Effects of volatile coatings on the laser-induced incandescence of soot

Abstract

We have measured time-resolved laser-induced incandescence (LII) from combustion-generated mature soot extracted from a burner and (1) coated with oleic acid or (2) coated with oleic acid and then thermally denuded using a thermodenuder. The soot samples were size selected using a differential mobility analyzer and characterized with a scanning mobility particle sizer, centrifugal particle mass analyzer, and transmission electron microscope. The results demonstrate a strong influence of coatings on the magnitude and temporal evolution of the LII signal. For coated particles, higher laser fluences are required to reach signal levels comparable to those of uncoated particles. The peak LII curve is shifted to increasingly higher fluences with increasing coating thickness until this effect saturates at a coating thickness of *75 % by mass. These effects are predominantly attributable to the additional energy needed to vaporize the coating while heating the particle. LII signals are higher and signal decay rates are significantly slower for thermally denuded particles relative to coated or uncoated particles, particularly at low and intermediate laser fluences. Our results suggest negligible coating enhancement in absorption cross-section for combustion-generated soot at the laser fluences used. Apparent enhancement in absorption with restructuring may be caused by less conductive cooling.

Effects of volatile coatings on the morphology and optical detection of combustion-generated black carbon particles.

“…When the remaining oleic acid is removed in the thermodenuder, the measured particle sizes shrink to the size distribution of the core soot particles. This effect is more pronounced for heavily coated particles, which is consistent with recent results based on laser-induced incandescence studies [75].…”

“…More compact aggregates have a smaller effective surface area. Our results [112] have shown that the LII signal decay rate can be substantially reduced for aggregates with larger fractal dimensions. Coating and denuding the particles causes the aggregates to become more compact, increasing the fractal dimension from an average of 1.88 for the uncoated particles (Fig.…”

“…In this report we present results of experiments focused on determining the effects of coatings of oleic acid on the morphology of combustion-generated mature soot particles [51] and on the magnitude and temporal response of LII signal from these particles [75]. Our results demonstrate that, for oleic-acid coatings, the dendritic soot particles become more compact when the coating is applied.…”

“…Nonabsorptive coatings are well known to change the optical properties of soot particles and have a strong effect on absorption and scattering cross sections [21, 50, 54-56, 59, 62, 63, 69], but the impact of aggregate morphology and restructuring on the optical properties of soot aggregates is not as well understood. Volatile coatings can also have an impact on the radiative emission from laser-heated soot and graphite particles [21,50,71,75].…”

“…This result is qualitatively consistent with the experimental results of Moteki and Kondo [71] and Slowik et al [50], which showed that the delay of the onset of the LII signal, once the particle has intersected with the laser beam, increased with increasing coating thickness. [75] None of these studies directly addressed the impact of coating-induced restructuring of the particle. Several studies have targeted the effects of aggregate size on LII signals via its influence on particle conductive-cooling rates [76][77][78][79][80] and optical properties [81,82], but none of these studies has addressed the effects of fractal dimension on LII.…”