Temperature-Dependent Viscosity of Organic Materials Characterized by Atomic Force Microscope

The viscosity of atmospheric aerosol particles determines the equilibrium timescale at which a molecule diffuses into and out of particles, influencing processes such as gas–particle partitioning, light scattering, and cloud formation that can affect air quality and climate. This particle viscosity is sensitive to environmental conditions such as relative humidity and temperature. Current experimental techniques mainly characterize aerosol viscosity at room temperature. The influence of temperature on the viscosity of organic aerosol remains underexplored. Herein, the viscosity of atmospherically relevant organic materials was examined at a range of temperatures from 15 °C to 95 °C using an atomic force microscope (AFM) equipped with a temperature-controlled sample module. Dioctyl phthalate and sucrose were selected for investigation. Dioctyl phthalate served as the proxy for atmospherically relevant primary organic materials while sucrose served as the proxy for secondary organic materials. The resonant frequency responses of the AFM cantilever within dioctyl phthalate and sucrose were recorded. The link between the resonant frequency and material viscosity was established via a hydrodynamic function. Results obtained from this study were consistent with previously reported viscosities, thus demonstrating the critical capability of AFM in temperature-dependent viscosity measurements.