Focus: Particles Favor Chaos Over Calm
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 and their colleagues have challenged this assumption by independently measuring the liquid and particle flows. They used 2.5-micron-diameter oil-filled beads as their colloid particles because the oil can be tracked with the same MRI-type system that can measure the water flow surrounding them.
The team pumped their colloid–about 10% beads by volume–through a glass tube about the size of a pencil lead. They applied 3.5-millisecond-long magnetic field pulses whose strength varied with position. According to a standard nuclear magnetic resonance (NMR) technique, the different field strengths cause protons to emit radio waves at different frequencies, providing information on their positions. And the oil frequencies are easy to separate from the water frequencies. The team used sequences of field pulses to characterize the motion of the particles and the water.
Then came their NMR trick: A sequence of four pulses, where two pulses have the reverse field directions from the other two, should cancel the NMR effects of ordinary particle flow along streamlines. If each of six lanes of car traffic has a different speed, this sequence effectively starts cars at a starting line, allows them to drive for a fixed time, and then forces all of them to reverse direction at the same speeds and for the same time period. If the cars don’t end up synchronized at the starting line, you know that someone wasn’t following their speed limit. When observing for more than 100 milliseconds, Seymour and his colleagues were surprised to find exactly this result–additional particle diffusion not accounted for if they assumed that particles remained in their lanes. And the discrepancy grew as they increased the observation time–a result that meets the mathematical definition for a chaotic system.
Seymour and Codd’s team concluded that interactions between particles must be more important than previously thought, and they are leading to frequent “lane changing” in the flow. “The simple models we’ve been using are probably not capturing the transport correctly,” says Seymour. “In these dilute colloids, maybe these many-body interactions that we have traditionally associated with concentrated suspensions have applicability in the less concentrated ones.”
The Montana team specifically chose bead sizes to mimic one important colloid: human blood. Seymour believes this research will have an impact on microfluidic devices and biosensors for blood. For now though, the basic science behind these flows is challenge enough. “These problems turn out to be very hard,” says Michael Brenner of Harvard University. He says studies of colloid physics go all the way back to Einstein’s seminal 1905 paper on the topic. “This is research [Einstein] would probably appreciate.”
The Basics of NMR–a web-based introduction with animations