Ultrasonically Levitated Granular Matter

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Non-Technical Abstract:The better understanding of granular matter, a class of materials comprised of large numbers of particles that collide and aggregate, is important for a wide range of industrial processes as well as natural phenomena, including the very beginning of planet formation from interstellar dust. The project introduces a new experimental platform, in which small particles are levitated acoustically, their arrangement can be manipulated under computer control, and their interactions can be measured with precision. The detailed investigation of such interactions among particles addresses many fundamental problems in physics and materials science, and improved knowledge about aggregate formation opens new opportunities for better control of industrial processes that involve the handling of fine particles. The project trains one graduate student, one postdoctoral scholar, and several undergraduate students, introduces them to forefront issues in materials research, and involves them in science outreach activities. Technical Abstract:The project focuses on sub-millimeter scale particles, which can easily become charged and as a result exhibit a rich interplay of short-range contact forces and longer-ranged electrostatic forces. To measure these forces with precision while providing a means for controlled particle manipulation, ultrasonic levitation generates a stable environment in which gravity is balanced by acoustic pressure so that minute particle-particle interactions become observable. A special aspect is the implementation of a sound pressure field that can be changed in real time under computer control. This enables experiments that previously have not been possible, such as the measurement of charge transfer during repeated particle-particle collisions. Suitable acoustic potentials also can produce lattices of particles of the same charge polarity, similar to Coulomb crystals in dusty plasmas. Importantly, ensembles of macroscopic particles are easily driven into the strongly interacting regime, where the interaction potential far exceeds the ambient thermal energy. Two thrusts of the project develop a programmable acoustic cavity for multi-particle trapping and manipulation, and investigate the collective behavior of systems comprised of large numbers of levitated particles.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.