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1. Where Is the Bottom of a Watershed?

   https://doi.org/10.1029/2019WR026010  

   Condon, Laura E.; Markovich, Katherine H.; Kelleher, Christa A.; McDonnell, Jeffrey J.; Ferguson, Grant; McIntosh, Jennifer C. (March 2020, Water Resources Research)
2. Observing Antarctic Bottom Water in the Southern Ocean

   https://doi.org/10.3389/fmars.2023.1221701  

   Silvano, Alessandro; Purkey, Sarah; Gordon, Arnold L; Castagno, Pasquale; Stewart, Andrew L; Rintoul, Stephen R; Foppert, Annie; Gunn, Kathryn L; Herraiz-Borreguero, Laura; Aoki, Shigeru; et al (December 2023, Frontiers in Marine Science)

   Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABW’s key role in regulating Earth’s climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope wherein situmeasurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, wherein situobservations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system. 

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3. DNA origami: The bridge from bottom to top

   https://doi.org/10.1557/mrs.2017.275  

   Xu, Anqin; Harb, John N.; Kostiainen, Mauri A.; Hughes, William L.; Woolley, Adam T.; Liu, Haitao; Gopinath, Ashwin (December 2017, MRS Bulletin)

   Over the last decade, DNA origami has matured into one of the most powerful bottom-up nanofabrication techniques. It enables both the fabrication of nanoparticles of arbitrary two-dimensional or three-dimensional shapes, and the spatial organization of any DNA-linked nanomaterial, such as carbon nanotubes, quantum dots, or proteins at ∼5-nm resolution. While widely used within the DNA nanotechnology community, DNA origami has yet to be broadly applied in materials science and device physics, which now rely primarily on top-down nanofabrication. In this article, we first introduce DNA origami as a modular breadboard for nanomaterials and then present a brief survey of recent results demonstrating the unique capabilities created by the combination of DNA origami with existing top-down techniques. Emphasis is given to the open challenges associated with each method, and we suggest potential next steps drawing inspiration from recent work in materials science and device physics. Finally, we discuss some near-term applications made possible by the marriage of DNA origami and top-down nanofabrication. 

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4. Photochemistry on the bottom side of the mesospheric Na layer

   https://doi.org/10.5194/acp-19-3769-2019  

   Yuan, Tao; Feng, Wuhu; Plane, John M.; Marsh, Daniel R. (January 2019, Atmospheric Chemistry and Physics)

   Abstract. Lidar observations of the mesospheric Na layer have revealed considerablediurnal variations, particularly on the bottom side of the layer, where morethan an order-of-magnitude increase in Na density has been observed below 80 kmafter sunrise. In this paper, multi-year Na lidar observations areutilized over a full diurnal cycle at Utah State University (USU) (41.8^∘ N,111.8^∘ W) and a global atmospheric model of Na with 0.5 kmvertical resolution in the mesosphere and lower thermosphere (WACCM-Na) to explorethe dramatic changes of Na density on the bottom side of the layer. Photolysis of the principal reservoir NaHCO₃ is shown to beprimarily responsible for the increase in Na after sunrise, amplified by theincreased rate of reaction of NaHCO₃ with atomic H, which is mainlyproduced from the photolysis of H₂O and the reaction of OH withO₃. This finding is further supported by Na lidar observation at USUduring the solar eclipse (>96 % totality) event on 21 August 2017, when a decrease and recovery of the Na density on thebottom side of the layer were observed. Lastly, the model simulation showsthat the Fe density below around 80 km increases more strongly and earlierthan observed Na changes during sunrise because of the considerably fasterphotolysis rate of its major reservoir of FeOH. 

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5. Bottom-Up Construction of the Interaction between Janus Particles

   https://doi.org/10.1021/acs.jpcb.2c07858  

   Popov, Alexander; Hernandez, Rigoberto (February 2023, The Journal of Physical Chemistry B)

   While the interaction between two uniformly charged spheres—viz colloids—is well- known, the interaction between non-uniformly charges spheres such as Janus particles is not. Specifically, the Derjaguin approximation relates the potential energy between two spherical particles with the interaction energy Vpl per unit area between two pla- nar surfaces. The formalism has been extended to obtain a quadrature expression for the screened electrostatic interaction between Janus colloids with variable relative orientations. The interaction is decomposed into three zones in the parametric space, distinguished by their azimuthal symmetry. Different specific situations are examined to estimate the contributions of these zones to the total energy. The effective potential Vpl is renormalized such that the resulting potential energy is identical to the actual one for the most preferable relative orientations between the Janus particles. The po- tential energy as a function of the separation distance and the mutual orientation of a pair of particles compares favorably between the analytical (but approximate) form and the rigorous point-wise computational model used earlier. Coarse-grained models of Janus particles can thus implement this potential model efficiently without loss of generality. 

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