Surface deposition and coagulation efficiency of combustion generated nanoparticles in the size range from 1 to 10 nm

Flame-Formed Carbon Nanoparticles: Morphology, Interaction Forces, and Hamaker Constant from AFM

“…The mechanism of collision and sticking of small particles can be viewed as due to a balance between the particle kinetic energy and the mutual interaction between the particles (van der Waals forces) (Narsimhan and Ruckenstein 1985); when particles are in the free molecular regime, at high temperature such as that reached in flames, the particle kinetic energy may be higher than the interaction energy, resulting in a thermal rebound effect after collision (Wang and Kasper, 1991). Experimental results (Minutolo et al 1999;Sgro et al 2003;D'Alessio et al 2005) agreed with this simple model showing that flame-formed nanoparticles with 2-5 nm size possess a rate of coagulation lower than that of larger soot particles (20-50 nm) and well below the value predicted by the gas-kinetic theory, thus showing that the coagulation efficiency depends on particle size, morphology, and chemical structure. Moreover, temperature has been demonstrated to play a key role in determining the rate of coagulation of different-sized nanoparticles (Sirignano and D'Anna 2013), also in different temperature regimes.…”

“…However, little is known about the forces, particularly van der Waals interactions, acting between carbon nanoparticles. Also, the Hamaker constants for flame-formed nanoparticles (Minutolo et al 1999;Sgro et al 2003;D'Alessio et al 2005), usually used in the calculation of the particle interaction potential, have been so far mainly assumed on the basis of particle chemical nature, since no direct experimental measurement can be found in the literature.…”

Charge Distribution of Incipient Flame-Generated Particles

“…The smaller particles have a charge fraction in agreement with Boltzmann curves near the maximum flame temperature while the f +1/−1 of larger particles in the distribution are in better agreement with B(T) at the cooler temperatures near the probe inlet. In order to analyze in more detail the smallest inception particles, we performed measurements as a function of height above the burner, H, thus varying flame residence time, in two flames, where the coagulation process is quite different (D'Alessio et al 2005;Minutolo et al 1999). Figure 7A and B shows the measured size distributions of negatively charged particles and total particles at various H for two ethylene-air flames (C/O = 0.61 and 0.65).…”

“…Losses are only significant in the orifice prior to dilution and only for d < 4 nm particles Sgro et al 2009). Previous works investigating the impact of flame-generated inception particles with surfaces at flame temperatures concluded that d < 5 nm particles may experience thermal rebound, and there is no currently accepted theory to calculate size-dependent losses of particles in tubes at the temperature of the orifice (800-1000 K) in the size range of interest in this study (1-5 nm) (D'Alessio et al 2005;De Filippo et al 2009;Sgro et al 2009). Losses in the orifice are significantly different depending on whether or not thermal rebound occurs, but in both cases they only affect d < 4 nm particles (see Figure 5 in De Filippo et al 2009).…”

“…Thermal and diffusion charging mechanisms are explored to explain the measured charge distributions, and we discuss the implications of the results for particle coagulation at high temperature. The particles are produced in atmospheric pressure premixed flames, which have been previously examined with many complementary particle diagnostics with sensitivity to particles/macromolecules as small as 1 nm, including multiwavelength optical, microscopy and mobility measurements (D'Alessio et al 2005Minutolo et al 1999;Sgro et al 2008). While the various measurement techniques are in excellent agreement in flame conditions that eventually produce particles larger than 3-5 nm, the DMA measurements are significantly lower than in situ optical measurements in flame conditions where the size distribution is unimodal with a modal diameter, d modal ∼ 2 nm ).…”