Soot aggregate restructuring during water processing

• We monitor the restructuring of soot during water processing.
• We observed soot restructuring takes place above certain critical saturation ratio.
• The mechanism of soot restructuring was explored.
• Experiments indicate that soot restructuring occurs during evaporation, not during condensation.
• Calculation shows the restructuring is driven by capillary forces.

Soot aggregate restructuring is explored in laboratory prepared soot particles upon exposure to high humidity environment followed by rapid water evaporation. Soot was generated in a Santoro style ethylene diffusion burner, and condensation of water on the soot particles was realized in a temperature-controlled water growth tube. The structure transformation of soot particles under different humidity conditions was monitored using a Differential Mobility Analyzer – Aerosol Particle Mass Analyzer (DMA-APM) and Tandem Differential Mobility Analyzer (TDMA) methods. The primary measured properties were mass-mobility scaling exponent, particle mass and mobility size before and after processing. A critical saturation ratio was observed above which aggregate restructuring occurred. The morphological change was visualized by taking electron microscopic images. X-ray photoelectron spectroscopy (XPS) analysis found that the chemical structures of different processed soot were indistinguishable. To assess if soot collapse occurred during water condensation or evaporation, water-coated soot was directly injected into water, where multi-angle light scattering showed that the structure was uncollapsed. This result indicates that soot restructuring driven by capillary forces occurs during evaporation, not condensation.