Ice nucleation properties of well characterized single particle-droplet pairs assessed using a microfluidic platform

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

Water droplets do not freeze spontaneously until -40 degrees C, unless there is a particle surface to act as a nucleus for the freezing process. Ice nucleation and the resulting freezing of water induced by particles and surfaces is important in many processes. It plays a critical role in organ preservation and in the manufacturing and purification of pharmaceuticals, food, biomolecules, cosmetics, and nanomaterials. Ice nucleation also plays a key role in cloud glaciation, the transformation of water droplets in a cloud to ice crystals. Glaciated clouds are responsible for most precipitation over land. Atmospheric ice nucleating particles that cause clouds to glaciate create significant changes in the structure and behavior of the clouds. This cloud freezing dramatically changes the lifetime of the cloud and increases its ability to precipitate. However, we lack a rigorous understanding of this heterogeneous ice nucleation process. We still do not understand what special properties make these rare one-in-a-million atmospheric particles effective at nucleating ice crystals. We also lack small transportable instruments that can determine the concentration and freezing temperature of atmospheric ice nucleating particles in real-time by sampling from suspended aerosol particles. This research addresses many of these short-comings through the development of a new microfluidic approach to continuously capture and measure ice nucleating particles. Individual particles are activated into liquid droplets, which are then captured into an oil flowing through a microchannel in a device fabricated from a soft polymer. By sending the microdroplets through a temperature gradient applied to the microfluidic device, the freezing temperature of each particle-droplet pair is determined optically as each droplet turns opaque upon freezing. The use of engineered carbon nanotubes and metal oxide nanoparticles as robust ice nucleating particle standards with well-defined and reproducible freezing temperatures will then be explored using our unique device. The importance of heterogeneous ice nucleation and phase transitions will be communicated to the public and to underrepresented K-12 students in particular by conducting hands-on educational activities at several local public schools through after-school programs. The students will conduct experiments where they induce freezing in a small cloud chamber. These activities will be developed into self-contained educational modules that will be shared with a larger number of educators. A new approach will be developed for transferring individual size-selected aerosolized particles in microdroplets into a continuous oil flow in a microfluidic chip. This will enable new experimental avenues in numerous fields. The capture of particles - first activated into droplets using a cloud condensation nuclei counter - into the continuous oil flow will be optimized using scaling analysis to tune the aerosol and oil flow rates to ensure droplet impaction and coalescence that avoids bouncing or droplet shattering. A linear temperature gradient will be applied along the microchannel using micro-Peltier elements, and measured using an array of thin-film thermocouples. Temperature measurements along with numerical simulations will provide the temperature gradient field along the chip, allowing us to correlate the observed position of droplet freezing to its critical ice nucleation temperature. The lattice and surface properties of engineered nanoparticles will be determined and correlated to the ice nucleation ability we determine using the microfluidic device. This will be used to test our hypothesis that engineering nanoparticles will have tight and robust freezing temperature spectra. The uniform characteristics of these engineered nanoparticles should result in very reproducible freezing properties, creating improved ice nucleating particle standards that are greatly needed for ice nucleation measurements. This project brings together experts in aerosol science and technology, atmospheric chemistry, transport phenomena, and microfluidics, to perform truly interdisciplinary research. As this research focuses on advancing fundamental understanding of a key phase transition, it will have immediate impact on a wide range of fields in science and engineering, such as materials synthesis and purification, physical chemistry, the geophysical sciences, and anthropogenic climate change. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.