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1. Unmanned aerial vehicles reveal the impact of a total solar eclipse on the atmospheric surface layer

   https://doi.org/10.1098/rspa.2019.0212  

   Bailey, Sean C.; Canter, Caleb A.; Sama, Michael P.; Houston, Adam L.; Smith, Suzanne Weaver (September 2019, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   We use unmanned aerial vehicles to interrogate the surface layer processes during a solar eclipse and gain a comprehensive look at the changes made to the atmospheric surface layer as a result of the rapid change of insolation. Measurements of the atmospheric surface layer structure made by the unmanned systems are connected to surface measurements to provide a holistic view of the impact of the eclipse on the near-surface behaviour, large-scale turbulent structures and small-scale turbulent dynamics. Different regimes of atmospheric surface layer behaviour were identified, with the most significant impact including the formation of a stable layer just after totality and evidence of Kelvin–Helmholtz waves appearing at the interface between this layer and the residual layer forming above it. The decrease in surface heating caused a commensurate decrease in buoyant turbulent production, which resulted in a rapid decay of the turbulence in the atmospheric surface layer both within the stable layer and in the mixed layer forming above it. Significant changes in the wind direction were imposed by the decrease in insolation, with evidence supporting the formation of a nocturnal jet, as well as backing of the wind vector within the stable layer. 

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2. Composition controls soft hydrogel surface layer dimensions and contact mechanics

   https://doi.org/10.1116/6.0002047  

   Johnson, Christopher L.; Dunn, Alison C. (November 2022, Biointerphases)

   Hydrogels are soft hydrated polymer networks that are widely used in research and industry due to their favorable properties and similarity to biological tissues. However, it has long been difficult to create a hydrogel emulating the heterogeneous structure of special tissues, such as cartilage. One potential avenue to develop a structural variation in a hydrogel is the “mold effect,” which has only recently been discovered to be caused by absorbed oxygen within the mold surface interfering with the polymerization. This induces a dilute gradient-density surface layer with altered properties. However, the precise structure of the gradient-surface layer and its contact response have not yet been characterized. Such knowledge would prove useful for designs of composite hydrogels with altered surface characteristics. To fully characterize the hydrogel gradient-surface layer, we created five hydrogel compositions of varying monomer and cross-linker content to encompass variations in the layer. Then, we used particle exclusion microscopy during indentation and creep experiments to probe the contact response of the gradient layer of each composition. These experiments showed that the dilute structure of the gradient layer follows evolving contact behavior allowing poroelastic squeeze-out at miniscule pressures. Stiffer compositions had thinner gradient layers. This knowledge can potentially be used to create hydrogels with a stiff load-bearing bulk with altered surface characteristics tailored for specific tribological applications. 

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3. Evolution of photoresist layer structure and surface morphology under fluorocarbon‐based plasma exposure

   https://doi.org/10.1002/ppap.201900026  

   Pranda, Adam; Gutierrez Razo, Sandra A.; Fourkas, John T.; Oehrlein, Gottlieb S. (March 2019, Plasma Processes and Polymers)

   Abstract Ion bombardment of photoresist materials during plasma etching results in the formation of a surface dense amorphous carbon (DAC) layer that contributes to both etch resistance and the development of surface roughness. Real‐time ellipsometric measurements/analysis reveals that a C₄F₈‐containing plasma interacts with an Ar‐plasma‐formed DAC layer to produce a modified DAC/fluorocarbon (FC) layer by FC deposition/diffusion of fluorine into the surface. The depletion of the DAC layer via modification and ion bombardment causes the etch rate of the bulk layer to increase. As the modified surface layer is formed, a noticeable decrease in surface roughness decrease is observed. These findings provide an understanding of the mechanisms of atomic layer etching processes in photoresist materials. 

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4. The effect of shear sheltering on trailing edge noise

   https://doi.org/10.2514/6.2020-2515  

   Jimenez, Ignacio; Glegg, Stewart A.; Devenport, William J. (June 2020, AIAA AVIATION FORUM)

   Shear sheltering is defined as the effect of the mean flow velocity profile in a boundary layer on the turbulence caused by an imposed gust. It has been studied extensively in applications involving boundary layer transition, where the primary concern is flow instabilities that are enhanced by turbulence in the flow outside the boundary layer. In aeroacoustic applications turbulent boundary layers interacting with blade trailing edges or roughness elements are an important source of sound, and the effect of shear sheltering on these noise sources has not been studied in detail. Since the surface pressure spectrum below the boundary layer is the primary driver of trailing edge and roughness noise, we will consider the effect that shear sheltering has on the surface pressure spectrum below a boundary layer. We will model the incoming turbulence as vortex sheets at specified heights above the surface and show, using classical boundary layer profiles and approximations to numerical results, how the mean flow velocity can be manipulated to alter the surface pressure spectrum and hence the radiated trailing edge noise. 

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5. Dynamical Importance of the Trade Wind Inversion in Suppressing the Southeast Pacific ITCZ

   https://doi.org/10.1029/2023JD039571  

   Gonzalez, Alex O.; Ganguly, Indrani; Osterloh, Marissa; Cesana, Gregory V.; DeMott, Charlotte A. (February 2024, Journal of Geophysical Research: Atmospheres)

   Sea surface temperature (SST) gradients are a primary driver of low‐level wind convergence in the east Pacific Inter‐Tropical Convergence Zone (ITCZ) through their hydrostatic relationship to the surface pressure gradient force (PGF). However, the surface PGF may not always align with SST gradients due to variations in boundary layer temperature gradients with height, that is, the boundary layer contribution to the surface PGF. In this study, we investigate the observed northern hemisphere position of the east Pacific ITCZ using a slab boundary layer model (SBLM) driven by different approximations of the boundary layer virtual temperature field. SBLM simulations using the entire boundary layer virtual temperature profile produce a realistic northern hemisphere ITCZ. However, SST‐only simulations produce excessive equatorial divergence and southern hemisphere convergence, resulting in a latitudinally confined double ITCZ‐like structure. Observed virtual temperature gradients highlight the importance of northward temperature gradients strengthening with height from the equator to 15°S below the trade wind inversion (TWI). Our interpretation is that the equatorial cold tongue induces relatively weak high surface pressure and double ITCZ‐like convergence because the resulting layer of cold air is shallow. Concurrently, relatively strong high surface pressure spreads out in the southern hemisphere due to interactions between stratocumulus clouds and the ocean surface. Together, the equatorial cold tongue and the TWI/stratocumulus clouds enable a more northern hemisphere dominant ITCZ. Thus, we provide evidence of a dynamical link between the equatorial cold tongue, low clouds, and double ITCZs, which continue to be problematic in Earth system models. 

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