Laboratory Studies Investigating the Influence of Particle Diameter on Viscosity

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Visibility, cloud formation, climate and human health are affected by suspended particles that are smaller than 100 nm in size, yet, many key particle properties remain ill-defined. In this project, novel experimental techniques are used to increase understanding of how particle viscosity changes with size. Results will help explain and predict atmospheric chemical processes with implications on climate and human health, as well as apply to a wide range of fields where nanoparticles are present. Graduate and undergraduate students will be an integral part of the work, and outreach activities are planned for high school students. As particle size decreases, strong curvature of the particle-air interface leads to deviations in physical properties from those observed in bulk materials. Viscosity is one such property, which dependence on particle size is poorly understood, yet, viscosity plays key roles in diffusion and reaction rates affecting atmospheric chemistry, global climate and human health. To shed light on the particle size-viscosity relationship, the present study seeks to quantify how solid-liquid phase transitions are influenced by particle diameter between 10 and 100 nm. Particular focus is on 50 nm particles composed of pure or multicomponent blends of a series of functionalized organic compounds that serve as models for oxidized atmospheric organic aerosol (OA). A shape-relaxation method is used that relies on the coagulation of laboratory-generated dumbbell-shaped dimer particles to single spheres. This transformation occurs at a temperature and viscosity that is representative of the solid-liquid phase transition. The change in particle morphology is measured in a differential mobility analyzer. Besides impacting climate and human health, findings will be transferable beyond the field of atmospheric sciences where nanoparticles are investigated. Through this project, graduate and undergraduate students will receive hands-on training on cutting edge instrumentation. High school students will be enticed to visit university sciences laboratories by participating in chess tournaments organized on the university campus.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.