| Laboratory
Investigation of Heterogeneous Ice Nucleation Ice formation in atmospheric clouds is crucial to our understanding of precipitation and cloud radiative properties. Laboratory experiments offer an opportunity to study fundamental atmospheric processes, such as freezing, with the advantage of having the ability to control variables such as temperature, pressure, and relative humidity. Recent experiments were performed in the MTU Cloud Physics Lab to understand heterogeneous ice nucleation in supercooled liquid water drops. We investigated the effect on the nucleation rate of adding a catalyst, or ice-forming nucleus (IN), to a drop of water. In our case we used volcanic ash particles and calibrated soda glass microspheres. We carried out experiments with pure water drops in an oil and air medium, and investigated the effect on nucleation rate of positioning the IN at the surface of the drop (contact freezing) versus fully immersed within the drop (immersion freezing). We also studied how freezing temperature varies during drop evaporation.Apparatus
Freezing Movie
Conclusions From our laboratory experiments, we concluded the following: (1) The nucleation rate at the drop surface is a factor of 1010 greater than in bulk water. (2) Interpretation of the data via classical nucleation theory shows that the free energy of formation of a critical ice germ is decreased by a factor of ~2 when the substrate is near the air-water interface. (3) Freezing temperature consistently increases by ~3-4 K regardless of whether an ice-forming nucleus comes into contact with the water surface from the outside-in, or from the inside-out, compared to immersion freezing involving the same ice-forming nucleus. (4) A liquid water drop containing an immersed ice-forming nucleus can undergo a sudden transition to a higher freezing temperature as the surface comes into contact with the ice-forming nucleus during evaporation.
Articles
Conference Proceedings Shaw, R. A., and A. J. Durant, Contact Nucleation Linked to ‘Evaporation Freezing’, Eos Trans. AGU, 86(52), Fall Meet. Suppl., Abstract A23C-0970, 2005. Durant, A. J., Y. Mi, R. A. Shaw, G. G. J. Ernst, and W. I. Rose, Ice Nucleation by Volcanic Ash: Influence of Composition and Morphology, paper presented at IAVCEI General Assembly, Chile, 2004. Mi, Y., A. J. Durant, and R. A. Shaw, Laboratory Measurements of Heterogeneous Ice Nucleation: Contact Nucleation Inside-Out, ICCP, 2004.
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