A team of researchers at the Georgia Institute of Technology (Georgia Tech) have discovered that coating the interior of glass microtubes with a smart polymer hydrogel material significantly changes the way capillary action draws water into these small structures.
Through capillary action, water and other liquids are drawn into limited spaces such as straws, tubes, paper towels, and wicks, and it is possible to predict the flow rate by utilizing a simple hydrodynamic analysis. However, the latest research would redefine these predictions for conditions, where hydrogel films coat the inside of narrow glass tubes transporting water-based liquids. The study, sponsored by the Air Force Office of Scientific Research (AFOSR) via the BIONIC center at Georgia Tech, has been published in the journal Soft Matter.
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. 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.
Andrei Fedorov, a professor in the George W. Woodruff School of Mechanical Engineering at Georgia Tech
When the thin glass tube opening is exposed to a water droplet, the liquid starts to flow within the tube, drawn by adhesion between the liquid and tube walls as well as the surface tension in the liquid. A curved water surface at the periphery of the water column also known as the meniscus, leads the way. Through capillary forces, a standard borosilicate glass tube fills at a slow decreasing rate with the meniscus propagation speed gradually decreasing as a square root of time. However, a complete transformation occurs when the tube’s interior is coated with a thin poly(N-isopropylacrylamide) layer, which is a kind of smart polymer (PNIPAM).
Furthermore, when water moves inside a tube, the interior of which is coated with a dry hydrogel film, it should first wet the film so that the film swells before it can continue farther down the tube. This wetting and swelling process does not occur continuously, but takes place with distinct steps wherein the water meniscus first adheres, its motion stops as the polymer layer deforms locally. The meniscus slides quickly for a small distance before the process starts again. Through this stick-slip phenomenon, water is forced to enter the tube in a step-by-step fashion.
The research team measured the flow rate in the coated tube which was found to be three orders of magnitude less when compared to the flow rate in an uncoated tube. Instead of a traditional quadratic equation that illustrates the filling of an uncoated tube, a linear equation illustrates the time dependence of the filling procedure.
Instead of filling the capillary in a hundredth of a second, it might take tens of seconds to fill the same capillary. Though there is some swelling of the hydrogel upon contact with water, the change in the tube diameter is negligible due to the small thickness of the hydrogel layer. This is why we were so surprised when we first observed such a dramatic slow-down of the filing process in our experiments.
The research group, which included senior research engineer Peter Kottke and graduate students Drew Loney, Ren Geryak and James Silva, again performed the experiment by utilizing glycerol, a kind of liquid that does not get absorbed by the hydrogel. With this liquid, the capillary action continues via the hydrogel-coated microtube similar to an uncoated tube in accordance with the traditional theory. Following the use of high-resolution optical visualization to analyze the meniscus propagation whilst the layer of polymer swelled, the research team ultimately discovered that this unique behavior can be put to good use.
When the materials are less than a particular transition temperature, water gets absorbed by the hydrogels. When materials are heated over that temperature, they stop absorbing water. This prevents the stick-slip process in the microtubes, and enables them to behave like standard tubes. This capability to change the stick-slip phenomenon with temperature may present a novel way to regulate the flow rate of water-based liquid in microfluidic instruments, such as labs-on-a-chip. The hydrogel’s chemical composition can also be changed to control the transition temperature.
By locally heating or cooling the polymer inside a microfluidic chamber, you can either speed up the filling process or slow it down. The time it takes for the liquid to travel the same distance can be varied up to three orders of magnitude. That would allow precise control of fluid flow on demand using external stimuli to change polymer film behavior.
The cooling/heating process can be carried out using small heaters, lasers, or thermoelectric systems mounted at certain areas of the microfluidic devices. This would promote a series of slow and fast reactions in microfluidic devices or facilitate accurate timing of reactions by regulating the speed of both reactant delivery and product removal. Yet another key application would be controlled drug release where the preferred rate of molecule delivery can be adjusted over time to realize improved therapeutic results.
In future studies, the researchers intend to gain a better understanding of the physics of the hydrogel-altered capillaries and explore capillary flow by utilizing partially-transparent microtubes. They would also study other smart polymers, which modify the rate of flow in response to different forms of stimuli, such as the induction of mechanical stress, exposure to electromagnetic radiation, and the liquid’s changing pH. All of these could alter the properties of a specific hydrogel developed to response to those triggers.
These experimental and theoretical results provide a new conceptual framework for liquid motion confined by soft, dynamically evolving polymer interfaces in which the system creates an energy barrier to further motion through elasto-capillary deformation, and then lowers the barrier through diffusive softening. This insight has implications for optimal design of microfluidic and lab-on-a-chip devices based on stimuli-responsive smart polymers.
The research group also included Rajesh Naik, Biotechnology Lead and Tech Advisor of the Nanostructured and Biological Materials Branch of the Air Force Research Laboratory (AFRL) and Professor Vladimir Tsukruk from the Georgia Tech School of Materials Science and Engineering.
The Air Force Office of Scientific Research BIONIC Center supported the study through FA9550-09-1-0162 and FA9550-14-1-0269 awards, AFOSR award FA-9550-14-1-0015, and by Renewable Bioproducts Institute Fellowship of Georgia Tech.