Integrated studies of nanodroplet freezing

Ohio State University, The, Columbus OH

Investigators

Abstract

With this award,the Chemical Structure, Dynamics and Mechanisms (CSDM-A) Program of the Division of Chemistry is funding Professor Barbara Wyslouzil at Ohio State University to pursue an integrated experimental and molecular modeling program exploring the physics of freezing. This includes determining the limits to which liquids can be supercooled, the transformation kinetics, and the structure of the emerging solid phase. The focus is on short chain-like molecules, i.e. n-alkanes and n-alcohols with less than 12 carbons, confined in nanodroplets. These chain molecules are the building blocks of materials as diverse as lipids, surfactants, and polymers. Freezing is a fundamental physical process that plays a central role in both industry and nature, such as in atmospheric aerosols. The freezing transitions will be quantitatively investigated with extremely well characterized samples under conditions that are far from equilibrium, and a powerful set of experimental tools probes conditions very close to the still idealized requirements of molecular simulations. Prof. Wyslouzil will also engage with high school students in her research and conduct public outreach in collaboration with the Steam Factory, an initiative at her institution to promote interdisciplinary collaboration across Science, Technology, Engineering, Arts, and Math. Prof. Wyslouzil and her research group will pursue the following specific experimental objectives: (1) measure liquid to solid nucleation rates as nanodroplets freeze, (2) explore the role of molecular structure on the relative importance of surface versus bulk freezing, and (3) further develop X-ray scattering techniques to follow this transformation directly. The objectives of the molecular modeling work are to (1) understand the driving force for surface freezing in chain-like molecules, and (2) calculate liquid-solid nucleation rates to compare directly to the measured values. Supersonic nozzle flows are to generate liquid droplets in highly non-equilibrium states with static pressure measurements to characterize the flow, spatially resolved small and wide angle X-ray scattering (SAXS/WAXS) to characterize the aerosol, and infrared spectroscopy to quantify the distribution of the condensable fraction among the vapor, liquid and solid states. Efforts to develop single nanodroplet/nanocrystal X-ray diffraction measurements to measure the structure of freezing clusters on a particle-by-particle basis using free electron lasers are to be extended. Molecular dynamics (MD) simulations are to directly test competing theories of surface freezing, and develop molecular level insight into homogeneous liquid-solid nucleation using available interaction potentials for both n-alcohols and n-alkanes at the united atom or all-atom level.