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Water Transport Through Metallic Nanotubes
Precise separation of molecules is involved in several commercial uses, from water desalination to pharmaceutical manufacturing. The efficiency of the separation process is impacted by how well the technologies can transport water, protons, and other molecules through membrane pores. In a study featured on the cover of the June 27, 2024, issue of Nature Materials, researchers from Lawrence Livermore National Laboratory, the Massachusetts Institute of Technology, the University of Texas at Austin, and the National Institute of Standards and Technology reported that transport in tiny membrane pores depends strongly on the electronic nature of the channel walls.
The scientists used a combination of experimental measurements and molecular dynamics simulations to compare water, ion, and proton transport efficiency through metallic and superconducting carbon nanotubes of identical diameter. At sub-nanometer scales, metallic carbon nanotubes were significantly better at transporting material than semiconducting carbon nanotubes. “These results emphasize the complex role of the electronic properties of nanofluidic channels in modulating transport under extreme nanoscale confinement,” explains Alex Noy, a Livermore scientist and co-corresponding author on the paper.
The scientists uncovered the likely origins of enhancing transport through metallic nanotubes. Wall polarizability differences produce a noticeable reduction in the water interaction energy with the channel walls, allowing water to slip easier through the metallic nanotubes. Because of the underlying electric interactions, protons also travel more closely to the walls in superconducting nanotubes than in metallic nanotubes, forcing them into a corrugated path and diminishing their transport efficiency in semiconducting channels.
Contact: Alex Noy (925) 423-3396 (noy1 [at] llnl.gov (noy1[at]llnl[dot]gov)).
Confined Water’s Electric Response
Confining water to understand how water’s unique properties change in small spaces—sometimes below 10 nanometers—has broad scientific relevance ranging from biological effects to desalination. Researchers from Lawrence Livermore and the University of Texas at Austin focus on how confinement impacts water’s dielectric response, or its ability to screen electric fields within nanometer-scale pores. The research was published in The Journal of Physical Chemistry Letters on June 27, 2024.
Using a combination of machine learning and molecular dynamics simulations, the researchers determined to quantum-level accuracy the response of nanoconfined water to an electric field. The team studied the potential energy of the system and how electric charge is separated within water molecules and found an increase in the ability of water to screen electric fields applied along the axis of the one-dimensional nanopore.
“Our work reveals peculiar impacts of one-dimensional hydrophobic nanoconfinement, not only on the dielectric constant, but also on the electronic structure of water that cannot be observed with simulations based on conventional parametric force fields,” explains Livermore scientist and lead author of the study Marcos Calegari Andrade. In particular, the team found that water confined within nanopores can better screen electric fields due to a long-range alignment of water dipoles under confinement. The same alignment is also responsible for exotic ice structures in these extreme conditions.
Contact: Marcos Calegari Andrade (925) 423-7381 (calegariandr1 [at] llnl.gov (calegariandr1[at]llnl[dot]gov)).
Symmetry a Major Factor in Pre-Ignition ICF Experiments
In 2021, indirect drive inertial confinement fusion (ICF) experiments at the National Ignition Facility (NIF) achieved a burning plasma state with neutron yields more than 170 kilojoules, a critical step toward fusion ignition. However, multiple sources of degradations were present in the experiments, hindering predictability in performance. A Livermore study led by ICF research physicists Joe Ralph, Steven Ross, and Alex Zylstra has confirmed that one degradation source, implosion asymmetry, was impactful to the extent that it prevented plasma from entering the burning state in many of the ICF experiments leading up to the burning plasma results. Their findings are published in the April 6, 2024, issue of Nature Communications.
Symmetry in ICF experiments is paramount to successfully achieving a burning plasma state. A degradation factor has already been determined for two degradation types: mode-1 asymmetry and radiative mix. The paper is the first to present an empirical degradation factor for mode-2 asymmetry that, when incorporated into the theoretical fusion yield scaling with the other two degradation factors, accounts for the measured fusion performance variability in the two highest-performing experimental campaigns at NIF to within error. “By identifying and accounting for these degradation factors, we can better assess the performance of our experiments and make more informed decisions,” says Ralph. “This work was a significant step toward achieving ignition.”
Contact: Joe Ralph (925) 423-3932 (ralph5 [at] llnl.gov (ralph5[at]llnl[dot]gov)).
