Focus: Freezing Water from the Outside In
Ice forms easily around a foreign nucleus such as a grain of dust, but how it does so is poorly understood. To examine this process, a pair of researchers used a layer of alcohol as the nucleation surface. They surrounded water drops with alcohol and measured their freezing points. The team discovered that the freezing process is strongly controlled by the boundary between the drop and the alcohol. Their experiments, reported in the 30 July print issue of PRL, are among the first to look at how ice forms on a foreign substance. The results could help atmospheric scientists determine if pollution leads to an observed abundance of ice in the lower atmosphere.
Cool a drop of water below its freezing point, and it may not freeze. A small cluster of molecules must first align to form a nucleus that the rest of the drop can crystallize around. The constant thermal motion in a drop can easily frustrate these attempts at solidarity and allows water to stay liquid–even at temperatures 10 degrees Celsius below its freezing point. This so-called supercooled water is common in the atmosphere where pure water drops float freely, but it is rarely found on earth because dust and other particles act as crystallization nuclei.
Although researchers have studied freezing around such nuclei for decades, they’ve had a hard time learning about it because the details of the process are difficult to observe. In their paper, Lane Seeley and Gerald Seidler of the University of Washington in Seattle used statistics to infer the exact process behind the freezing of drops coated by a single layer of alcohol molecules. The alcohol acted as the nucleus, freezing the water from the surface inward. The researchers froze and re-froze single drops of water over 500 times, each time recording the freezing point of the drop. The freezing point varied slightly each time, and from that variation they could determine the rate at which the drop would freeze for a given temperature. They used that rate to infer other properties of the drop via a classical theory of nucleation. Specifically, Seeley and Seidler determined the energy barrier that the drop must overcome to freeze and the diffusion rate of water molecules at the surface of the drop.
The team found that the diffusion of water molecules at the freezing point times slower when the alcohol was present. This result suggested to them that a single, two-dimensional layer of water molecules interacts with the alcohol. “A two-dimensional system always has much slower molecular kinetics than a three-dimensional system,” says Seidler, so it could explain the sudden loss of mobility. This view contrasts sharply with traditional theories that the freezing process starts in a three-dimensional volume of water near a point at the surface.
“I think it’s a very interesting result from a beautiful experiment,” says David Oxtoby of the University of Chicago. Oxtoby points out that this is one of the first times statistics has been used to study the nucleation process, and he believes the technique will be very fruitful. Seidler hopes that this research can provide some insight into a poorly understood phenomenon in the earth’s atmosphere. According to recent studies, there is a larger than expected amount of ice in the lower atmosphere, and atmospheric scientists believe pollutants similar to the alcohols Siedler and Seeley studied might be causing drops to freeze at unusually high temperatures.