Experimental Investigations of the Contact Freezing Properties of Mineral Particles Using Optical Tweezers

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The Environmental Chemical Sciences Program in the Chemistry Division at the National Science Foundation supports the research of Professor Ryan Sullivan at Carnegie-Mellon who will determine the contact freezing properties of atmospherically relevant mineral particle compositions using optical tweezers. Professor Sullivan and his students will determine the ability of various types of mineral dust particles to catalyze the spontaneous heterogeneous freezing of supercooled droplets through contact freezing. The droplets will be suspended in a focus laser beam using a refrigerated optical tweezers system. The retrieved Raman spectrum will provide key information on the droplet?s physical and chemical state just prior to and when freezing occurs. The number of dust particle-droplet collisions that occur before freezing will be explicitly known, and contact freezing will also be discriminated from immersion freezing mechanisms. This research will help to resolve long outstanding questions regarding the prevalence of contact freezing in the atmosphere, and its potential importance for modifying cloud properties and inducing precipitation.The proposed research will help fill an important knowledge gap regarding a fundamental phase transition of a ubiquitous molecule in our environment: water. The results will impact a wide range of fields from physical chemistry to biogeochemistry and atmospheric science, and increase our understanding of aerosols-cloud-climate interactions. The findings will be particularly important for improving the outdated descriptions of heterogeneous ice nucleation currently used in weather forecast and global climate models. The project will provide students with valuable experience in the novel use of optical tweezers to study heterogeneous ice nucleation for the first time. Our international collaboration with scientists at the University of Bristol will afford our students with the unique opportunity to be trained by experts in the use of optical tweezers for atmospheric aerosol particle research. The project will provide training opportunities for students in a research area of great societal importance.