Phononics: Structural dynamics of materials and implications to fluid dynamics, heat transfer, and beyond

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• Prof Mahmoud I. Hussein, Alvah and Harriet Hovlid Professor of Aerospace Engineering Sciences, University of Colorado Boulder
• Friday 03 February 2023, 14:00-15:00
• Department of Engineering - LR4.

If you have a question about this talk, please contact Hilde Hambro.

Phononics is an emerging field that seeks to elucidate the nature of intrinsic mechanical motion in both conventional and artificially structured materials, and uses this knowledge to extend the boundaries of physical response at either the material or structural/device level or both. The field bridges multiple disciplines across applied physics and engineering, and spans multiple scales reaching the atomic scale where a rigorous definition of phonons originates–quanta of lattice vibrations. In this talk, I will present two distinct contributions of phononics, one to the classical field of fluid dynamics and the other to the emerging field of nanoscale heat transfer. In both cases, intervention that causes critical changes in fundamental physical behaviour is demonstrated.

In fluid dynamics, I will show that phonon motion underneath a surface interacting with a flow may be engineered to cause the flow to stabilize, or destabilize, as desired [Hussein et al., Proc. R. Soc. A, 2015]. The underlying control mechanism utilizes the principle of destructive or constructive interferences and the notion of symmetry breaking, core concepts in phononics. This is realized by installing a “phononic subsurface” (PSub), which is an architectured periodic structure placed in the subsurface region and configured to extend all the way such that its edge is exposed to the flow, forming an elastic fluid-structure interface. I will present results showcasing perfectly synchronized, passive, and responsive, phased response and energy exchange between the elastic domain of a PSub and the perturbation (instability) field within an interfacing flow. One outcome of this state of response is delay of laminar-to-turbulent transition.

In heat transfer, I will present the concept of a locally resonant nanophononic metamaterial (NPM) [Davis and Hussein, Phys. Rev. Lett., 2014], of which one realization is a freestanding silicon membrane (thin film) with a periodic array of nanoscale pillars extruding out of one or both free surfaces. Heat is transported along the membrane portion of this nanostructured material as a succession of wavenumber-dependent propagating vibrational waves, phonons. The atoms making up the minuscule pillars, on their part, generate wavenumber-independent resonant vibrational waves, which we describe as vibrons. These two types of waves interact causing a mode coupling for each pair leading to (1) the generation of new modes localized in the nanopillar portion(s) and (2) the reduction of the base membrane phonon group velocities around the coupling regions. These effects take place across the full spectrum and bring rise to a unique form of heat conduction, namely, resonant thermal transport. The outcome is an inherent reduction in the in-plane thermal conductivity of the base membrane material.

PSubs provide a new paradigm of flow control for drag reduction in air, sea, and land vehicles, and NPMs offer a new route for high-efficiency solid-state thermoelectric energy conversion.

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Bio: Mahmoud I. Hussein is the Alvah and Harriet Hovlid Professor at the Smead Department of Aerospace Engineering Sciences at the University of Colorado Boulder. He holds a courtesy faculty appointment in the Department of Physics and an affiliate faculty appointment in the Department of Applied Mathematics, and he has formally served as the Engineering Faculty Director of the Pre-Engineering Program and the Program of Exploratory Studies. He received a BS degree from the American University in Cairo (1994) and MS degrees from Imperial College London (1995) and the University of Michigan‒Ann Arbor (1999, 2002). In 2004, he received a PhD degree from the University of Michigan‒Ann Arbor, after which he spent two years at the University of Cambridge as a postdoctoral research associate.

Dr. Hussein’s research focuses on the dynamics of materials and structures, especially phononic crystals and metamaterials, at both the continuum and atomistic scales. His research considers areas that range from vibrations and acoustics of engineering materials and structures and passive flow control to lattice dynamics and thermal transport in semiconductor-based nanostructured materials. His studies are concerned with physical phenomena governing these systems, associated theoretical and computational treatments, and analysis of relevant mechanisms such as dispersion, resonance, dissipation, and nonlinearity. His team also conducts experiments to support some aspects of the theoretical work.

Dr. Hussein received a DARPA Young Faculty Award in 2011, an NSF CAREER award in 2013, and in 2017 was honored with a Provost’s Faculty Achievement Award for Tenured Faculty at CU Boulder. He has co-edited a book titled Dynamics of Lattice Materials published by Wiley. He is a Fellow of ASME and has served as an associate editor for the ASME Journal of Vibration and Acoustics. In addition, he is the founding vice president of the International Phononics Society and has co-established the Phononics 20xx conference series which is widely viewed as the world’s premier event in the emerging field of phononics.

This talk is part of the Engineering - Mechanics Colloquia Research Seminars series.

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