(296a) Measurements of the Rate of Solid Nucleation in Drops of Supercooled Water and Hexadecane Using a High-Throughput Microfluidic Apparatus

We will report on measurements of the rates of nucleation of solids in supercooled water and hexadecane. These measurements were performed using a microfluidic apparatus that can record the individual freezing temperatures of tens of thousands of microdrops in a measurements cycle that lasts approximately 10 minutes; to our knowledge, this apparatus has the highest data throughput among instruments designed for the study of nucleation of solids in supercooled liquids. This apparatus is a versatile tool, allowing the study of both homogenous and heterogeneous nucleation of ice in water, and of solid nucleation in other substances such as hydrocarbon liquids. Our measurements of the rate of ice nucleation in pure water span the temperature range between -38.5 ºC and -36 ºC. The large size of our statistical ensembles of freezing temperatures and an accurate temperature-measurement system allowed us to perform accurate measurements of the homogenous nucleation rate of ice over a wide dynamic range. These measurements agree with independent results, showed that ice nucleation is homogenous in the volume of the microdrops, and showed that the ice nucleation rates had a simple exponential dependence on supercooling temperature. Solid nucleation in hexadecane had a more complex kinetics. Preliminary measurements indicate that freezing of microdrops of hexadecane occurs through two different processes that are dominant at different supercoolings. Direct imaging of freezing hexadecane microdrops suggests that the mechanism of freezing is epitaxial growth of the solid phase starting from the surface of the drop.