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1. Discovery of two-dimensional binary nanoparticle superlattices using global Monte Carlo optimization

   https://doi.org/10.1038/s41467-022-35690-8  

   Zhou, Yilong; Arya, Gaurav (December 2022, Nature Communications)

   Abstract Binary nanoparticle (NP) superlattices exhibit distinct collective plasmonic, magnetic, optical, and electronic properties. Here, we computationally demonstrate how fluid-fluid interfaces could be used to self-assemble binary systems of NPs into 2D superlattices when the NP species exhibit different miscibility with the fluids forming the interface. We develop a basin-hopping Monte Carlo (BHMC) algorithm tailored for interface-trapped structures to rapidly determine the ground-state configuration of NPs, allowing us to explore the repertoire of binary NP architectures formed at the interface. By varying the NP size ratio, interparticle interaction strength, and difference in NP miscibility with the two fluids, we demonstrate the assembly of an array of exquisite 2D periodic architectures, including AB-, AB₂-, and AB₃-type monolayer superlattices as well as AB-, AB₂-, A₃B₅-, and A₄B₆-type bilayer superlattices. Our results suggest that the interfacial assembly approach could be a versatile platform for fabricating 2D colloidal superlattices with tunable structure and properties. 

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2. Interfacial flow around a pusher bacterium

   https://doi.org/10.1017/jfm.2023.905  

   Deng, Jiayi; Molaei, Mehdi; Chisholm, Nicholas G.; Stebe, Kathleen J. (December 2023, Journal of Fluid Mechanics)

   Motile bacteria play essential roles in biology that rely on their dynamic behaviours, including their ability to navigate, interact and self-organize. However, bacteria dynamics on fluid interfaces are not well understood. Swimmers adsorbed on fluid interfaces remain highly motile, and fluid interfaces are highly non-ideal domains that alter swimming behaviour. To understand these effects, we study flow fields generated byPseudomonas aeruginosaPA01 in the pusher mode. Analysis of correlated displacements of tracers and bacteria reveals dipolar flow fields with unexpected asymmetries that differ significantly from their counterparts in bulk fluids. We decompose the flow field into fundamental hydrodynamic modes for swimmers in incompressible fluid interfaces. We find an expected force-doublet mode corresponding to propulsion and drag at the interface plane, and a second dipolar mode, associated with forces exerted by the flagellum on the cell body in the aqueous phase that are countered by Marangoni stresses in the interface. The balance of these modes depends on the bacteria's trapped interfacial configurations. Understanding these flows is broadly important in nature and in the design of biomimetic swimmers. 

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3. Global Regularity for Gravity Unstable Muskat Bubbles

   https://doi.org/10.1090/memo/1455  

   Gancedo, Francisco; García-Juárez, Eduardo; Patel, Neel; Strain, Robert (December 2023, Memoirs of the American Mathematical Society)

   In this paper, we study the dynamics of fluids in porous media governed by Darcy’s law: the Muskat problem. We consider the setting of two immiscible fluids of different densities and viscosities under the influence of gravity in which one fluid is completely surrounded by the other. This setting is gravity unstable because along a portion of the interface, the denser fluid must be above the other. Surprisingly, even without capillarity, the circle-shaped bubble is a steady state solution moving with vertical constant velocity determined by the density jump between the fluids. Taking advantage of our discovery of this steady state, we are able to prove global in time existence and uniqueness of dynamic bubbles of nearly circular shapes under the influence of surface tension. We prove this global existence result for low regularity initial data. Moreover, we prove that these solutions are instantly analytic and decay exponentially fast in time to the circle. 

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4. Effects of misfit dislocations and dislocation mobility on thermal boundary resistance of PbTe/PbSe interfaces

   https://doi.org/10.1016/j.commatsci.2024.112828  

   Taormina, Nicholas; Li, Yang; Phillpot, Simon; Chen, Youping (February 2024, Computational Materials Science)

   We present a molecular dynamics study of the thermal transport properties of PbTe/PbSe (111) and PbTe/PbSe (100) interfaces at room temperature. The PbTe/PbSe heterostructures are obtained through simulations of the kinetic processes of direct bonding of PbTe and PbSe crystals. The atomic-scale dislocation core structures and the misfit dislocation networks in the heterostructures obtained in the simulations are found to closely match experimental data. Two types of heat transfer experiments are then simulated: a heat-sink heat-source experiment and an ultrashort heat pulse experiment. Thermal boundary resistance is calculated for three distinct interface types: coherent, semi-coherent, and semi-coherent with pinned dislocations. Both types of simulations consistently capture the significant role of the misfit dislocations on thermal resistance. The effect of the mobility of dislocations on thermal resistance is demonstrated for the first time through comparing the thermal boundary resistance of interfaces containing pinned dislocations and with those containing unpinned dislocations. In addition, the thermal boundary resistance is found to strongly depend on the length of the specimen and the area of the interface. 

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5. Interplay between dynamic heterogeneity and interfacial gradients in a model polymer film

   https://doi.org/10.1063/5.0165650  

   Hartley, Austin D; Drayer, William F; Ghanekarade, Asieh; Simmons, David S (November 2023, The Journal of Chemical Physics)

   Glass-forming liquids exhibit long-lived, spatially correlated dynamical heterogeneity, in which some nm-scale regions in the fluid relax more slowly than others. In the nanoscale vicinity of an interface, glass-formers also exhibit the emergence of massive interfacial gradients in glass transition temperature Tg and relaxation time τ. Both of these forms of heterogeneity have a major impact on material properties. Nevertheless, their interplay has remained poorly understood. Here, we employ molecular dynamics simulations of polymer thin films in the isoconfigurational ensemble in order to probe how bulk dynamic heterogeneity alters and is altered by the large gradient in dynamics at the surface of a glass-forming liquid. Results indicate that the τ spectrum at the surface is broader than in the bulk despite being shifted to shorter times, and yet it is less spatially correlated. This is distinct from the bulk, where the τ distribution becomes broader and more spatially organized as the mean τ increases. We also find that surface gradients in slow dynamics extend further into the film than those in fast dynamics—a result with implications for how distinct properties are perturbed near an interface. None of these features track locally with changes in the heterogeneity of caging scale, emphasizing the local disconnect between these quantities near interfaces. These results are at odds with conceptions of the surface as reflecting simply a higher “rheological temperature” than the bulk, instead pointing to a complex interplay between bulk dynamic heterogeneity and spatially organized dynamical gradients at interfaces in glass-forming liquids. 

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