Caution: Slippage may occur

for tightly confined aqueous Newtonian fluids. Fluid mechanics is one of the most mature and successful branches of physics. Its success for Newtonian liquids—whose viscosity is constant—rests in part on the often-assumed no-slip boundary condition, in which the fluid molecules adjacent to a surface are always stationary with respect to that surface. Now, two teams of researchers—one from the Australian National University and one from the University of Illinois at Urbana-Champaign—have demonstrated that the assumption is sometimes wrong. The Australian researchers measured the motion of a 10-micron silica sphere as they drove it through sugar water toward a wall. The Illinois group studied a system in which one cylinder oscillated toward another, with various fluids between them. In both sets of experiments, the classical no-slip model could not explain the data. Furthermore, the inferred amount of slip depended on the fluid’s flow or shear rate. A complete theory will also need to incorporate the fluid’s viscosity, its surface wettability, and the wall’s roughness. Slippage is already known to occur at times for both non-Newtonian liquids and nonaqueous Newtonian ones. The new studies might have implications for capillary blood flow, lubricants in nanomachines, and filtration. (V. S. J. Craig et al. , Phys. Rev. Lett.   87 , 054504, 2001  https://doi.org/10.1103/PhysRevLett.87.054504 .Y Zhu, S. Granick, Phys. Rev. Lett.   87 , 096105, 2001  https://doi.org/10.1103/PhysRevLett.87.096105 .)