Quantifying Viscosity of Semisolid Submicrometer and Nanosized Individual Aerosols Using Atomic Force Microscopy with Varying Cantilever Velocity.

Atmospheric aerosol viscosity governs diffusion time scales of reactive gases into aerosols, thereby influencing gas uptake, heterogeneous chemistry, and atmospheric aging. However, direct viscosity measurements of individual semisolid aerosols have been restricted to a relatively high viscosity range and submicrometer size. Herein, a new atomic force microscopy-based nanorheological experimental approach is presented that utilizes force-indentation distance and indentation rate measurements at varying cantilever velocities to quantify the viscosity of individual semisolid aerosols as a function of relative humidity (RH). Using glucose and sucrose as model systems, we demonstrate accurate single particle viscosity quantification spanning ∼102·5 to 107 Pa s under atmospherically relevant RH conditions. We establish an iterative cantilever velocity framework that enables the determination of the appropriate velocity required for accurate viscosity characterization of aerosols without prior knowledge of their composition. Additionally, we demonstrate that the method enables accurate viscosity quantification of individual sub-100 nm aerosols. We anticipate that future viscosity measurements of atmospheric aerosols from various sources at different RHs will provide valuable insights into aerosol viscosity variability and reveal the extent of particle-to-particle heterogeneity.