Tikalon Blog by Dev Gualtieri

Low Density Ice, Shedding Surface Ice

April 4, 2016

Now that the first day of spring has passed, we anticipate no more icy weather at Tikalon's home base in Northern New Jersey. Spring starts with the passing of the vernal equinox that occurred this year at 04:30 UT on March 20. This equinox is the point at which the Sun crosses from the southern hemisphere to the northern hemisphere, and it's a day when night time darkness and day time light are equal. Daylight increases after that point.

Water is very abundant on the Earth, and ice, the solid phase of water has always interested scientists. The most interesting property of this hydrogen-bonded solid is that the density of ice is less than that of water. Just below freezing, ice has a density of just 0.9167 g/cc, which is 8.3% less dense than water just above freezing (0.99984 g/cc). The maximum density of water is 0.999973 g/cc at 3.98 °C (see graph).

Density of water from 0-8 degrees Celsius.

You can see why 1.0 g/cc works well in most cases.

(Graphed using Gnumeric from available data.)

The density of ice just below freezing is impressively low, and it's the reason that ice floats on water. This fact must have been important for Earth's climatic history and the evolution of life. As if inspired by the phrase, "How low can you go," in the Chubby Checker song, Limbo Rock, scientists at the Dalian University of Technology (Dalian, China), the University of Nebraska (Lincoln, Nebraska), the Chinese Academy of Sciences (Beijing, China), and the University of Science and Technology of China (Hefei, China) have done calculations that show the possibility of attaining an ice phase with a density of just 0.593 g/cc.[1-2]

The trick to making such an ice is to start first with a clathrate, a crystal of mostly water containing a guest molecules, such as methane, that stabilizes the structure. It appears that once such a clathrate forms, the guest molecules can be extracted, leaving just the low density ice behind.[1-2] The structure of the resultant ice crystal is shown below.

s-III ice, with an without a caged C20H40 molecule

Upper left, a water cage containing a molecule of C₂₀H₂₀, and upper right, the same water cage with the molecule removed.

Lower image, the configuration of an ice crystal built from these units.

(Upper images are from ref. 1, published under a Creative Commons Attribution-NonCommercial license.

Lower image from University of Nebraska-Lincoln/Yingying Huang and Chongqin Zhu)

Says study co-author, Xiao Chen Zeng,

“Water and ice are forever interesting because they have such relevance to human beings and life... If you think about it, the low density of natural ice protects the water below it; if it were denser, water would freeze from the bottom up, and no living species could survive. So Mother Nature's combination is just so perfect.”[2]

Clathrate compounds can assume various crystal structures, including cubic, hexagonal, and tetragonal.[1] In previous research, a guest-free clathrate, now known as ice XVI, was synthesized, and this motivated scientists to look for other low density ice clathrates.[1] Clathrates containing methane are found at high pressure conditions on the ocean floor, but the contained water structure was not thought to be stable upon removal of the guest molecule.[2]

The research team used a Monte Carlo packing algorithm and other techniques to predict the cubic clathrate structure, s-III, that has two large icosihexahedral cavities and six small decahedral cavities per unit cell.[1] This structure was found to be dynamically stable with the guest molecule removed.[1] Synthesis of the clathrate will be difficult, since it would require a pressure of about four megabars at -10°F.[2] The guest molecules could then be extracted by a vacuum process.[2]

If this ice can be synthesized, it will be the 18th known phase of water; however, it will be known as "Ice XVII," not Ice XVIII. That's not because the ice phases, like elements of a data array, start at zero. It's because there are two versions of "normal" ice (Ice I), one hexagonal, and the other cubic. The team's research was funded in part by the National Science Foundation.[1-2]

Ordinary people, including most scientists, are most interested in a specific aspect of ice; namely, how best to remove it from surfaces, such as automobile windshields. I've written articles about a method for producing ice-resistant surfaces in an earlier article (Icing-Resistant Surfaces, August 8, 2012). In that technique, called SLIPS, for "Slippery Liquid Infused Porous Surfaces," a liquid is held in place on a nanostructured surface. Since frost and ice can't adhere to the liquid, it will slide off.[3-4]

Scientists and engineers from the University of Michigan (Ann Arbor, Michigan) and the Air Force Research Laboratory (Edwards Air Force Base, California) have developed a different approach to ice resistance. They found that they could render surfaces ice repellent, or "icephobic," by coating them with a clear, rubbery material that causes ice to quickly slide off because of a mechanical principal called interfacial cavitation.[5-8]

Surfaces are classed as icephobic if ice can be removed with a shear force of ≤ 100 kPa, but passive ice removal in which the ice will just slide off a surface will happen only when the shear force is ≤ 20 kPa.[5] By using a blend of common synthetic rubbers, the research team was able to produce coatings with a shear force of ≤ 0.2 kPa, and durable coatings able to withstand severe mechanical abrasion, acid/base exposure, 100 icing/deicing cycles, temperature cycling, and accelerated corrosion.[5-6]

Slip-sliding away. A demonstration of the icephobic coating. The effect is dramatic in this case, since the mass of the thick block of ice generates considerable shear force at the interface. (Still images from a YouTube Video.[8])

As happens often in research, this material property was discovered by accident.[6] Says materials science graduate student, Kevin Golovin,

"Researchers had been trying for years to dial down ice adhesion strength with chemistry, making more and more water-repellent surfaces... We've discovered a new knob to turn, using physics to change the mechanics of how ice breaks free from a surface."[6]

The research team has been able to fine-tune the effect by changing the rubber mix. Softer coatings, while more ice repellent, are less durable, and the opposite is true for harder coatings.[6] University of Michigan Materials Science and Engineering associate professor, Anish Tuteja, who led the study, is looking forward to an immediate application.

"I think the first commercial application will be in linings for commercial frozen food packaging, where sticking is often a problem. We'll probably see that within the next year... Using this technology in places like cars and airplanes will be very complex because of the stringent durability and safety requirements, but we're working on it."[6]

This research was funded by the Office of Naval Research, the Air Force Office of Scientific Research, and the National Science Foundation under its Nanomanufacturing Program.[6]

Real-world demonstration. Keeping your school colors showing in Michigan's icy winter. (University of Michigan image.)

Low Density Ice, Shedding Surface Ice

References: