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Events on Friday, December 9th, 2022

Thesis Defense
Physical structure of tooth enamel at the nano- and micro-scales, revealed by x-ray linear dichroism, and displayed by polarization-dependent imaging contrast mapping
Time: 10:30 am
Speaker: Cayla Stifler, Physics Graduate Student
Abstract: Tooth enamel is an extremely tough and wear resistant material, withstanding hundreds of Newtons of force every day during mastication. Enamel’s superior mechanical performance is especially important in animals that have particularly high biting forces or use their teeth to break hard materials like wood, nuts, bone, and shells. Enamel is hierarchical, meaning that it has different structures at different scales, from centimeter to Ångstrom. This hierarchy confers toughness and longevity to enamel, especially the micro- and nanoscale structure and crystal orientations. We developed Polarization-dependent Imaging Contrast (PIC) mapping of hydroxyapatite and fluorapatite (Ca10(PO4)6(OH)2) and Ca10(PO4)6F2), based on a physical effect we discovered: x-ray linear dichroism in all apatite crystals. With PIC mapping, we reveal for the first time the complex and diverse crystal orientations in enamel from modern and fossil animals, with nanoscale resolution. Crystal misorientation of adjacent pixels in PIC maps is converted to toughness, producing the first ever toughness maps. Surprisingly, T. rex, a dinosaur that had an extremely high biting force (15,000 N), has the least tough enamel of the 30 animals we measured. The toughest enamel is the saltwater crocodile, with the greatest biting force of all living animals (16,000 N).
Host: Pupa Gilbert
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Thesis Defense
New Frontiers in Collisionless Reconnection: Exploring Magnetosphere-Relevant Reconnection with Experiments and Custom Kinetic Simulations
Time: 1:00 pm
Place: B343 Sterling or
Speaker: Samuel Greess, Physics Graduate Student
Abstract: Magnetic reconnection is a ubiquitous phenomenon throughout the universe, but in terms of proximity, its occurrence at the day-side magnetopause is the instance that is closest to Earth both spatially and in importance to human life. At the day-side magnetopause, the solar magnetic field reconnects with the magnetic field of the Earth, beginning the process that results in the transfer of energized solar wind particles into the Earth's upper atmosphere. Usually, the result of these incursions is only the ethereal beauty of the auroras (borealis and australis); however, larger quantities of incident plasma can and have had devastating effects on terrestrial and space-based electronic systems. Predicting these geomagnetic storm events depends on an understanding of both how and when large quantities of plasma are emitted from the Sun (also a reconnection-based event) and how long it will take for these particles to enter the Earth's atmosphere via the magnetopause reconnection process. To that end, in addition to satellite missions created to measure the in situ process, experiments and simulations here on Earth are studying reconnection in the relevant parameter regimes, particularly in plasmas whose collisionality is low enough to mimic the space environment. One such experiment is the Terrestrial Reconnection EXperiment (TREX), which is based as the University of Wisconsin-Madison as a partner of the Wisconsin Plasma Physics Laboratory (WiPPL) collaborative research facility. TREX is designed to access the kinetic regime, which is typified by thin current layers, anisotropic pressure distributions, and fast reconnection. In conjunction with TREX, the newly developed Cylindrical VPIC (Vectorized Particle-in-Cell) code from Los Alamos National Laboratory has been used to simulate TREX in manner that preserves the experiment's cylindrical symmetry while optimizing computational efficiency. Different modified versions of the basic TREX VPIC setup have been successfully used to confirm and complement experimental findings, as well as to investigate plasma regimes the experiment cannot (presently) reach and to model different proposed TREX drive coil geometries. This thesis will present work from both the TREX laboratory and TREX VPIC simulations, with an emphasis on comparing the measured properties of reconnection in both scenarios and demonstrating how these data align with theoretical predictions about the kinetic reconnection parameter regime. Significant background to the construction and operation of TREX, Cylindrical VPIC, and relevant portions of the WiPPL facility will also be included.
Host: Jan Egedal
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