Instrument for studies of homogeneous and heterogeneous ice nucleation in free-falling supercooled water droplets

Abstract

We have developed an instrument to study the homogeneous and heterogeneous freezing of droplets in free fall. The advantages of this technique are high repetition rates, telemicroscopic imaging of frozen and unfrozen droplets, and the elimination of possible contamination and nucleation effects induced by substrates. Droplets are ejected at a rate of about 5 per s from a generator at the top of a temperature controlled freezing tube. They fall in a stream down the center of the tube as their images are recorded using video-telemicroscopy. The fraction of drops frozen is measured as a function of height (and, hence, as a function of temperature) by illuminating slices of the stream with linearly polarized laser light and monitoring the depolarization of the backscattered light; ice particles depolarize the scattered light while the liquid droplets do not. The use of depolarization for phase discrimination is unique in this context. We have demonstrated the usefulness of our instrument with pure water droplets and droplets containing water and a bionucleant (Pseudomonas Syringae, or “SNOMAX”). The observed homogeneous freezing temperature of pure water droplets is about −37.0 °C while heterogeneously frozen water droplets containing SNOMAX freeze at around −8.0 °C. We find that the homogeneously frozen pure water droplets tend to be more irregular and bumpy than those heterogeneously frozen.