Effects of volatile coatings and coating removal mechanisms on the morphology of graphitic soot

We have measured morphological changes of combustion-generated mature soot with various quantities of hydrocarbon coating and different coating-removal mechanisms. We made these measurements on soot extracted from a burner and then (1) coated with oleic acid, (2) coated with oleic acid and then denuded using a thermodenuder, (3) coated with oleic acid and then heated with a laser, and (4) coated with oleic acid, denuded with a thermodenuder, and then laser heated. We compared these results to results for untreated soot from the burner. The soot samples were size selected using a differential mobility analyzer prior to coating. Uncoated, coated, and denuded particles were characterized by electric mobility size, particle and coating mass, and particle morphology. Our results show that the particles are restructured (become compact) when coated. Particles sent through the thermodenuder are irreversibly restructured. Laser desorption of coatings with thicknesses >= 20% by mass, however, returns the soot particles to a less compact morphology with some fragmentation as the coating rapidly vaporizes. A majority of laser-heated heavily coated particles stay associated with unvaporized oleic acid droplets despite some fragment ejection from the droplet. Thermally denuded particles neither return to a less compact morphology nor fragment when laser heated. (C) 2013 The Authors. Published by Elsevier Ltd. All rights reserved.