A high-speed microscopy technique explains the behavior of fluids by observing how micron-size suspended particles dance in real time. Knowledge of how fluids work will help scientists and engineers handle complex fluids ranging from biological liquids such as lymph and blood to industrial materials such as paint, detergents and pastes. Cornell University physicists have unveiled how these particles respond to fluid flows from shear – a specific way of stirring. The first to link direct imaging of the particle motions with changes in liquid viscosity, the scientists combined high-speed 3-D imaging techniques with a sensitive force-measuring device to track the motions of tiny particles suspended in the fluids while monitoring the thickening or thinning behaviors under shear. They observed that fluids become thinner when the particles – which normally move in a random way – get swept by the induced fluid flows. They also studied how fluids became viscous when particles were driven past one another too quickly for the fluid between them to drain or get out of the way. At such high speeds, the particles form clusters that lock together, making the fluid more viscous. These observations refute theories that such changes in fluid viscosity result from the formation and destruction of particle layers under shear. These theories hold that streamlining particle trajectories reduces random collisions, enabling particles to flow past each other more smoothly. When the particles form layers at low shear rates, their viscosity decreases, causing the fluid to become thin; when the particle layers break up at high shear rates, the viscosity increases, causing the fluid to thicken. Instead, by directly imaging the layering and measuring the fluid viscosity, the physicists observed that, although the amount of layering and delayering was comparable, the changes in viscosity were substantially different in the thinning and thickening regimes. Because delayering occurred at shear rates much lower than those leading to thickening, they proved that layering is not the major reason for viscosity changes in these suspensions. The findings were published in the Sept. 2 issue of Science (doi: 10.1126/science.1207032).