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Molecular Dynamics Cornerstone Breaks Downs
For over a century, the Nernst-Einstein (NE) relation has linked charged particles’ ability to diffuse through liquids (diffusion coefficient) to movement in the presence of an electric field (electrophoretic mobility). However, new research finds this correspondence is not universal but breaks down inside carbon nanotube porins (CNTPs). Appearing December 30, 2022, in Nature Nanotechnology, research by scientists at Lawrence Livermore and the Massachusetts Institute of Technology found that strongly confining ions inside CNTPs alters expected molecular behavior.
The study, involving experimentation and simulation, confined potassium ions (K+) and water molecules inside straw-shaped nanotubes (0.8 nanometers in diameter) inserted into phospholipid membranes, forming transmembrane pores. K+ ions could not freely diffuse due to obstruction from a water molecule chain filling the CNTP; but under an electric field, the chain broke down, allowing formation of individual ion-water clusters that migrated quickly. “The extreme spatial confinement in these pores hinders the diffusion of K+ ions, reducing the diffusion coefficient by three orders of magnitude relative to its bulk value,” says Livermore scientist Aleksandr Noy, the study’s co-lead author. “Surprisingly, the same confinement has a negligible effect on the electrophoretic mobility, leading to a complete breakdown of the NE relation.”
The study suggests that the NE relation does not hold universally as Nernst theorized in 1888. At the extreme nanoscale, diffusion and electrophoretic mobility values may not correlate at all when confinement forces a different electromigration mechanism. These new insights are especially relevant to cell physiology and drug discovery because the ion channels mimicked by CNTPs are critical to cellular functioning.
Contact: Aleksandr Noy (925) 423-3396 (noy1@llnl.gov).
Deep Learning Explains Photocatalyst Surface Chemistry
Interaction between liquid water and metal oxide surfaces is applicable to several technologies. In the presence of sunlight, surfaces of titanium oxide (TiO2) can accelerate separation of water into the molecular species H2 and O2. This process, photocatalysis, drives hydrogen production and photooxidation of organic matter in self-cleaning devices. Predicting how these reactions may proceed requires understanding the chemical form of water at the material’s surface—molecular or dissociated. Published January 3, 2023, in Proceedings of the National Academy of Sciences, a multi-institutional research team including Lawrence Livermore used deep learning-based simulations to characterize associated structures and interactions of water on TiO2 surfaces.
Using ab initio-based atomistic simulations with nanosecond timescale, the team assessed the dynamic equilibrium of molecular and dissociated water at the surface of rutile TiO2(110), the material’s most common crystal polymorph. “Our simulations revealed how sensitive the surface chemistry of TiO2 is relative to the thickness of thin TiO2 films, and they provided atomic-level detail into the structure of water close to the TiO2 surface,” says co-author Marcos Felipe Calegari Andrade from the Quantum Simulations Group at Lawrence Livermore.
Researchers found a larger water dissociation fraction than previously estimated, supporting the greater photooxidative capacity of rutile TiO2 over its other anatase phase. Greater understanding of surface interaction between water and TiO2 could enable discovery of efficient, affordable materials for clean hydrogen production and medical uses.
Contact: Marcos Calegari Andrade (925) 423-7381 (calegariandr1@llnl.gov).
Turbulence-Measuring Telescope Sees Smooth Launch
For vehicles traveling at hypersonic speeds (above Mach 5, 6,174 kilometers per hour), slight variations in encountered air patterns have magnified effects on drag and heat factors. To better understand these impacts on hypersonic flight, the Stellar Occultation Hypertemporal Imaging Payload (SOHIP), a prototype telescope designed and built by Livermore researchers, was launched March 14, 2023, from Cape Canaveral, Florida, to the International Space Station (ISS).
“If we [can] accurately predict the conditions that trigger erratic gravity waves or hypersonic flows, we could inform better vehicle design, reduce costs, and improve overall hypersonic flight performance,” says principal investigator Matthew Horsley. SOHIP employs Livermore’s patented monolithic optics to investigate upper-atmosphere turbulence using starlight. Focusing on one star above the atmosphere and another in the wake of the ISS’s trajectory, the telescope will monitor for minute fluctuations in air refractivity. Capturing roughly 1,000 images per second, it will work in concert with other ISS instruments to calculate encountered turbulence at the finest spatial resolution to date.
The shoebox-size design weighs only 13.6 kilograms and is constructed from off-the-shelf parts, making it readily integrable with existing ISS equipment. Data captured using specialized firmware will enhance development of algorithms for modeling turbulence. SOHIP represents the first Livermore instrument to operate on the ISS, and the project fulfilled strict NASA safety requirements while being completed on time and within budget.
Contact: Matthew Horsley (925) 423-0141 (horsley1@llnl.gov).
