Researchers have pushed water’s freezing point lower than ever before and altered our understanding of how ice originates by generating ice from tiny droplets only a few hundred molecules in size.
The general rule is that water freezes at 32 degrees Fahrenheit (0 degrees Celsius). However, in some situations, water can remain liquid at lower temperatures.
In a study published on Nov. 30 in the journal Nature Communications, researchers managed to keep water droplets in a liquid state at temperatures as low as minus 47.2 F (minus 44 C).
Their breakthrough was made possible by two factors: very minute droplets and a very soft surface. They started with droplets as small as 150 nanometers, or about the size of an influenza virus particle, as large as 2 nanometers, or about the size of a cluster of only 275 water molecules.
The researchers determined the impact of size in the change from water to ice using this spectrum of droplet sizes.
“We covered all of these ranges so that we can understand at which condition ice is going to form—which temperature, which size of the droplets,” study co-author Hadi Ghasemi, a mechanical engineering professor at the University of Houston, told Live Science. “And more importantly, we found that if the water droplets are covered with some soft materials, the freezing temperature can be suppressed to a really low temperature.”
The soft material they used was octane, an oil that surrounded each droplet within the nanoscale pores of an anodized aluminum oxide membrane. As a result, the droplets could take on a more rounded shape with more pressure, which the researchers claim is necessary to prevent ice formation at these low temperatures.
Because observing the freezing process at such small scales is nearly impossible. The researchers used electrical conductance measurements—ice is more conductive than water—and light emitted in the infrared spectrum to capture the precise moment the temperature at the droplets turned from water to ice.
They discovered that the smaller the droplet, the colder it had to be for ice to form and that the rate of ice formation reduced substantially for droplets smaller than 10 nanometers. Frost didn’t develop until the water hit a bone-chilling minus 44 C in the tiniest drops scientists measured, Live Science reported.
Is this to say that minuscule droplets in clouds and biological cells can grow even colder than previously thought?
Ghasemi stated, “As a scientist, I would say we don’t know yet.”
This discovery, however, could have significant implications for ice prevention on human-made materials, such as those used in aviation and energy systems, according to Ghasemi.
Engineers could use a mix of soft and hard materials in their designs to prevent ice from forming on soft surfaces if water on soft surfaces takes longer to freeze.