Quantifying the Viscosity of Individual Submicrometer Semisolid Particles Using Atomic Force Microscopy

Abstract

Atmospheric aerosols' viscosities can vary significantly depending on their composition, mixing states, relative humidity (RH) and temperature. The diffusion time scale of atmospheric gases into an aerosol is largely governed by its viscosity, which in turn influences heterogeneous chemistry and climate-relevant aerosol effects. Quantifying the viscosity of aerosols in the semisolid phase state is particularly important as they are prevalent in the atmosphere and have a wide range of viscosities. Currently, direct viscosity measurements of submicrometer individual atmospheric aerosols are limited, largely due to the inherent size limitations of existing experimental techniques. Herein, we present a method that utilizes atomic force microscopy (AFM) to directly quantify the viscosity of substrate-deposited individual submicrometer semisolid aerosol particles as a function of RH. The method is based on AFM force spectroscopy measurements coupled with the Kelvin−Voigt viscoelastic model. Using glucose, sucrose, and raffinose as model systems, we demonstrate the accuracy of the AFM method within the viscosity range of ∼10 4 −10 7 Pa s. The method is applicable to individual particles with sizes ranging from tens of nanometers to several micrometers. Furthermore, the method does not require prior knowledge on the composition of studied particles. We anticipate future measurements utilizing the AFM method on atmospheric aerosols at various RH to aid in our understanding of the range of aerosols' viscosities, the extent of particle-to-particle viscosity variability, and how these contribute to the particle diversity observable in the atmosphere.

Effects of Wind Speed on Size-Dependent Morphology and Composition of Sea Spray Aerosols

“…A molecular force probe three-dimensional (3D) AFM (Asylum Research, Santa Barbara, CA) was used for imaging individual substrate-deposited SSA at ambient temperature (20–25 °C) and pressure as described previously. ,, A custom-made humidity cell was used to control RH within a range of ∼20 to 80% . Silicon nitride AFM tips (MikroMasch, model CSC37, typical tip radius of curvature of ∼10 nm, nominal spring constant of 1.0 N/m) were used to image individual SSA.…”