122 research outputs found
The Many Faces of Heterogeneous Ice Nucleation: Interplay Between Surface Morphology and Hydrophobicity
What makes a material a good ice nucleating agent? Despite the importance of
heterogeneous ice nucleation to a variety of fields, from cloud science to
microbiology, major gaps in our understanding of this ubiquitous process still
prevent us from answering this question. In this work, we have examined the
ability of generic crystalline substrates to promote ice nucleation as a
function of the hydrophobicity and the morphology of the surface. Nucleation
rates have been obtained by brute-force molecular dynamics simulations of
coarse-grained water on top of different surfaces of a model fcc crystal,
varying the water-surface interaction and the surface lattice parameter. It
turns out that the lattice mismatch of the surface with respect to ice,
customarily regarded as the most important requirement for a good ice
nucleating agent, is at most desirable but not a requirement. On the other
hand, the balance between the morphology of the surface and its hydrophobicity
can significantly alter the ice nucleation rate and can also lead to the
formation of up to three different faces of ice on the same substrate. We have
pinpointed three circumstances where heterogeneous ice nucleation can be
promoted by the crystalline surface: (i) the formation of a water overlayer
that acts as an in-plane template; (ii) the emergence of a contact layer
buckled in an ice-like manner; and (iii) nucleation on compact surfaces with
very high interaction strength. We hope that this extensive systematic study
will foster future experimental work aimed at testing the physiochemical
understanding presented herein.Comment: Main + S
Benchmarking the performance of Density Functional Theory and Point Charge Force Fields in their Description of sI Methane Hydrate against Diffusion Monte Carlo
High quality reference data from diffusion Monte Carlo calculations are
presented for bulk sI methane hydrate, a complex crystal exhibiting both
hydrogen-bond and dispersion dominated interactions. The performance of some
commonly used exchange-correlation functionals and all-atom point charge force
fields is evaluated. Our results show that none of the exchange-correlation
functionals tested are sufficient to describe both the energetics and the
structure of methane hydrate accurately, whilst the point charge force fields
perform badly in their description of the cohesive energy but fair well for the
dissociation energetics. By comparing to ice Ih, we show that a good prediction
of the volume and cohesive energies for the hydrate relies primarily on an
accurate description of the hydrogen bonded water framework, but that to
correctly predict stability of the hydrate with respect to dissociation to ice
Ih and methane gas, accuracy in the water-methane interaction is also required.
Our results highlight the difficulty that density functional theory faces in
describing both the hydrogen bonded water framework and the dispersion bound
methane.Comment: 8 pages, 4 figures, 1 table. Minor typos corrected and clarification
added in Method
Crystal Nucleation in Liquids: Open Questions and Future Challenges in Molecular Dynamics Simulations
The nucleation of crystals in liquids is one of nature's most ubiquitous
phenomena, playing an important role in areas such as climate change and the
production of drugs. As the early stages of nucleation involve exceedingly
small time and length scales, atomistic computer simulations can provide unique
insight into the microscopic aspects of crystallization. In this review, we
take stock of the numerous molecular dynamics simulations that in the last few
decades have unraveled crucial aspects of crystal nucleation in liquids. We put
into context the theoretical framework of classical nucleation theory and the
state of the art computational methods, by reviewing simulations of e.g. ice
nucleation or crystallization of molecules in solutions. We shall see that
molecular dynamics simulations have provided key insight into diverse
nucleation scenarios, ranging from colloidal particles to natural gas hydrates,
and that in doing so the general applicability of classical nucleation theory
has been repeatedly called into question. We have attempted to identify the
most pressing open questions in the field. We believe that by improving (i.)
existing interatomic potentials; and (ii.) currently available enhanced
sampling methods, the community can move towards accurate investigations of
realistic systems of practical interest, thus bringing simulations a step
closer to experiments
Crumbling Crystals: On the Dissolution Mechanism of NaCl in Water
Life on Earth depends upon the dissolution of ionic salts in water,
particularly NaCl. However, an atomistic scale understanding of the process
remains elusive. Simulations lend themselves conveniently to studying
dissolution since they provide the spatio-temporal resolution that can be
difficult to obtain experimentally. Nevertheless, the complexity of various
inter- and intra-molecular interactions require careful treatment and long time
scale simulations, both of which are typically hindered by computational
expense. Here, we use advances in machine learning potential methodology to
resolve for the first time at an ab initio level of theory the dissolution
mechanism of NaCl in water. The picture that emerges is that of a steady
ion-wise unwrapping of the crystal preceding its rapid disintegration,
reminiscent of crumbling. The onset of crumbling can be explained by a strong
increase in the ratio of the surface to volume of the crystal. Overall,
dissolution is comprised of a series of highly dynamical microscopic
sub-processes, resulting in an inherently stochastic mechanism. These atomistic
level insights now pave the way for a general understanding of dissolution
mechanisms in other crystals, and the methodology is primed for more complex
systems of recent interest such as water/salt interfaces under flow and salt
crystals under confinement
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