Monday, October 27, 2025

3:00 PM LR3

Zoom Link: https://northwestern.zoom.us/j/99215996751

ABSTRACT

My research group explores high-resolution optical techniques to investigate elastic wave and heat transport phenomena in condensed matter. In the first half of my talk, I will describe our work on thermal conductivity imaging near single grain boundaries (GBs) in thermoelectric materials. GBs are critical microstructural components that control the performance of thermoelectric materials by reducing the bulk thermal conductivity through the scattering of lattice waves. To date, most GB-thermal conductivity studies have primarily focused on grain size as a fundamental structural property that is important for reducing thermal conductivity. However, it is unclear if the right strategy for optimizing the thermoelectric performance is suppressing the thermal conductivity using small (or nanosized) grains or proliferating thermally resistive GBs. Addressing this fundamental question will be critical to developing next-generation thermoelectric generators for deep space exploration and commercial solid-state refrigerators for mobile phone and electric vehicle applications. In our work, we examine the impact of the GB morphology (e.g., misorientation angle, roughness of the GB plane, nanotwinning, porosity, etc.) on the local thermal conductivity suppression near individual GBs. Our measurements show that the "all GBs are the same" picture adopted in thermal conductivity homogenization modeling is not accurate. Alternatively, each GB is best described as a unique complexion that differs from the surrounding bulk phase. The GB complexion may be controlled by adjusting the thermodynamic parameters of processing methods, leading to complexion transitions similar to those in bulk materials, which can result in drastic changes in bulk transport properties and enhance thermoelectric performance. In the last half of my talk, I will present a dynamic optical coherence elastography (OCE) technique that relies on optical measurement of bulk shear wave propagation for the characterization and mapping of shear viscoelastic properties. My group has adapted the method for viscoelastic characterization of wastewater biofilm membranes and beads. These materials are heterogeneous and may be layered, leading to guided or interfacial elastic waves. I will discuss how we harnessed these wave modes to probe spatial variations in viscoelastic properties, track changes in pH and crosslinking time, and address the influence of stretch-dependent properties.

BIO

Oluwaseyi Balogun is an Associate Professor of Mechanical Engineering and Civil and Environmental Engineering at Northwestern University. He received the B.S. degree from the University of Lagos, Nigeria, and the M.S. and Ph.D. degrees from Boston University, all in Mechanical Engineering. Dr. Balogun's research focuses on micro- and nanoscale heat transport, elastic wave phenomena, and high-resolution optical and scanning probe microscopy. His research is relevant to applications that involve heat conduction in condensed matter, material characterization based on optical and elastic wave measurements, and high-frequency nanoacoustic devices. He currently serves as the co-director for the Center for Smart Structures and Materials at Northwestern University. He is a member of the IEEE UFFC and IEEE Nanotechnology Societies and a recipient of the 2020 & 2021 IEEE Nanotechnology Council Distinguished Lecturer awards.