The way liquids flow through tubes is presumably the most basic challenge in hydrodynamics, no matter if it is oil flowing through pipelines or blood circulating through arteries.
The task is to increase transport efficiency by reducing the loss of energy to friction between the moving liquid and the stationary tube surfaces. Contrary to what was expected, incorporating a small amount of large, slow-moving polymers to the liquid, making it a “complex liquid,” results in more efficient and rapid transport.
It was hypothesized that this phenomenon originates from the formation of a thin layer around the inner wall of the tube, called split layer or depletion layer, where the polymer concentration was considerably lower than in the bulk solution.
However, considering the natural thinness of this layer, which has a thickness of just a few nanometers, on the order of the polymer size, direct experimental observation was not easy, and hence advancement in the field was greatly dependent on computer simulations and bulk measurements.
Scientists from the Center for Soft and Living Matter, of the Institute for Basic Science (IBS, South Korea), made an important advancement in the area by successfully imaging the depletion layer in polymer solutions that flow through microchannels.
Their research, reported in the Proceedings of the National Academy of Sciences USA, was dependent on the advent of a new super-resolution microscopy method that enabled the scientists to observe this layer with unparalleled spatial resolution.
This phenomenon was first observed almost a century ago. Experimental studies on high molecular weight polymer solutions showed a confusing observation: there was a clear difference between the measured viscosity of the polymer solution and the rate at which it flowed through a narrow tube.
The rate at which the polymer solution flows would always be higher than expected. Moreover, as the tube becomes narrower, this difference becomes greater. This induced an interest which remains to date.
Depletion layer dynamics was a problem we found very interesting, but it was challenging to make progress with current experimental techniques. We knew the first step needed to be the development of a technique that could provide new information.
John T. King, Study Corresponding Author, Center for Soft and Living Matter, Institute for Basic Science
Using his proficiency in super-resolution microscopy, Seongjun Park, the first author of the study, created a new adaptation of stimulated emission depletion (STED) microscopy that has adequate contrast sensitivity and spatial resolution to directly see depletion layers.
Meanwhile, Anisha Shakya, the co-author of the study, utilized her understanding of polymer physics to optimize an appropriate imaging system. The group took a decision that the best strategy would be to apply the newly developed STED-anisotropy imaging to a solution of high molecular weight polymer, polystyrene sulfonate (PSS), flowing through silica microfluidic channels with a width of 30 μm.
The behavior of PSS was monitored using fluorescent dyes. Transient interactions between the side-chains of PSS and the dye slow down the rotational movement of the dye molecule. These minor variations show the position and concentration of PSS with a spatial resolution of tens of nanometers.
The scientists first substantiated the formation of depletion layers at the wall and measured that the dimensions of the depletion layer were in agreement with PSS size. After that, they noticed that the thickness of the depletion layer reduced when the solution began to flow.
Fascinatingly, variations to the depletion layer dimension only start after a critical flow rate that denotes known changes in the polymer conformation. This was the first direct experimental confirmation of this phenomenon, which was estimated from molecular dynamics simulations years ago.
Remarkably, it was also noticed that variations to the composition of the depletion layer take place at surprisingly low flow rates. Particularly, polymer segments are dragged away from the wall, leaving nearly pure solvent, without polymers, near to the wall. This can be ascribed to hydrodynamic lift forces, such as aerodynamic lift in airplanes that occurs due to asymmetric flow at the wall.
Although hydrodynamic lift has been well-characterized using computer simulations, and noticed in macroscopic systems (for example, flounders fight against this lift better than other animals because of their flatter shape), direct experimental observations on nanoscopic length scales have remained obscure.
It is expected that this potential method can offer new information on complex fluids under flow in various systems (for example, turbulent flow) similar to what is observed in rapidly flowing rivers, or flow through nanofluidic devices.