Supercooled Nanodroplets Research Articles

A new insight into the interaction of hydroxyl radical with supercooled nanodroplet in the atmosphere

• The interaction between OH* and supercooled nanodroplet led to delocalized states caused by surface effects. • The OH* shows a high tendency for accepting H-bond from neighboring water molecules around (1.77 Å, 122°). • RDF and ring analysis results confirmed the LDA ice for OH*-supercooled nanodroplet. • The preference of six-membered rings in supercooled nanodroplet based on ring analysis. • The 3-fold and 4-fold coordination were revealed as dominant structures. Fallstreak clouds are types of clouds with supercooled water droplets, meaning that they are still liquid at sub-freezing temperatures. In the atmosphere, hydroxyl radicals serve as an important source of oxidants. The reactivity of the hydroxyl radicals in ice environments is significant, affecting other atmospheric reactions. However, the interaction between OH radical with supercooled water nanodroplet remains unknown. This work presents a comprehensive computational study of OH radicals at the air-ice interface. Ab initio molecular dynamics simulation showed that despite previous assumptions, the hemibond could also exist in large OH*-nanodroplet systems under supercooling process. Two hydrogen-bonded water molecules along with the hemibond have been found to be the reason for OH* stability. There is a significant correlation between our RDF and ring analysis results and the structural information from X-ray crystallography, confirming LDA ice for supercooled nanodroplets. The hydrogen bond analysis revealed that there is no radical effect on the hydrogen bond formation of water nanodroplets at 250 K. Moreover, the time evolution of critical bonds based on distances revealed that the OH* could be a movable and reactive radical during the supercooling process. Results from this research contribute to a better understanding of natural phenomena and rational design of anti-icing systems for aviation, wind energy, and infrastructures along with cryopreservation.

Supercooled nano-droplets of water confined in hydrophobic rubber.

Hydrophobic elastomers are capable of absorbing a small amount of water that forms droplets around hydrophilic sites. These systems allow the study of confinement effects by a hydrophobic environment on the dynamics and thermodynamic behaviour of water molecules. The freezing-melting properties and the dynamics of water inside nano-droplets in butyl rubber are affected, as revealed by differential scanning calorimetry (DSC) and deuterium nuclear magnetic resonance (2H-NMR). Upon cooling down, all water crystalizes with a bimodal droplet population (da = 3.4 nm and db = 4.4 nm) in a temperature range associated with the droplet size distribution. However, the melting temperature is not shifted according to the Gibbs-Thomson equation. The relative decrease of the 2H-NMR longitudinal magnetization is not a single exponential and, by inverse Laplace transformation, it was deduced to be bimodal in agreement with the DSC measurements (T1,a ∼ 10 ms and T1,b ∼ 200 ms). The deduced correlation time of molecular reorientation is longer than that of bulk water and the behaviour with temperature follows the Vogel-Fulcher-Tammann (VFT) equations with a changing fragility as the droplet size is reduced when reducing hydration.

Directional Ostwald Ripening for Producing Aligned Arrays of Nanowires.

The remarkable electronic and mechanical properties of nanowires have great potential for fascinating applications; however, the difficulties of assembling ordered arrays of aligned nanowires over large areas prevent their integration into many practical devices. In this paper, we show that aligned VO2 nanowires form spontaneously after heating a thin V2O5 film on a grooved SiO2 surface. Nanowires grow after complete dewetting of the film, after which there is the formation of supercooled nanodroplets and subsequent Ostwald ripening and coalescence. We investigate the growth mechanism using molecular dynamics simulations of spherical Lennard-Jones particles, and the simulations help explain how the grooved surface produces aligned nanowires. Using this simple synthesis approach, we produce self-aligned, millimeter-long nanowire arrays with uniform metal-insulator transition properties; after their transfer to a polymer substrate, the nanowires act as a highly sensitive array of strain sensors with a very fast response time of several tens of milliseconds.

Ice nucleation rates near ∼225 K.

We have measured the ice nucleation rates, Jice, in supercooled nano-droplets with radii ranging from 6.6 nm to 10 nm and droplet temperatures, Td, ranging from 225 K to 204 K. The initial temperature of the 10 nm water droplets is ∼250 K, i.e., well above the homogeneous nucleation temperature for micron sized water droplets, TH ∼235 K. The nucleation rates increase systematically from ∼1021 cm-3 s-1 to ∼1022 cm-3 s-1 in this temperature range, overlap with the nucleation rates of Manka et al. [Phys. Chem. Chem. Phys. 14, 4505 (2012)], and suggest that experiments with larger droplets would extrapolate smoothly the rates of Hagen et al. [J. Atmos. Sci. 38, 1236 (1981)]. The sharp corner in the rate data as temperature drops is, however, difficult to match with available theory even if we correct classical nucleation theory and the physical properties of water for the high internal pressure of the nanodroplets.

Freezing of supercooled n-decane nanodroplets: from surface driven to frustrated crystallization.

Whether crystallization starts at the liquid-vapor interface or randomly throughout the bulk has been the subject of intense debate. In our earlier work, we investigated the freezing of supercooled nanodroplets of short chain (C8, C9) n-alkanes formed by homogeneous condensation in a supersonic nozzle. The rate at which the solid appeared suggested freezing starts at the droplet surface well before the rest of the droplet freezes. Experiments were, however, limited to a single condition for each compound and it was not clear whether freezing of n-alkanes always occurs in this two step manner. Here, we expand our work to include freezing of a third n-alkane, n-decane, and, furthermore, we vary the temperatures at which droplets are formed and freeze. The phase transitions are again characterized using three experimental techniques - pressure trace measurements (PTM), Fourier Transform Infrared Spectroscopy (FTIR), and Small Angle X-ray Scattering (SAXS). We also use Wide Angle X-ray Scattering (WAXS) to confirm, for the first time, the crystalline nature of our frozen n-alkane nanodroplets. As the temperature at which the droplets form and freeze decreases, the kinetics of the phase transition changes. At higher temperatures, the phase transition occurs in two steps characterized by different rates, whereas at lower temperatures we observe only a single step. Finally, in the lowest temperature experiment, where droplets start to form and freeze ∼50 K below the bulk melting temperature, we found that the particles develop a fractal structure and appear locked in a "frustrated" crystalline state.

Experimental evidence for surface freezing in supercooled n-alkane nanodroplets

Intermediate chain length (16 ≤i≤ 50) n-alkanes are known to surface freeze at temperatures that are up to three degrees higher than the equilibrium melting point [B. M. Ocko et al., Phys. Rev. E, 1997, 55, 3164-3182]. Our recent experimental results suggest that highly supercooled nanodroplets of n-octane and n-nonane also surface freeze, and subsequently bulk crystallization occurs. The data yield surface and bulk nucleation rates on the order of ~10(15) cm(-2) s(-1) and ~10(22) cm(-3) s(-1), respectively, at temperatures between 180 K and 200 K. Molecular dynamics simulations at the united atom level were used to follow the freezing of a supercooled n-octane drop and show that an ordered monolayer develops on the surface of the droplet almost immediately, and the rest of the droplet then freezes in a layer-by-layer manner.

Simulations of Crystallization in Supercooled Nanodroplets in the Presence of a Strong Exothermic Solute

Molecular dynamics simulations of crystallization in supercooled nanodroplets of binary Lennard-Jones mixtures with a solvent and a strong exothermic solute corresponding to a sulfuric acid aerosol demonstrate that the presence of the solute controls the nucleation. A strongly exothermic solute prefers to be surrounded at all sides by solvent molecules, and for this reason the outermost 1–2 surface layers are depleted of solute and with a diluted concentration of the solvent. The crystallization is observed to initiate a few layers beneath the surface and to result in a crystal growth that encapsulates the droplet by an “egg shell” of a crystal of solvent particles. Our findings are discussed in relation to experimental data of ice formation in polar stratospheric clouds.