Mechanical properties of bare and coated soot aggregates probed by atomic force microscopy

Abstract

Soot from incomplete combustion of carbonaceous materials is a major constituent of atmospheric aerosols. Individual soot particles are aggregates of primary carbon spherules connected together by carbon necks. Freshly released soot aggregates have lacey fractal morphology, but in the atmosphere they undergo compaction, induced by capillary forces exerted by liquid coatings that act against the covalent, cohesive and friction forces between the carbon spherules. Since compaction alters the optical properties and atmospheric lifetime of soot, an ability to model this process is important for predicting the soot’s environmental impacts. To inform and validate our recently developed discrete element method (DEM) model of a soot aggregate, we employed force spectroscopy by atomic force microscopy to measure the forces and other mechanical properties related to the bonding between the spherules in the individual soot aggregates. Fractal and compact aggregates, both bare and with liquid coatings were examined. We observed a characteristic sawtooth pattern on force–displacement curves and collected statistics on bonding forces within individual fractal aggregates, as they were fractured and unraveled. Contrary to fractal aggregates, compact aggregates could not be unraveled due to multiple cohesive interactions between spherules. An increase in bonding forces and energies due to capillarity was observed in coated aggregates. The sawtooth pattern was interpreted with the help of a simple conceptual model and the rigorous DEM model was used to show that only one or two necks need to be fractured for a fractal aggregate to yield, and that mechanical failure will most likely be in shear.