Stringy Soot

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.