Inspired by naturally occurring air bubbles in glaciers, researchers have developed a method to encode messages in ice. Publishing June 18 in the Cell Press journal Cell Reports Physical Science, the paper explains how the team encoded frozen messages in binary and Morse code by manipulating the size and distribution of bubbles in ice. The method could be used to store short messages in very cold regions such as Antarctica and the Arctic, where conventional information storage is difficult or prohibitively expensive.
"In naturally cold regions, the use of trapped air bubbles as a means of message delivery and storage uses less energy than telecommunication and is more covert than paper documents," says mechanical engineer and author Mengjie Song, affiliated with the Beijing Institute of Technology. "These ice messages can be preserved for a long time and the messages they carry are easy to visualize and read."
As water freezes, dissolved gases are squeezed out and pushed together, forming trapped pockets of air (e.g., bubbles). These bubbles are either egg-shaped or needle-shaped and can usually be found within three-dimensional chunks of ice. To investigate how these air bubbles form in ice, the team used a cold plate to freeze a two-dimensional layer of water between two transparent sheets of plastic. Then, they tested different temperatures and orientations to examine the impact of freezing rate and direction on bubble formation.
They found that suddenly increasing the freezing rate by sharply decreasing the cold plate temperature resulted in a single bubble layer. Faster freezing rates resulted in egg-shaped bubbles, so by gradually reducing the freezing rate, they were able to produce consecutive layers containing differently shaped bubbles: the first layer contained only egg-shaped bubbles, followed by a layer with egg- and needle-shaped bubbles, followed by a layer of needle-shaped bubbles—and finally a bubbleless layer of clear ice.
"Since bubble position and shape are determined by the freezing rate, it is possible to manually control the freezing rate to manipulate the shape and distribution of bubbles in ice," says Song.
Next, the researchers tested whether they could use this information to encode messages in ice. To do this, they assigned bubble sizes, shapes, and positions to distinct characters within Morse and binary codes. Then, they programmed their cold plate to control the rate and direction of freezing, resulting in a slice of ice with appropriately positioned and sized air bubbles.
To read the frozen message, the team took a photo of the ice and converted it to gray scale. Then, they trained a computer to automatically detect the position and size of the air bubbles based on their gray value (bubbleless regions are dark gray, whereas bubbles are almost white). Based on these grayscale values, the computer decoded the frozen message into binary or Morse code and then converted the message into a readable format—in this case, as English letters and Arabic numerals.
After comparing Morse and binary coding methods, the researchers concluded that binary coding is the better option because it is able to store messages that are around ten times longer.
Being able to control the position and distribution of bubbles in ice could have applications beyond messaging, the researchers say. For example, since bubbles decrease ice's mechanical strength, placing bubbles in a straight line could enable ice sheets to be neatly snapped, like the perforated line on a graham cracker. The method could also help scientists understand how bubbles form in other solid materials, such as aluminum, which cannot be imaged internally.
"Our findings can be widely applied in many areas," says Song. "In our daily life, we can manipulate bubbles to efficiently produce ice with different bubble contents and create beautiful ice sculptures. In industry, our research can help with metal smelting and manufacturing, as well as de-icing for aircraft and ships."
In the future, the team plans to investigate the impact of gas type and concentration on bubble ice characteristics and to further examine bubble formation in three-dimensional contexts.
