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Nanoscale evaporation of liquids plays a key role in several applications including cooling, drag reduction and liquid transport. This research investigates the Leidenfrost effect at the nanoscale as a function of substrate material, droplet size and temperature using molecular dynamics models. Water droplets ranging from 4 nm to 20 nm were simulated over gold and silicon substrates at 293 K, 373 K, 473 K, and 573 K. A significant increase in the kinetic energy (>5000 kcal mol −1 ) was observed for molecules in the vicinity of the substrates, indicating the presence of a vapor barrier layer between substrate and liquid. Higher droplet velocities were tracked for hydrophobic gold substrates as compared to hydrophilic silicon substrates indicating the influence of the surface wettability on the Leidenfrost effect. Droplets over silicon substrates had a higher number of fluctuations (peaks and valleys) as compared to gold due to the cyclic behavior of vapor formation. An increase in the interfacial kinetic energies and translatory velocities (>10 m s −1 ) were observed as the droplet sizes reduced confirming the Leidenfrost effect at 373 K. This research provides understanding of the Leidenfrost effect at the nanoscale which can impact several applications in heat transfer and droplet propulsion.
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In situ SERS detection of dissolved nitrate on hydrated gold substrates
The accurate and fast measurement of nitrate in seawater is important for monitoring and controlling water quality to prevent ecologic and economic disasters. In this work we show that the in situ detection of nitrate in aqueous solution is feasible at nanomolar concentrations through surface enhanced Raman spectroscopy (SERS) using native nanostructured gold substrates without surface functionalization. Spectra were analyzed as collected or after standard normal variate (SNV) normalization, which was shown through Principal Component Analysis (PCA) to reduce spectral variations between sample sets and improve Langmuir adsorption model fits. An additional normalization approach based on the substrate silicon template showed that silicon provided an internal standard that accounted for the spectral variance without the need for SNV normalization. Nitrate adsorption was well-described by the Langmuir adsorption model, consistent with an adsorbed monolayer, and a limit of detection of 64 nM nitrate was obtained in ultrapure water, representing environmentally relevant concentrations. Free energy calculations based on the Langmuir adsorption constants, approximating equilibrium adsorption constants, and calculated self-energy arising from image charge, accounting for electrostatic interactions with a polarizable nanostructured substrate, suggest that nitrate adsorption was partially driven by an entropy gain presumably due to dehydration of the gold substrate and/or nitrate ion. This work is being extended to determine if similar statistical and normalization methods can be applied to nitrate detection in complex natural waters where non-target ions and molecules are expected to interfere.
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- Award ID(s):
- 1655221
- PAR ID:
- 10340499
- Date Published:
- Journal Name:
- Nanoscale Advances
- Volume:
- 3
- Issue:
- 14
- ISSN:
- 2516-0230
- Page Range / eLocation ID:
- 4098 to 4105
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d1na00156f
