YSM Issue 94.2

NEWS
Chemistry
NANOSCALE
NUCLEATION
Insight into the unusual
mechanism of contact
freezing
BY KRISHNA DASARI
IMAGE COURTESY OF FLICKR
Freezing: a simple phase transition with a surprisingly complex With these advancements, the researchers were able to
set of mechanisms. Researchers use freezing dynamics to study simulate the nanoscale dynamics of contact freezing for two
the crystallization of various materials, but not all kinds of mW-based liquids, one that had a surface freezing propensity
freezing are mechanistically understood. Graduate student Sarwar
Hussain and professor Amir Haji-Akbari in the Department of
Chemical and Environmental Engineering at Yale recently simulated
contact freezing to unravel its unique mechanism.
In contact freezing, a water droplet that is supercooled—below
freezing temperature but still liquid—collides with an external
particle that promotes nucleation, the initiating step in the process
of freezing. This results in unusually rapid ice formation. However,
why exactly freezing proceeds so quickly remains unclear. Previous
studies have suggested two key points. First, the rate of contact
freezing is related to the liquid’s tendency to surface freeze—that
is, to rapidly freeze its surface relative to the interior. Second, the
nucleating particle doesn’t necessarily have to directly disrupt the
liquid-vapor interface to increase the freezing (nucleation) rate.
The researchers’ results ruled out the prevailing theories on freezing
mechanism. “[These findings] were not necessarily explained by the
previous mechanisms which said that there has to be some sort of
mechanical disturbance in the free interface between the liquid and
the vapor for the increased freezing rate to take place,” Hussain said.
Further experiments into this nucleation mechanism are
hindered by the limits of technology. Freezing events occur on
the nanosecond timescale and create ice nuclei containing only
hundreds to thousands of molecules. They cannot be observed
with sufficient resolution, so the mystery of high nucleation rates
during contact freezing is difficult to approach experimentally.
What can’t yet be accomplished in a physical lab, though,
can now be done computationally. Two major advancements
allowed for the accurate simulation of contact freezing. The first
was the mW model, a water-like tetrahedral model liquid that
enables rapid simulation while still considering interactions
like hydrogen bonding. The second was the development of
jumpy forward flux sampling, a novel sampling technique
conceptualized by Haji-Akbari to address the shortcomings
of the original forward flux sampling method. The original
and one that did not. By simulating supported nanofilms of
the liquids, they were able to exclusively investigate the effect
of the proximity of the nucleating particle to the vapor-liquid
interface on the nucleation rate.
From their simulations, they observed that only the surface
freezing liquid’s nucleation rate was affected by proximity
to the nucleating particle, increasing the nucleation rate by
orders of magnitude. Moreover, they discovered that in such a
case, freezing proceeds through the formation of hourglassshaped
nuclei, a unique property that mechanistically explains
the increased nucleation rate. Compared to the spherical capshaped
nuclei of classical models, hourglass-shaped nuclei have a
smaller surface area exposed to the liquid. According to classical
nucleation theory, the smaller liquid-exposed surface area lowers
the nucleation barrier, allowing nucleation to proceed faster.
Their findings have implications for understanding contact
freezing of water in the atmosphere, a phenomenon important
for predicting weather patterns. These results support the
theory that water may be capable of surface freezing, giving it
access to rapid nucleation through contact freezing. However,
the authors also note that other agents in the atmosphere
may also contribute to nucleation rate, making it difficult to
conclude that the proximity effect of surface freezing liquids is
necessarily the main cause for atmospheric nucleation rates.
More broadly, their discovery of this non-classical nucleation
mechanism provides a roadmap to study other non-classical
processes. For example, scientists may investigate nucleation in
the vicinity of surfaces with different ice-forming properties,
such as a protein with hydrophilic and hydrophobic patches.
“The question is, if you put that protein inside supercooled
water, what will happen? How will the critical nucleus look like,
and how will the rate be sensitive to temperature, for example?
These are all questions that our work gives a good conceptual
framework to pursue,” Haji-Akbari said. ■
method could be used to estimate the likelihood of freezing
events progressing toward complete freezing, but Haji-Akbari’s
Hussain, S., & Haji-Akbari, A. (2021). Role of Nanoscale
version accounts for the jumpiness of freezing progression Interfacial Proximity in Contact Freezing in Water. Journal of
caused by the coagulation of ice particles. “If you do not take the American Chemical Society, 143(5), 2272–2284. https://
into account these jumps, you can underestimate the nucleation doi.org/10.1021/jacs.0c10663
rate by several orders of magnitude,” Haji-Akbari said.
8 Yale Scientific Magazine May 2021 www.yalescientific.org