Stringy Soot

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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.
As a graduate student, Rajan Chakrabarty, now a postdoctoral fellow in the Reno lab, made soot in a standard premixed oxygen-ethylene flame. He sprayed the particles with charges from a radioactive source, and then passed the charged soot through an apparatus that selects particles of a specific size and charge. Finally, he used an electron microscope to measure the shapes of the selected particles. Surprisingly, doubly-charged particles about half a micron in size had a fractal dimension that ranged from about 1.2 to 1.5, far from the usual value of 1.8. The researchers suggest that the requisite two electrons can more easily cohabit these small rod-like particles because they can stay farther apart than on a more compact particle.
But how did these anomalous particles, which they found make up about 3% of the total, arise in the first place? The researchers speculate that slightly elongated particles pick up positive and negative charges in the flame, causing them to align with random electric fields. Moving along electric field lines, they aggregate to form even longer particles. In support of this model, the team saw less of the stringy soot when the flame was hotter, perhaps because the electric field has less influence when the particles bounce around more rapidly. The researchers plan to further test their idea by adding an external electric field, with the hope of making even more of the rod-like particles.
Other researchers, Chakrabarty notes, have found that such elongated clusters are blacker because they scatter less light than their bushy cousins. So controlling their numbers within soot could be useful in the production of many flame-generated products, he says. These products include the "carbon black" that is added to car tires and used as a pigment in toner for printers and copiers. Atmospheric soot affects the amount of solar radiation absorbed by the earth, so the variety of particle shapes is important to atmospheric scientists as well.
Many previous experiments have found that the entire population of soot particles has a dimension of 1.8, notes Christopher Sorensen of Kansas State University in Manhattan, Kansas. The new work is "alerting us to the fact that the ensemble may have various sub-populations that have different morphologies," he says. "It's good to be aware that there's a deeper story than what we've been saying for 20 or 30 years."--Don Monroe Don Monroe is a freelance science writer in Murray Hill, New Jersey.