Mahmoud I. Hussein, Ph.D.
Alvah and Harriet Hovlid Professor
Smead Department of Aerospace Engineering Sciences
University of Colorado Boulder

Friday, Jan. 31st at 3:00pm
MEK 3550

ABSTRACT: The phenomenon of resonance is central to structural dynamics, impacting a wide range of applications in engineering such as oscillations of buildings shaken by an earthquake (in Hz) and vibrations of aircraft wings subject to aerodynamic boundary-layer forces (in kHz). Two key characteristics associated with a resonance are the natural frequency and mode shape.

A decade ago, we proposed the engineering of intrinsic nanoscale substructures over ultrathin membranes to alter the thermal properties of the underlying crystalline material [1]. These miniature substructures exhibit resonances at extremely high frequencies, in the THz, such that they interfere with the anharmonic oscillations of the atoms comprising the host material. We have shown that such coupling, between the natural vibrations of atoms on the one hand and the resonances of the engineered nanostructures on the other, causes fundamental changes to the lattice thermal conductivity. These nanoscale dynamical effects were elucidated by conducting molecular dynamics simulations followed by Fourier analysis of the atomic velocity field, yielding an anharmonic dispersion diagram characterizing the nature of wave propagation within a nonlinear thermalized environment [2].

Recently, we supplemented these early investigations with equilibrium molecular dynamics simulations where wave-packet excitations are introduced at the resonance frequencies to reveal the emergence of phonon localization [3]. The deterministic and subwavelength nature of this new form of phonon localization offers superior attributes compared to the classical form of Anderson localization due to disorder.

All these calculations, however, fall short of elucidating the nature of thermal evolution in the presence of the resonances. Here we present an analysis framework for the characterization of the entropic signature of the mechanism induced by the nanoresonators, using nonequilibrium atomistic simulations [4]. Specifically, we examine the effects of the underlying local phonon resonances in room-temperature nanostructured silicon, and quantify how these resonances modify the rate of entropy production and thermal relaxation. We reveal that the presence of the resonances spanning the full spectrum enables a highly ordered regime of heat conduction, where irreversible evolution and entropy maximization are severely hindered by extensive mode hybridizations. This behavior points to a most intriguing natural competition between the structural dynamics and the statistical thermodynamics, where deterministic vibrations (the former) are shown to slow down the path to thermal equilibrium (the latter).

This research was done in collaboration with A. Beardo, C.-N. Tsai, and P. Rawte, among others.

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 has previously 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. He 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 was awarded as PI two large-scale grants, both on concepts he discovered—nanophononic metamaterials (NPMs, Phys. Rev. Lett., 2014; ARPA-E, $3 million, 2019-2023) and phononic subsurfaces (PSubs, Proc. R. Soc. A, 2015; ONR MURI, $7.5 million, 2024-2029). 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 biennial Phononics 20xx conference series which has helped create a new multidisciplinary research community and is widely viewed as the world’s premier event in the emerging field of phononics.

References:

[1] Davis, B.L. and Hussein, M.I., Phys. Rev. Lett. 112, 055505 (2014)

[2] Hussein, M.I., Tsai, C.-N., and Honarvar, H., Adv. Func. Materials 30, 1906718 (2020)

[3] Beardo, A., Desmarchelier, P., Tsai, C.N., Rawte, P., Termentzidis, K. and Hussein, M.I., “Resonant phonons: Localization in structurally ordered crystals,” Phys. Rev. B 110, 195438 (2024)

[4] Beardo, A., Rawte, P., Tsai, C.N. and Hussein, M.I., “Entropic signature of resonant thermal transport: Ordered form of heat conduction,” arXiv:2404.15831 (2024)