Caution: Slippage Might Occur

Caution: slippage might occur in tightly confined fluids. Fluid mechanics is one of the most mature and successful branches of physics---a study vital for understanding phenomena ranging from ocean currents to the lift generated by butterfly wings. It would likely startle many physicists to learn that one of the venerated discipline's fundamental assumptions is sometimes wrong.

But that's exactly what a group of researchers from the Australian National University found in an experimental test of fluid behavior in small spaces. It has long been thought that the fluid molecules adjacent to a surface are stationary, regardless of the motion of the rest of the fluid. This no-slip boundary condition, as the assumption is known, leads to precise and detailed descriptions of nearly all fluid mechanical systems.

Recently, however, a number of studies have suggested that the assumption breaks down for flow in confined spaces, such as the insides of capillaries or the channels of microfluidic chips used in cutting-edge bioanalysis. The Australian researchers (V. S. J. Craig, vince.craig@anu.edu.au, 61-2-6125-3359) set out to explicitly test the age-old conjecture by measuring the motion of a ten micron silica sphere as they drove it through liquid toward a flat wall. Subsequent analysis showed that a fluid model incorporating boundary slip explained the data better than the classical no-slip model. No-slip boundaries are still close enough to the truth in most circumstances.

The new study, however, reveals that descriptions of the blood moving through our capillaries, lubricants in nanomachines, and flow in other tiny systems must include boundary slip conditions to achieve precision at such small scales. (V. S. J. Craig et al., Physical Review Letters, 30 July; text at Physics News Select).