Crystallization of water to ice has for a long time remained exciting and mind-boggling phenomenon among scientists. This conundrum, especially when it occurs in ‘No Man’s Land’ (NML), which typically ranges from ~150K(-123°C) (below which amorphous ice is detectable) to ~235K(-38°C), above which homogeneous nucleation is realized, any experimentation becomes elusive owing to the fast nucleation experienced at this temperature range. The phenomenon, however, is the key to comprehending and predicting numerous puzzling environmental phenomena, such as: ice-fog formation in cirrus clouds and atmospheric-cloud formation. So far, the available experimental approaches have been seen to lack the spatial and temporal resolution to yield fundamental microscopic insights and identify various ice polymorphs formed. Consequently, researchers are now inclining towards the use of molecular dynamics, as it is capable of providing sufficient microscopic resolution required to shed light on nucleation. Recent studies have highlighted the influence of external electric fields in this phenomenon. Nonetheless, the issue is still under investigations as the final polymorph adopted under NML environments is still unresolved.
Recently, University College Dublin researchers: Dr. Prithwish Nandi, Dr. Christian Burnham and Dr. Niall English from the School of Chemical and Bioprocess Engineering conducted a study in which, through non-equilibrium molecular-dynamics simulations of sub-microsecond formation of rhombus-shaped ice Ic nano-crystallites from aggressively-quenched supercooled water nano-droplets in the gas phase, in external static electric fields, they tackled the aforementioned difficulties related to water-to -ice freezing, particularly within NML. Specifically, they explored droplets’ nano-confined geometries and the entropic-ordering agent of external electric fields as a means of realizing cubic-ice formation, especially with very few stacking faults and defects. Their work is currently published in the research journal, Physical Chemistry Chemical Physics.
The researchers performed NE-MD simulations under periodic boundary conditions with an abruptly-NML-quenched-from-ambient spherical liquid-water droplet of radius ~2.5 nm. As the secret of nanoscale electro-nucleation lies in the rings; the essential mechanism was rooted in field-mediated torques undermining pentamers’ entropic dominance in preserving the supercooled-liquid state. Generally, they scrutinized supercooled-water nano-droplets’ freezing under NML conditions, where among other things, the droplets’ nano-confined geometries were explored.
The authors observed that gross morphological shape transformation was achieved through collective partial dipolar alignment. Remarkably, the fact that essentially pure and near-perfectly ordered cubic ice, bereft of stacking faults with suppressed ice, realized an important, experimentally-elusive goal. As such, manipulation of pentamer’s competing entropic favorability was seen to help suppress Ih and realize fewer defects and stacking faults.
In summary, the University College Dublin study elucidated, for the first time, on the kinetic and mechanistic intricacies of electric-field-driven ice nano-crystallisation in No Man’s Land, of tropospheric relevance. They showed that the absence of any applied electric field resulted in amorphization of the suddenly-quenched droplets leading to the adoption of an amorphous, LDA-type ‘arrested’ state, on the cusp of transition towards crystalline (Ih) ice, but frustrated from so doing.
“Our study offers various insights into the multi-facted world of electro-nucleation, from a tentative and speculative ‘nod’ to Scheiner’s Halo to external-field manipulation of entropy’s guiding hand in molecular self-assembly” said Dr. Niall English in a statement to Advances in Engineering.
Prithwish K. Nandi, Christian J. Burnham, Niall J. English. Electro-nucleation of water nano-droplets in No Man’s Land to fault-free ice Ic†. Phys.Chem.Chem.Phys., 2018, volume 20, page 8042Go To Phys.Chem.Chem.Phys.