Pulsed lasers aid in the homogeneous nucleation of oriented ice

Ice nucleation typically initiates at a preexisting heterogeneous interface. This process exists everywhere from frost on bacteria to ice accretion on airplanes and is well understood. Homogeneous nucleation, on the other hand, occurs without an interface, making the time and place of its origin difficult to predict and the mechanism hard to study. Using a pulsed laser, Nevo et al. were able to induce homogeneous nucleation of water at temperatures significantly higher than its non-irradiated freezing point.

By exposing water droplets to 30-180 seconds of 10-hertz laser pulses with a corresponding field of about 10⁷ V/m, the authors found the irradiated drops froze up to 11 degrees Celsius higher than those not exposed to the laser pulses. Furthermore, the hydrogen-bonded arrays comprising the ice crystals were oriented parallel to the laser’s field polarization. Once X-ray diffraction measurements ruled out the possibility of heterogeneous nucleation, the group determined the laser-induced process was homogeneous.

“It was intuitive and clear to us that if the laser beam induced ice nucleation, then for sure the laser polarization has a central role,” said author Iftach Nevo. “It took us time to establish a link between the laser field polarization and the X-ray diffraction vectors, and subsequently with that of the H-bonding arrays in ice.”

The mechanism of how the laser pulses induce homogeneous nucleation remains an open question. Looking forward, the authors plan to further their study to include other types of molecules.

“Without restricting ourselves to a particular material, understanding homogeneous nucleation processes of molecular materials paves the way to develop methods to control solidification processes in which the interface with foreign media does not affect the final outcome,” Nevo said.

Source: “Evidence for laser-induced homogeneous oriented ice nucleation revealed via pulsed x-ray diffraction,” by Iftach Nevo, Sabrina Jahn, Norman Kretzschmar, Matteo Levantino, Yishay Feldman, Nir Naftali, Michael Wulff, Dan Oron, and Leslie Leiserowitz, Journal of Chemical Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0006100 .