Topological Elastic Metamaterials: An introduction and application to the analog Quantum Spin Hall Effect

Date: April 25, 2025; Time: 2:30 PM Location: PWEB 175

Abstract: Inspired by recent discoveries of topological phases of matter in quantum physics, there has been a rapidly growing research effort to uncover analog mechanisms in classical wave physics, including acoustics and elastodynamics. By properly acting on time reversal, chiral, and particle-hole symmetries, material systems obeying the laws of classical mechanics can deliver dispersion properties reminiscent of selected quantum mechanical systems. Among the many remarkable characteristics of these materials, their ability to support unidirectional propagating waves is particularly significant and it could serve as a foundational property to achieve waveguides that are robust even in presence of disorder and defects. This talk will discuss the general concept of a topological elastic metamaterial, the foundational role played by the geometric phase, and finally present a selected example of a topological elastic metamaterial from recent works conducted in Prof. Semperlotti’s group. More specifically, a topological elastic waveguide inspired by a mechanism analog to the quantum Spin Hall effect will be discussed. A combination of theoretical, numerical, and experimental results will be presented to illustrate how unidirectional propagating guided modes can be achieved at the interface between elastic material phases having different topological order. These so-called edge states are topologically protected against backscattering, hence allowing efficient elastic energy transmission even in presence of defects and disorder. Such unique propagation properties could have a profound impact on the development of many real-world applications and on the performance of practical devices whose operating mechanism is rooted in the physics of acoustic and elastic waves.

Biographical Sketch: Dr. Fabio Semperlotti is a Professor in the School of Mechanical Engineering and the Perry Academic Excellence Scholar at Purdue University; he also holds a courtesy appointment in the School of Aeronautics and Astronautics Engineering. He directs the Structural Health Monitoring and Dynamics laboratory (SHMD) where he conducts, together with his group, research on several aspects of structures and materials design including structural dynamics and wave propagation, elastic metamaterials, structural health monitoring, and computational and experimental mechanics. His research has received financial support from a variety of sources including the National Science Foundation, the Department of Defense, the Department of Energy, and industrial sponsors. Dr. Semperlotti was the recipient of the National Science Foundation CAREER award (2015), the Air Force Office of Scientific Research Young Investigator Program (YIP) (2015), the DARPA Young Faculty Award (YFA) 2019, and the ASME C.D. Mote Jr. Early Career Award 2019. Dr. Semperlotti received a M.S. in Aerospace Engineering (2000), and a M.S. in Astronautic Engineering (2002) both from the University of Rome “La Sapienza” (Italy), and a Ph.D. in Aerospace engineering (2009) from the Pennsylvania State University (USA). In 2010, he was a postdoctoral research associate in the Mechanical Engineering department at the University of Michigan. Prior to joining Penn State, Dr. Semperlotti served as a structural engineer for a few European aerospace industries, including the French Space Agency (CNES), working on the structural design of space launch systems and satellite platforms.

PM_2.5 in Health and Atmospheric Science

Date: April 11, 2025; Time: 2:30 PM Location: PWEB 175

Abstract: Air pollution has been a problem as long as there have been cities.  Major air pollution disasters in the mid-20^th century prompted efforts to understand and control air quality.  Episodes in Europe and the eastern USA were linked to primary emissions from coal combustion and heavy industries, while those in the western USA, especially the infamous Los Angeles smog involved photochemical generation of secondary pollutants, both gases like ozone and the smog that degraded visibility as the day progressed.   A study of the smog aerosol revealed characteristics that, when combined with efforts to understand the health impacts of inhaled particles, formed the foundation for the first regulations regulate airborne particulate matter concentrations, ultimately leading to PM_2.5 as the primary particulate air quality standard.  PM_2.5, the mass concentration of particles smaller than about 2.5 µm, describes the fine particles that can penetrate to the lower airways, including the alveolar region, when inhaled.  This seminar will explore this history and examples of its impact, including the recent southern California fires.  Since PM_2.5 first appeared in the scientific literature, PM_2.5 has been referenced in more than 50,000 papers.  With this prominence, many studies limit their focus to PM_2.5, or to chemical composition, black carbon, and other properties of that size fraction.  Health studies report exacerbations of respiratory and cardiovascular health effects close to roadways, and some suggest that a common suspect, black carbon, is not the culprit, but that ultrafine particles (< 100 nm diameter) may be responsible, but the mass concentration of such particles is usually too small to be detectable within PM_2.5.  The exclusion of these small particles, and of larger ones, also means that the growing PM_2.5 bias in atmospheric particle measurements misses parts particles that govern atmospheric aerosol dynamics, confounding efforts to understand even PM_2.5.  We shall, therefore, also examine some of the limitations of PM_2.5, and highlight research opportunities that this bias creates.

Biographical Sketch: Richard Flagan is the Irma and Ross McCollum/William H Corcoran Professor of Chemical Engineering and Environmental Science and Engineering at the California Institute of Technology.  He received his BS in Mechanical Engineering from the University of Michigan, and his SM and PhD from MIT, also in Mechanical Engineering.  He joined the Environmental Engineering Science department of Caltech after a couple of years as a postdoc and Lecturer at MIT.  Although his PhD research with John Appleton in Mechanical Engineering, and postdoctoral studies were with Adel Sarofim and John Heywood in Chemical Engineering, Flagan shifted his focus to aerosols upon joining the Caltech Faculty.  Primarily an experimentalist, his contributions include numerous advances in aerosol measurements, particularly in the nanoparticle regime with the first low pressure impactor, the scanning mobility particle sizer (SMPS), and others.  He pioneered chamber studies that determined aerosol yields of a wide range of anthropogenic and biogenic hydrocarbons, and the SMPS to measure size distributions of nanoparticles in airborne measurements.   Flagan has published over 420 papers, and holds 28 patents.  Though out of print, his textbook has been downloaded over 350,000 times.  Flagan has received numerous awards, including the Fuchs Memorial Award of the International Aerosol Research Assembly, the highest award in the field of aerosol science, and the Haagen-Smit Clean Air Award from the California Air Resources Board.  He is a member of the National Academy of Engineering, and has received two honorary doctorates.  He has served as Chair of the Faculty at Caltech, and is a member of the Board of Directors of the California Council on Science and Technology.