Particles in flows are found everywhere, from sediment in rivers to blood cells in veins and even to windblown leaves in the backyard. Determining how these systems behave, and what causes some particles to mix well while others segregate from the fluids through which they’re flowing, can have profound implications.

“If you’re pumping thousands of pounds per hour of material from one process to another, and you assume it’s well mixed but it’s not, that can ruin everything,” says James Gilchrist, the P.C. Rossin Assistant Professor of chemical engineering. “For example, pharmaceutical companies make hundreds of millions of pills every year. If segregation takes over a system, one pill might have none of the active ingredient while another has 100 times what it should have — enough to kill someone.”

Gilchrist studies the flow behavior of small particles that range in size from nanoparticles to grains of sand. He has fabricated channels as fine as a human hair and added ridges that enhance mixing by stirring the fluid as it passes through the channel. The use of such small systems allows him to measure the mixing and segregation of small particles more accurately and will have direct impact on the design of microscale “lab-on-a-chip” systems used as chemical and biological sensors.

Other researchers have demonstrated that simple Newtonian fluids such as water mix exponentially faster in chaotic flows generated by channels similar to the ones Gilchrist has fabricated. Gilchrist, though, has come up with different findings. Typically, adding energy to a system of fluids causes a faster, more uniform mixing. In a system containing both particles and fluids, however, the energy can produce a more efficient separation.

“Not all stirring is equal,” says Gilchrist. “Even within an ideal fluid without particles, stirring that leads to chaotic mixing can still produce large regions in the flow that internally mix poorly, and do not mix externally with the rest of the fluid except via diffusion.”

Gilchrist’s research is funded by NSF, the American Institute of Chemical Engineers’ North American Mixing Forum and the American Chemical Society’s Petroleum Research Fund, which seeks the efficient removal of impurities from crude oil.

Gilchrist fabricates microchannels in which he measures the mixing and segregation of small particles.