New microscopy technique provides unprecedented insight into nanoscopic slip layers formed in flowing complex liquids
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.
Researchers at the Center for Soft and Living Matter, within the Institute for Basic Science (IBS, South Korea), made a significant advance in the field by successfully imaging the depletion layer in polymer solutions flowing through microchannels. Their study, published in the Proceedings of the National Academy of Sciences, relied on the development of a novel super-resolution microscopy technique that allowed the researchers to see this layer with unprecedented spatial resolution.
The first observation of this phenomenon was made nearly a century ago. Experimental studies on high molecular weight polymer solutions revealed a puzzling observation: there was an apparent discrepancy between the measured viscosity of the polymer solution and the rate at which it flowed through a narrow tube. The polymer solution would always flow faster than expected. Furthermore, the narrower the tube, the larger this discrepancy. This sparked an interest which persists to this day.