Some flame conditions create straight sooty aggregates. Their special properties could have effects in the atmosphere.

The soot from a flame consists mostly of tiny, bush-like particles with many branches, whose shape is predicted by theory. In the 12 June Physical Review Letters, however, researchers report that some soot particles are straighter. These rare, oddly shaped particles have optical properties that could be important for industrial flame-generated materials like pigments and printer toner, and they might also affect the amount of sunlight reflected by the atmosphere.

Soot particles grow inside a flame when tiny, carbon-rich spheres stick together to form larger, tenuous aggregates. As they grow, the particles take on a characteristic branched shape because two colliding clusters are most likely to attach at their protruding "fingers."

These bushy shapes are conveniently described as fractals--geometric objects whose mass grows as a fractional power of their linear size, rather than the third power that characterizes ordinary solids like spheres and cubes. Theory predicts that virtually all clusters should have a fractal dimension very close to 1.8, and past experiments agree. But a collaboration led by Hans Moosmüller of the Desert Research Institute in Reno, Nevada, found many clusters with a much lower dimension, characteristic of a more rod-like shape.

Read the full "Stringy Soot" article on Physical Review Focus >>
Phys. Rev. Lett. 102, 235504
(issue of 12 June 2009)