02/06/2018
Ice Formed by Contact Freezing: Pressure Matters, Not Just Temperature
Laboratory measurements show that distortion of water droplet surface may increase the likelihood of the droplet freezing.
The Science
The formation of ice in clouds influences precipitation rate, large-scale cloud motions, and cloud optical properties. This research helps to address a long-standing mystery about why water droplets will freeze at a warmer temperature when they are impacted by a particle than when the same particle is immersed in the drop.
The Impact
In most predictive models of cloud formation and growth, temperature is the most important variable in determining whether or not a cloud is composed of water droplets or ice crystals. These new results imply that in addition to temperature and aerosol material properties, it may also be important to consider dynamic properties that influence the water surface.
Summary
In the atmosphere, droplets of liquid water are frequently found at temperatures below the freezing temperature of water (0°C) down to temperatures as cold as around -40°C. These cold liquid droplets are referred to as “supercooled”. Supercooled drops can freeze into ice crystals in the presence of a class of small particles known as ice-nucleating particles. Understanding which particles serve as ice-nucleating particles under which circumstances and at which temperatures is an active research area because the formation of ice in clouds influences precipitation rate, large-scale cloud motions, and cloud optical properties. Much of the ice formation in the atmosphere is a result of catalysis by ice-nucleating particles, and the many different types of ice-nucleating materials are usually characterized by the temperature at which they trigger freezing. A long-standing mystery is the observation that supercooled water droplets freeze at a warmer temperature when an ice-nucleating particle impacts the water surface (contact nucleation), compared to the same particle being immersed in the droplet (immersion nucleation). Researchers performed laboratory experiments where they held the droplet temperature constant and agitated pure water drops and drops contaminated with a small amount of oil on two different surfaces. The drops were mechanically agitated using a frequency-controlled speaker and photographed with a high-speed camera to detect whether freezing occurred and if so, when and where on the drops. These experimental results show that ice nucleation initiated by mechanical agitation is strongly related to the moving and distorted three-phase contact line, suggesting pressure perturbations as a cause.
Principal Investigator(s)
Raymond A. Shaw
Michigan Technological University
[email protected]
Funding
This work was supported by DOE Office of Science as part of the Atmospheric System Research program through Grant No. DE-SC0011690. A. Kostinski acknowledges support from NSF Grant No. AGS-1639868. We thank Dr. W. Cantrell for helpful discussions and Dr. D. Knopf for pointing out the supersaturation possibility. We thank Dr. A. Barnard for assistance with the laser vibrometer measurements, Yujin Sun and Dr. J.W. Drelich for assisting with surface characterization, and Dr. M. C. Frost for providing polymer substrates.
References
Yang, F., O. Cruikshank, W. He, A. Kostinski, and R. Shaw, “Nonthermal ice nucleation observed at distorted contact lines of supercooled water drops.” Physical Review E, 97(2), 023103, (2018). [doi:10.1103/PhysRevE.97.023103]