Development of a High-Resolution Microfluidic Device in Atmospheric Ice Nucleation Research and Integrated Science Teaching

Ambient ice-nucleating particles (INPs) represent an important and unique subset of aerosol particles that facilitate the heterogeneous formation of ice crystals under ice supersaturation conditions. To date, aerosol radiative forcing and feedback associated with INPs remains highly uncertain and not well understood. Particularly, the understanding of atmospheric ice formation in mixed-phase clouds, where immersion freezing predominates the ice formation mechanism, represents a major challenge and motivates the community's current interest in quantifying and improving prognostic skills for INP number concentrations. In this work, we developed an affordable, offline microfluidic freezing assay system to measure INP concentration and verified its performance with known ice nucleation active compounds in immersion mode as well as high-latitude soil samples. Two types of new microfluidic droplet trapping circuits, consisting of a static droplet array with 60 interconnected loops (15 nL) and 720 loops (1.5 nL), were fabricated to simulate the cloud-droplet-relevant size. We examined the freezing properties of suspensions of Snomax^®, illite NX, microcrystalline cellulose (MCC), and three Alaskan soil samples to see if previous laboratory results with µL freezing assay were reproducible within marginal uncertainties with our microfluidic device for these two different droplet volumes. Our results suggest that the highest freezing temperature and efficiency of Snomax^®, as well as the lowest freezing temperature and efficiency of MCC, is consistent for both techniques, and positive controls with known suspension samples were established for the microfluidic device. With the INP detection limit per unit mass of 10⁵ to 10¹³ g^-1 over the temperature range of -5 to -35 °C, the new microfluidic device is verified for its applicability to atmospherically relevant freezing conditions, providing accurate data for parameterization development. Lastly, the tools and approaches developed in this work are to be integrated into science teaching at a primarily undergraduate and Hispanic-serving institute. Our interim effort will be presented.