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Capillary Control with Polymer Hydrogel

Peter Kottke adjusting optics
Georgia Tech Senior Research Engineer Peter Kottke adjusts the optics used in a study of how hydrogel coatings affect capillary action in a narrow glass tube.
Photo: Candler Hobbs

Coating the inside of glass microtubes with a polymer hydrogel dramatically alters the way capillary forces draw water into the tiny structures, researchers have found. The discovery could provide a new way to control microfluidic systems, including popular lab-on-a-chip devices.

Capillary action draws water and other liquids into confined spaces such as tubes, straws, and wicks, and the flow rate can be predicted using a simple hydrodynamic analysis. But a chance observation by Georgia Tech researchers could change those predictions for conditions in which hydrogel films line the tubes carrying water-based liquids.

“In hydrogel-coated tubes, rather than moving according to conventional expectations, water-based liquids slip to a new location in the tube, get stuck, then slip again — and the process repeats over and over again,” explained Andrei Fedorov, a professor in Georgia Tech’s George W. Woodruff School of Mechanical Engineering. “Instead of filling the tube at a rate that slows with time, the water propagates at a nearly constant speed into the hydrogel-coated capillary.”

When the opening of an ordinary thin glass tube is exposed to a droplet of water, the liquid begins to flow into the tube, pulled by a combination of surface tension and adhesion between the liquid and the walls of the tube. Leading the way is a meniscus, a curved surface of the liquid at the leading edge of the water column. An ordinary borosilicate glass tube fills by capillary action at a gradually decreasing rate with the speed of meniscus propagation slowing as a square root of time.

But when the inside of a tube is coated with a very thin layer of a so-called “smart” polymer, everything changes.

Water entering a tube coated on the inside with a dry hydrogel film must first wet the film and allow it to swell before it can proceed farther into the tube. The wetting and swelling take place not continuously, but with discrete steps in which the water meniscus first sticks and its motion remains arrested while the polymer layer locally deforms. The meniscus then rapidly slides for a short distance before the process repeats. This “stick-slip” process forces the water to move into the tube in a step-by-step motion.

The findings resulted from research sponsored by the Air Force Office of Scientific Research (AFOSR) through the BIONIC center at Georgia Tech, and were reported in the journal Soft Matter.​ 

— John Toon

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