Liquid flattens ice’s nanoscale surface

Where there’s ice, there’s often liquid water, and that liquid can affect the ice’s properties. To investigate the interface between solid and liquid water, researchers have used atomic force microscopy: When the microscope’s cantilever arm with its tiny probe passes closely over a surface, small deflections caused by various forces between the probe and the surface are measured. The resulting image depicts features as small as a fraction of a nanometer.

Many of the investigations into ice–liquid interfaces have been done under vacuum, but measurements in more realistic scenarios are challenging. At water’s freezing point, both ice and liquid water are rapidly changing phase. As a result, the interface fluctuates in space too quickly for the microscope to obtain reliable measurements. Now researcher Hiroshi Onishi at Kobe University, Taketoshi Minato of the Institute for Molecular Science (both in Japan), and their colleagues have conducted experiments in subfreezing conditions to measure the interface between water ice and a liquid.

To make clear observations, the researchers had to place their atomic force microscope in a cold, stable environment. Even after nitrogen gas was pumped into the enclosure, the temperature still fluctuated too much for them to collect images. Only after they added an additional cooling system of copper tubing filled with antifreeze did the temperature remain low enough—at about −8 to −3 °C—to make clear observations. In the subfreezing container housing the atomic force microscope, shown in the photo below, the researchers added drops of water to a hydrophilic mica surface, and the drops froze within 30 minutes.

Onishi, Minato, and colleagues chose to measure the interface of water ice with alcohol instead of with liquid water. Alcohol is similar to liquid water in many ways, but because its freezing point is substantially lower than that of water, the researchers could clearly measure the ice–alcohol interface without it constantly moving.

In the absence of alcohol, the ice formed a rough surface with frost pillars a few tens of nanometers tall. With a liquid layer of alcohol added on top, it instead formed a flat surface, whose height varies only as much as 0.3 nm. Even though the temperature is below water’s melting point, the researchers suspect that the flat ice–alcohol surface is formed from the exposed ice partially dissolving in the liquid alcohol and then recrystallizing as a flat structure. (R. Yanagisawa et al., J. Chem. Phys. 161, 024702, 2024 .)