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1. Programmable viscosity metamaterials: Designing fluid properties using temporal superposition of shear and acoustics

   https://doi.org/10.1103/PhysRevResearch.6.043107  

   Sehgal, Prateek; Ramaswamy, Meera; Ong, Edward_Y X; Ness, Christopher; Cohen, Itai; Kirby, Brian J (November 2024, Physical Review Research)

   Metamaterials are composite structures whose extraordinary properties arise from a mesoscale organization of their constituents. Here, we introduce a different material class—viscosity metafluids. Specifically, we demonstrate that we can rapidly drive large viscosity oscillations in shear-thickened fluids using acoustic perturbations with kHz to MHz frequencies. Because the timescale for these oscillations can be orders of magnitude smaller than the timescales associated with the global material flow, we can construct metafluids whose resulting time-averaged viscosity is a composite of the thickened, high-viscosity and dethickened, low-viscosity states. We show that viscosity metafluids can be used to engineer a variety of unique properties including zero, infinite, and negative viscosities. The high degree of control over the resulting viscosity, the ease with which they can be accessed, and the variety of exotic properties achievable make viscosity metafluids attractive for uses in technologies ranging from coatings to cloaking to 3D printing. Published by the American Physical Society2024 

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2. Fermionic tensor network contraction for arbitrary geometries

   https://doi.org/10.1103/PhysRevResearch.7.023193  

   Gao, Yang; Zhai, Huanchen; Gray, Johnnie; Peng, Ruojing; Park, Gunhee; Liu, Wen-Yuan; Kjønstad, Eirik F; Chan, Garnet Kin-Lic (May 2025, Physical Review Research)

   We describe our implementation of fermionic tensor network contraction on arbitrary lattices within both a globally ordered and a locally ordered formalism. We provide a pedagogical description of these two conventions as implemented for the quimb library. Using hyperoptimized approximate contraction strategies, we present benchmark fermionic projected entangled pair state simulations of finite Hubbard models defined on the three-dimensional diamond lattice and random regular graphs. Published by the American Physical Society2025 

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3. Measurement of pressure gradients near the interface in the viscous fingering instability

   https://doi.org/10.1103/PhysRevFluids.9.053902  

   Gowen, Savannah D; Videbæk, Thomas E; Nagel, Sidney R (May 2024, Physical Review Fluids)

   The viscous fingering instability, which forms when a less-viscous fluid invades a more-viscous one within a confined geometry, is an iconic system for studying pattern formation. For both miscible and immiscible fluid pairs the growth dynamics change after the initial instability onset and the global structures, typical of late-time growth, are governed by the viscosity ratio. Here we introduce an experimental technique to measure flow throughout the inner and outer fluids. This probes the existence of a new length scale associated with the local pressure gradients around the interface and allows us to compare our results to the predictions of a previously proposed model for late-time finger growth. Published by the American Physical Society2024 

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4. Pattern formation in acoustically levitated particle systems with competing near-field interactions

   https://doi.org/10.1103/PhysRevResearch.7.023017  

   Wu, Brady; Esposito, Edward P; Mao, Qinghao; Jaeger, Heinrich M (April 2025, Physical Review Research)

   Acoustic levitation in air provides a containerless, gravity-free platform for investigating driven many-particle systems with nonconservative interactions and underdamped dynamics. In prior work the interactions among levitated particles were limited to attractive forces from scattered sound and repulsion from hydrodynamic microstreaming. We report on experiments in which contact cohesion provides a third type of interaction. When particle size and separation are both much smaller than the sound wavelength, this interplay of three interactions results in forces that are attractive over several particle diameters, become repulsive at close approach, and are again attractive at contact. In the presence of sound-induced athermal fluctuations that generate particle collisions, the interplay of these three forces enables the formation of particle chains with anisotropic interactions that depend on chain size and shape due to multibody effects. With the control of the kinetic pathways and the strength of the contact cohesion, different patterns can be assembled, from triangular lattices to labyrinthine patterns of chains to lacelike networks of interconnected rings. These results shed light on the multibody character of acoustic interactions and can be utilized to direct the self-assembly of particles. Published by the American Physical Society2025 

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5. Surface acoustic waves and lattice vibrations in two-dimensional ${\mathrm{Ti}}_3{\mathrm{C}}_2{T}_x$ $(T=\mathrm{O}, \mathrm{F})$ MXene films

   https://doi.org/10.1103/PhysRevB.111.214103  

   Hamilton, Parker; Kalia, Rajiv; Wixom, Ryan; Dingreville, Rémi (June 2025, Physical Review B)

   We investigated surface acoustic wave (SAW) propagation and lattice vibrations in two-dimensional (2D) titanium carbide $\left({\mathrm{Ti}}_3{\mathrm{C}}_2{T}_{\mathrm{x}}\right)$MXene films as a function of surface termination and layer stacking, using atomistic simulations. We found that SAW propagation velocity is highly sensitive to both single-layer properties and interlayer bonding. Surface terminations significantly modulate wave behavior, with oxygen and fluorine terminations producing distinct effects on wave propagation, with oxygen-terminated monolayers exhibiting 20% higher wave speeds than fluorine counterparts due to strengthened intralayer bonds. Key observations include the transition from one to two layers causing wave speed variations, and the development of interlayer modes that generate more dispersed lattice vibrations. As the film layer thickness increases, SAW propagation becomes predominantly confined to the upper surface, with coherence of vibrational modes diminishing in multilayer structures. These findings suggest MXene terminations and layer stacking are crucial parameters for controlling SAW behavior, offering promising avenues for novel acoustic wave device applications. Published by the American Physical Society2025 

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