Researchers have simultaneously observed the flow of both the
liquid and solid components of a colloid–a fluid-particle mix
akin to blood or contaminated ground water. The team used an
MRI-like imaging system and found a surprise: even in slowly
flowing, dilute colloids, the particles refuse to remain on
purely parallel paths, like calm drivers on a highway. Instead,
they change lanes erratically, as the researchers report in the
14 December Physical Review Letters. The new insights should
improve researchers’ understanding of flows in many kinds of
colloids in biology and industry. For high concentrations and
flow speeds, particles in a colloid bump into each other
frequently, leading to chaotic flow of the particles. But for a
slow, dilute colloid flowing through a tube, researchers have
thought that each particle remains on the fluid’s local
streamline–moving faster in the middle, slower near the
edges–literally going with the flow. Now Joseph Seymour and
Sarah Codd of Montana State University in Bozeman a