An ice dendrite grown in space (Credit: Hokkaido Univ./JAXA).Although ice crystals are all around us, both on Earth and in space, it isn’t fully understood how their beautiful structures form. Japanese scientists have blasted their experiments to the International Space Station in a quest to fathom out how ice crystals grow.
Filling this knowledge gap would aid climate change and other meteorology research. It would also help researchers understand how other commercially-important dendritic silicon and metal crystals grow, that are even harder to study directly.
Such fundamentals of crystal growth form the basis of the first volume of the Handbook of Crystal Growth, published by Elsevier and available on ScienceDirect.
The International Space Station is an orbiting laboratory that hosts a wide range of experiments that require microgravity. The precise observation of ice crystal growth requires the elimination of heat convection, which changes the environment around the crystals and effects their growth. “The growth rates of crystals are strongly modified by the convection effect and the patterns become asymmetric,” explains the lead researcher Yoshinori Furukawa at Hokkaido University. “On the Earth, it is not possible to eliminate the effect of thermal convection. In space, we can completely remove this effect and highly symmetric dendrite patterns are realized.”
The first set of ice experiments took place between December 2008 and February 2009 in the Japanese experiment module known as Kibo. These experiments were 15 years in the making, having originally been selected for the mission in 1994. Furukawa blames “dilly-dallying” for reasons such as the economic status and shuttle accidents for the delay. “We were about ready to give up many times,” he explains.
But it was worth the wait. The team were able observe the growth process of perfectly hexagonal ice crystals, in supercooled heavy water, 134 times during this first three month period using two onboard interference microscopes.
The growth process was already known to start with the formation of smooth disc-shaped ice crystals. These then grow and eventually become unstable at the edges, causing branches – known as dendrites – to start to form. The reason for the instability was previously not understood.
These space experiments showed that the instability occurs at the point when the crystal’s growth has faltered and then started again. “A change from stopping growth to beginning growth may initiate the instability along the edge of ice circular disk. This is the beginning of the instability and after this moment the dendrite pattern continues to develop,” explains Furukawa. “This result means that the growth of basal face [the disc-like crystal] is a key process for beginning the instability.” The team were also able to measure the growth rate at the dendrite tips of the ice crystals very precisely.
The experimental apparatus floating inside the Kibo module (Credit: Hokkaido Univ./JAXA).Between November 2013 and June 2014, Furukawa’s team co-ordinated a second set of ice experiments onboard Kibo. They repeated the same experiment, but this time a small amount of an antifreeze glycoprotein was added to the water to see how this affected the growth process. “This is a functional protein with the capability to prevent the freezing of living fishes in subzero condition underneath the sea ice,” explains Furukawa. The results of this second set of experiments have not yet been published.
All these experiments were controlled from the ground. “We have an operation room at the Tsukuba Space Center of JAXA [The Japan Aerospace Exploration Agency], near Tokyo, and we can send all the commands from the operation desk to our apparatus,” says Furukawa. “We could observe the ice crystal growth in space in almost real time.” The time delay was only a few seconds. At the end of each experiment the apparatus is warmed, to melt the ice crystal, and then it’s ready to start again. “The astronauts never get involved in the experiment operation. In fact we carried out our experiments at the night when they were sleeping to prevent the effect of gravity fluctuation originating from their movements.”
The first volume of the Handbook of Crystal Growth, contains more information on the fundamentals of crystal growth.