String formation in complex fluids

Many everyday materials, such as shampoo, ink, and coffee, are complicated mixtures of fluids and suspended particles with complex and often surprising internal interactions (see the articles by Alice Gast and William Russel, Physics Today, December 1998, page 24 , and by Amy Shen and Perry Cheung, September 2010, page 30 , and the Quick Study by Peter Yunker, Doug Durian, and Arjun Yodh, August 2013, page 60 ). For example, under shear flow (like the flow field you generate when you stir your coffee), the particles can attract each other and align in chains, as seen in this microscope image of 40-µm-diameter polystyrene beads in a solution of polyisobutylene and heptane. First reported nearly four decades ago, the chain behavior has been extensively explored, but a conclusive explanation for it has been elusive. Experiments by Jan Vermant and colleagues at the University of Leuven, Belgium, and ETH Zürich offer new insights into the underlying causes. The team prepared various fluids with polystyrene beads and sheared them between counterrotating top and bottom plates or between counterrotating, concentric cylinders and imaged the particles’ motion. The researchers found that although anisotropic normal-stress differences and confinement can each promote string formation, neither is essential. But shear thinning—the decreasing of viscosity at higher shear rates—is. Shear flow naturally causes suspended particles to spin. In “normal,” Newtonian fluids, particles that get sufficiently close will tumble together and eventually separate. In shear-thinning fluids, however, the viscosity decreases in the thin gaps between adjacent particles, enabling the particles to spin separately yet stay in line. (S. Van Loon et al., J. Rheol. 58, 237, 2014, doi:10.1122/1.4853455 .)