Propulsion
Underwater robots can maneuver more precisely if they use pulsed jets instead of propellers to generate thrust. But in situations where multiple jets are needed - like when a robot needs to dock to a structure, or when a swarm of robots need to coordinate their activities - it is unclear how to best coordinate all the jets involved. In order to design and control underwater robots for efficient motion, it is critical to understand how the jets' wakes can interact, and how these interactions affect the thrusters' performance. To do this, I built an experiment to measure thruster performance when pulsed jets are ejected simultaneously. As the jets develop, vortex rings are formed in the wake; when the jets are close enough, the vortex interactions reduce thruster performance. To explain this behavior, I developed a physical model that agrees well with the data. My experiments show how some pulsing strategies can hurt performance, suggesting new strategies that leverage the vortex interactions to improve performance.
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Accessible Sleep Apnea Screening Device
Between 7 and 18 million Americans suffer from sleep disordered breathing (SDB), including those who suffer from obstructive sleep apnea (OSA). Despite this high prevalence of SDB, existing diagnostic techniques remain impractical for widespread screening. Furthermore, once a diagnosis is made, there are no readily available tools for routine monitoring of treatment efficacy on a longitudinal basis.
As part of the ReDx initiative, I mentored a team in Mumbai to develop a wearable sleep mask with sensors for at-home screening and longitudinal monitoring of patients at risk for OSA. We developed a few prototypes and ran an early round of clinical testing to compare the performance of our device with existing at-home monitors.
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Interfacial Flow Patterns in Wine Tears
In this quick experiment, Karim Khalil and I looked into the complex flow patterns within the tears of wine. While it has been known that alcohol evaporation drives interfacial flows within wine, producing well know 'tearing' instability, the structure of these flows has not been studied. Interesting phenomena have previously been observed, such as the 'nipping' and recoil of wine tears as they contact the main body of fluid. We capture these phenomena, and provide the first demonstration that there is relatively large-scale transport within the tears themselves!
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X-Ray Imaging of Granular Packings During Compression
To directly investigate how particle shape and particle interactions affect the mechanical properties of granular packings, I designed and built an x-ray tomography system that is able to image the 3D details of a granular packing while it is compressed in an Instron materials tester. This way, a packing's mechanical response (measured by the Instron) can be directly related to the internal geometry and rearrangements of grains.
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Granular Mechanics & Particle Shape
When you load a pile of sand (whether in the lab, at the beach, or on a construction site), the packing will slowly weaken until a slip-plane forms and the system cannot support the stress. However, if the shape of the grains is changed, the packing's behavior can change drastically, like we saw with the chains. An important question for designing granular systems is how the choice of grain shape will impact the mechanical response. To understand this relationship, we 3D-printed tens of thousands of mm-scale grains in 14 different shapes, and measured how bulk packings of each shape responded to mechanical compression. In addition to the broad collection of characteristic responses that we collected, we observed that local interactions between grains makes some packings more sensitive to confining pressure, suggesting that certain shapes can provide a more widely-tunable response for dynamic applications.
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Entanglement in Granular Chains
When a pile of chains is compressed, it will sometimes exhibit the unusual behavior of getting stronger the more it is compressed. This behavior is known as 'strain stiffening.' To figure out what causes strain stiffening in chains, we collected 3D x-ray measurements of the internal structure of chain packings at different stages during compression. These measurements revealed that the packings' increasing strength comes from interlocked clusters of chains that span the system. Moreover, if the chains are too short, they do not form large clusters, and we observed a corresponding loss of the strain-stiffening behavior.
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