United States. The capillary action extracts water and other liquids in confined spaces such as tubes, straws, wicks and paper towels, and the flow rate can be predicted by simple hydrodynamic analysis. But a chance observation made by researchers at the Georgia Institute of Technology will prompt a recalculation of those predictions for the conditions under which hydrogel films line the tubes that carry water-based liquids.
"Instead of advancing according to conventional expectations, the water-based liquids slide to a new location in the tube, get stuck, then slide again – and the process is repeated over and over again," explained Andrei Fedorov, a professor in the mechanical engineering school at Georgia Tech. "Instead of filling the tube with a rate of liquid penetration that decreases over time, water spreads at an almost constant rate in the coated hydrogel capillary. This was very different from what we expected."
The findings resulted from research sponsored by the Air Force Office of Scientific Research (AFOSR) through the BIONIC center at Georgia Tech.
When the opening of a thin glass tube is exposed to a drop of water, the liquid begins to flow into the tube, pulled by a combination of surface tension in the liquid and adhesion between the liquid and the walls of the tube. Leading the way is a meniscus, which is a curved surface of water at the leading edge of the water column. An ordinary borosilicate glass tube is filled by capillary action to a gradual decreasing range with the speed of propagation of the meniscus slowing down like a square root of time.
But when the inside of a tube is coated with a very thin layer of poly (N-isopropylarylamide), a so-called "smart" polymer (Pnipam), everything changes. Water entering a tube coated inside with a dry hydrogel film must first wet the film and let it swell before it can go further into the tube.
Wetting and swelling are carried out non-continuously, but with discrete steps in which the menisci of the water and its movement remain stopped while the polymer layer is locally deformed. The meniscus slides quickly for a short distance before the process is repeated. This "stick-slip" process forces the water to move in the tube in a step-by-step motion.
The flow range measured by the researchers in the coated tube is three orders of magnitude less than the flow range in an uncoated tube. A linear equation describes the time dependency of the filling process rather than a classical quadratic equation that describes the filling of an uncoated tube.
