At relatively low polymer concentrations, granular hydrogel particles, commonly called microgels, undergo a jamming transition and behave like a soft elastic solid with reversible yielding. Over the past decade, these microgel packings have been utilized as sacrificial scaffolds to enable studies of cell behavior in a controlled 3D-environment. Here, cells can either be randomly dispersed within the microgel packings or precisely structured into well-controlled geometric shapes using 3D-printing to systematically study cell migration and invasion, differentiation pathways, collective cell behavior, and tissue/organoid development in 3D. Similar to 2D cell assays, the behavior of cells in 3D depends on the material properties of their microenvironment. Thus, controlling the macroscopic elastic behavior and the interstitial pore structures of microgel packings is critical for their application as a 3D scaffolds. Here, we present our recent progress in understanding the origins of elasticity in granular microgel packings. We prepare charged polyelectrolyte microgels with varying charge density to investigate the effects salts have on elasticity of microgel packings and relate their behavior to classic polymer physics scaling laws. In addition, we investigate the effects crosslinking concentrations have on the bulk rheological properties and the interstitial pore structure of concentrated packings of uncharged polyacrylamide microgel particles at the onset of jamming. Finally, we share our recent efforts in designing microgel particles as a transparent soil medium to enable spatial temporal studies of plant roots and plant-microbe interactions within rhizosphere. 

Accelerating the development and deployment of advanced energy systems is crucial in an increasingly “energy-hunger” world, especially with the advent of AI era. In today’s U.S. energy market, nuclear power accounts for 20% of the total electricity generation capacity and half of its clean energy production. Therefore, further enhancing the safety and economic efficiency of both existing and future nuclear systems is pivotal for achieving our national energy demand. The swift advancement of advanced nuclear technologies has heightened the need for precise, spatiotemporally resolved, multi-scale multi-physics coupled fundamental modeling of fluid flow and heat transfer, along with numerically robust, reliable, and efficient analysis to support the lifecycle of nuclear power plants – both for the legacy and new constructions. The more in-depth understanding of the phenomenological behavior of fluid flow and heat transfer allows for a more cost-effective implementation of new innovations. 

In the present work, leveraging the high-resolution two-phase flow experimental data uniquely obtained from the RBHT reflood test facility in collaboration with U.S. Nuclear Regulatory Commission (NRC), detailed physics-based theoretical modeling, comprehensive numerical analyses and code validations are performed to understand the fuel-to-coolant thermal-hydraulic responses during quenching transients. Valuable insights obtained from these analyses are further enhanced through high-resolution visualization study and physics-based modeling of two-phase flow interface behavior under natural-circulation film boiling conditions, utilizing a small-scale quench test facility as well as extensive system-scale numerical simulations. The use of advanced image processing techniques allows an in-depth investigation to probe the boundary-layer scale as well as to facilitate the development of sophisticated theoretical flow/heat transfer models for code applications. The present work presents a thorough investigation of the transient fluid flow and heat transfer behaviors, as well as reactor thermal-hydraulic responses, thus extending the boundaries of our current knowledge. The experimental and modeling techniques that we are developing and demonstrating provide an exciting avenue for high-spaciotemporal resolution flow and heat transfer characterization. The results obtained are particularly valuable for the future development of thermal-fluid models and for numerical code validation, ultimately supporting the sustainability of nuclear energy. 

TBD

Abstract: Advances in supercomputing capabilities and electronic structure calculations based on density-functional theory (DFT) now make it possible to design materials with new properties starting from the atomic scale and guide their experimental synthesis and characterization. Concurrent advances in scanning transmission electron microscopy (STEM) enable imaging and spectroscopy of materials with unprecedent spatial and energy resolution. Naturally then, the combination of theory and microscopy provides an unparalleled probe to unravel the atomic-scale structure-property correlations in complex materials with defects and disorder. In this presentation, I will discuss my group’s efforts to develop new ferroic materials with defects and disorder for energy and optical applications. Examples will include the discovery of chalcogenide perovskites with colossal optical anisotropy [1,2], unraveling the origin of ferroelectricity in nanoscale hafnia [3]; and the realization of a relatively understudied class of multiferroic that combines ferroelectricity and chirality [4]. 

Bio: Rohan Mishra is an Associate Professor of Mechanical Engineering & Materials Science, and Physics (by courtesy) at Washington University in St. Louis. He is also an affiliate faculty at the Institute of Materials Science & Engineering at Washington University, where he serves as the Director of Graduate Studies. From 2012-2015, he was a postdoctoral researcher in the Scanning Transmission Electron Microscopy group at Oak Ridge National Laboratory with a joint-affiliation from the Department of Physics at Vanderbilt University. He has a Bachelor in Technology in Metallurgical and Materials Engineering from National Institute of Technology Karnataka in India (2008) and a PhD in Materials Science and Engineering from The Ohio State University (2012). He leads the Materials Modeling and Microscopy group (mcube.wustl.edu) that works on establishing quantitative structure-property correlations in materials using a synergistic combination of electronic structure theory and electron microscopy. Their end goal is the rational design of materials with properties tailored for various energy applications. Mishra has coauthored over 100 journal articles. He received the NSF CAREER award in 2022.

References:

[1]          B. Zhao et al., Advanced Materials 36, e2311559 (2024).

[2]          H. Mei et al., Advanced Materials 35, e2303588 (2023).

[3]          X. Li et al., 2024), p. arXiv:2408.01830.

[4]          G. Ren et al., Advanced Functional Materials  (2024).

Abstract:

"Mixed-dimensional heterostructures are a unique class of material combining components of distinct dimensionalities through van der Waals interactions, e.g., zero-dimensional organic molecules self-assembled on two-dimensional (2D) materials. In addition to the versatile tunability of organic molecules and advantageous properties of 2D materials, the interface coupling has demonstrated exotic electronic and excitonic properties beyond those of the individual component, promising for next-generation electronic and optoelectronic applications. Here I will introduce some intriguing interfacial phenomena in mixed-dimensional heterostructures, our understanding on the physics and their potential applications. I will show that organic molecules and interlayer coupling provides an incredible avenue to engineer the band structure and interlayer exciton dynamics in mixed-dimensional heterostructures, such as that consisting of titanyl phthalocyanine molecules and monolayer MoS2. Finally, I will introduce a computational approach for the heterogeneous and incomplete screening at the interface for mixed-dimensional heterostructures, advancing our understanding and capabilities in this emerging field."

TBA

Somewhat to my surprise, two independent research projects we are pursuing take place in or with suspended atomically thin membranes. In the first case, we will review thermal transport measurements inspired by a search for physics of the Kitaev quantum spin liquid; and for the second, we'll show how suspended membranes can lead to quantum sensing of rotations with remarkable precision, sufficient for lab-scale sensing of gravitation and variations in the length of a day. 

In contemporary condensed matter physics and photonics, four length scales are fundamentally interesting and intertwined: 1) Polaritonic wavelength λ in infrared (IR) and terahertz (THz) frequencies ω (e.g. plasmon, phonon, exciton, or magnon polaritons), which defines the scale of the light confinement and light-matter interaction; 2) Magnetic lengths  l_B =√(ℏ/eB)=257Å/√(B[T]), (with B the magnetic field), which defines the restricted electron motion in a B field; 3) Diffusion length D of the hot carriers at interfaces and the edges, which defines the scale of energy relaxation, and 4) Periodicities of superlattices induced by moiré engineering, which defines the energy scale of emerging quantum phases. In this talk, I will report 1) A new type of optical near-field nanoscopy technique (Landau level nanoscopy) to tackle all four above-mentioned ‘lengths’ simultaneously; 2) A new type of infrared polaritons that can be tuned via magnetic field; 3) A nanoscale probe of the many-body physics through the excitations of magnetoexcitons in graphene across the allowed and forbidden optical transitions. Our approach establishes the Landau-level nanoscopy as a versatile platform for exploring magneto-optical effects at the nanoscale. Our preliminary research also sets the stage for future spectroscopic investigations of the topological and chiral photonic phenomena in complex quantum materials using low-energy photons.

In this talk, we will discuss experimental and theoretical results that suggest the occurrence of a new type of quasi-particle in nanomagnets. The persistent quasi-particle kinetics results into a highly viscous magnetic fluid state in two-dimensional nanomagnetic lattice. We argue that the vortex-shaped quasi-particle can be detected in any nanomagnet with geometrical dimension smaller than a characteristic length e.g. domain wall or coherence. The finding is expected to have implication to the design of next generation spintronic devices.

Two dimensional (2D) nanomaterials have been intensively investigated as emerging materials for future devices, including electronics, photonics, and electrochemical energy storage devices. The mechanical stability of each 2D component is critical to the reliability of the fabricated devices. Currently, research on experimental mechanics of 2D materials has been focused on quantifying mechanical properties and understanding fracture behaviors using different techniques. Confined to 2D geometry, cracks in 2D materials generally favor a brittle behavior with minimum plasticity at room temperature, which continues the dilemma of mutually exclusive fracture toughness and mechanical strength in bulk materials.

Considerable research has been devoted to improving fracture toughness of 2D materials. For example, carbon nanotubes (CNTs) were integrated into graphene as an extrinsic toughening strategy. The fabricated rebar graphene displays a zigzag fracture surface, guided and redirected by the embedded CNTs. Such toughening mechanism is similar to improving fracture resistance extrinsically by introducing fiber/lamella bridging, oxide wedging, transformation toughening, etc. In addition to rebar graphene, h-BN has been carefully investigated as it has the same structure of graphene but is composed of two elements. The fracture behavior of monolayer single crystalline h-BN has long been taken as an ideal brittle material subject to Griffith’s law. By combining computational analysis and in situ tensile test, the monolayer h-BN has an exceptionally high fracture toughness. The crack deflection and branching occur repeatedly due to asymmetric edge elastic properties at the crack tip and edge swapping during crack propagation, which toughens h-BN tremendously and enables stable crack propagation not seen in graphene.

Abstract: The physics of low-dimensional magnetic systems has gained significant global attention over the last decade. Especially, two-dimensional (2D) layered magnetic systems are of present research interest due to their unusual magnetic properties, arising from the reduction in magnetic dimensionality and consequently, the geometrical spin frustrations. Such magnetic states are highly sensitive to the underlying magnetic lattice geometry. The talk will delve into the diverse magnetic properties of layered transition metal oxide compounds having variety of 2D magnetic lattices, viz., (a) triangular lattice [Na3Fe(PO4)2], (b) maple leaf lattice (Na2Mn3O7), and (c) honeycomb lattice [A2Ni2TeO6(A=Na/Li)] etc. The origin of unique long-range magnetic ground states, magnetic excitations in view of the spin-Hamiltonian of the system, 2D short range magnetic ordering, etc will be discussed. It will be demonstrated that how different types of 2D magnetic lattices (triangular lattice, maple leaf lattice, and honeycomb lattice) and their distortions result variations in geometrical spin frustrations, leading to the possibility of multiple spin structures. 

Furthermore, the layered materials are chosen in such a way that the magnetic layers are well-separated by the non-magnetic alkali-metal ions (A=Li/Na/K) alone. Such layered materials provide high ionic conduction and improved intercalation/de-intercalation properties, making them suitable for battery applications. The talk will discuss the role of underlying crystal structure on the ionic conduction properties within the context of functional battery applications.