(July 27, 2015) Coating
the inside of glass microtubes with a polymer hydrogel material 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, wicks and paper towels, and the flow rate can be
predicted using a simple hydrodynamic analysis. But a chance observation by
researchers at the Georgia Institute of Technology will cause a recalculation
of those predictions for conditions in which hydrogel films line the tubes
carrying water-based liquids.
“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 the George W. Woodruff School of Mechanical Engineering at
Georgia Tech. “Instead of filling the tube with a rate of liquid penetration
that slows with time, the water propagates at a nearly constant speed into the
hydrogel-coated capillary. This was very different from what we had expected.”