By Uzman Rizwan
University of Calgary chemistry PhD candidate Payman Pirzadeh is researching ice formations. Understanding how ice forms has important applications. Thousands of people every year undergo organ transplants to replace failing organs like livers, kidneys and corneas. As organ donation becomes more common, it is important to find better ways to preserve them.
Ice can form into two types of structures, hexagonal ice and cubic ice. Hexagonal ice takes well-known forms like freezer ice, or the ice that forms on a lake. In contrast, cubic ice does not form in nature and its crystals have been mainly created in laboratories under specific conditions. Cubic ice crystals are less harmful to organs.
The difference between the two types of ice is how they form, and studying this formation gives an understanding into the effects of ice on frozen objects, such as organs.
“It has been found in experiments with organ preservation that if cubic ice is present, then the tissues are less damaged,” said Pirzadeh.
Cell membranes or tissue can be damaged when organs are frozen, due to the formation of ice. Pirzadeh said the crystals of hexagonal ice are larger and sharper compared to cubic ice and can damage cell membranes.
Ice structures can be compared to a brick wall. Each molecule is a brick, and the molecules are stacked on top of one another. Sometimes these molecules, like bricks, slide and move. These shifts can cause defects in ice called stacking faults which occur when a new layer of ice grows on top of another, but the molecules are not lined up properly. It is this stacking that makes the ice jagged and sharp.
Pirzadeh began studying ice formation and stacking faults for his PhD.
“Research, I would say, has two parts. One part is aims that you usually plan for. Another part is luck, chance, random problems and observations that you make. Stacking faults was one of those random ones,” said Pirzadeh.
U of C chemistry professor and Pirzadeh’s supervisor Peter Kusalik gave Pirzadeh the inspiration to begin researching this topic.
“He was telling me that as you grow the ice, there is a chance that ice layers are shifted with respect to each other,” said Pirzadeh.
Kusalik said that stacking faults only occur when the ice grows in one direction.
“My question for him at that point of time was why we don’t see stacking faults when we are growing ice in other [directions],” said Pirzadeh. “Our simulations are 3D, so when we are growing ice in one direction, it is by default also growing in other directions.”
During the summer of 2007, Pirzadeh checked the progress of the simulations he left running and observed an unusual growth he was not expecting.
Pirzadeh and Kusalik found that instead of the molecular stacking in hexagonal form, cubic ice resulted when stacking faults occurred in their experiment.
The change in stacking of molecules was because of the formation of the ice molecules and the amount of rings created in their compounds.
Five- and eight-member rings were created, and they interfere with the growth of the ice crystal. These faults cause a shift in the subsequent ice layer.
“Ice does not like five-member rings. It prefers six-member rings. But if ice wants to accommodate five-member rings then it has to come up with a compromise. That produces the defect. In this case, we observed five- and eight-member rings that particularly shifted the subsequent layer,” said Pirzadeh.
In his simulations, Pirzadeh also found that the presence of solutes at the water-ice boundary can increase the probability of stacking faults. Molecular simulations using a methane molecule showed an increase in the formation of five- and eight-member rings.
Pirzadeh is extending his research to understand how other solutes cause stacking faults in order to apply them to cryopreservation, the storage of living organisms at extremely low temperatures.
The role that stacking faults play in shifting ice layers can also help glaciologists and planetary scientists studying the ice surfaces of the moons of Jupiter.