Slip layer dynamics reveal why some fluids flow faster than expected

Whether it is oil gushing through pipelines or blood circulating through arteries, how liquids flow through tubes is perhaps the most fundamental problem in hydrodynamics. The challenge is to maximize transport efficiency by minimizing the loss of energy to friction between the moving liquid and the stationary tube surfaces. Counterintuitively, adding a small amount of large, slow moving polymers to the liquid, thus forming a "complex liquid," leads to faster, more efficient transport. This phenomenon was speculated to arise from the formation of thin layer around the internal wall of the tube, known as depletion layer or split layer, in which the polymer concentration was significantly lower than in the bulk solution. However, given the inherently thinness of this layer, which is only a few nanometers thick, on the order of the polymer size, direct experimental observation was difficult, and so progress in the field relied heavily on bulk measurements and computer simulations.