DFG: MINION - Microfluidic device for Ice Nucleation analysis In cONtinuous flow

Ice nucleating particles (INP) are atmospheric aerosol particles that catalyze the ice nucleation process in cloud droplets at temperatures between 0 and -37 °C. The type and quantity of INPs influence primary ice formation in clouds and their properties. Measuring INP concentration, i.e., the number of INPs that are active per unit volume of air at a specific temperature, poses an experimental challenge due to the large variability of approximately 10 orders of magnitude. Consequently, there is a wide range of instruments available for determining INP concentrations, each covering a specific concentration and temperature range based on its measurement principle. None of the currently available devices capture both rare high-temperature INPs (> -10 °C) and more common low-temperature INPs (< -25 °C). One way to expand the measurable concentration and temperature range is to investigate a large number of droplets in the nanoliter volume range. These can be generated using microfluidic droplet generators in an oil matrix and subsequently cooled. Some devices already utilize this approach for INP measurements and can be categorized into two methods: Firstly, there are instruments that produce a fixed number of droplets and store them in a microfluidic chip. This chip is cooled, and the freezing of the droplets is observed using a microscope and camera. Secondly, there are devices that continuously generate droplets and guide them through a microfluidic channel over a cooled surface. Advantages over the first method include an unrestricted number of droplets and the potential to connect with a collector that separates aerosol particles into water, allowing for quasi-online measurements of INP concentration. Therefore, in this project proposal, we suggest further development of the second method. The new system, MINION (Microfluidic device for Ice Nucleation analysis In cONtinuous flow), features a novel form of optical phase-state detection, which replaces the time-consuming and often error-prone analysis of camera images from previously developed devices. This optical method can be easily and cost-effectively scaled up, enabling high-resolution INP measurements between 0 °C and the homogeneous freezing limit when combined with an appropriate cooling system. The measurable INP concentration range expands by at least 2 to 6 orders of magnitude compared to existing instruments, allowing for the detection of both high- and low-temperature INPs. In the proposed project, we plan not only the development of MINION but also comprehensive characterization and measurements of atmospheric INP concentrations based on filter samples from various geographical origins.