A small water droplet sitting on a piece of fabric is not as simple as it looks. How that droplet behaves, whether it soaks in or beads up and rolls away, depends on forces that scientists are still working to fully understand.
A new paper published in the journal Nature Physics takes a close look at one of those forces, called line tension. The research was written by Warren Jasper, a professor at NC State's Wilson College of Textiles, and reviewed the current state of scientific knowledge about how liquid droplets interact with solid surfaces.
According to a report by Phys.org, most people are familiar with surface tension, the cohesive force between two phases that allows small insects to walk on water. Line tension is different. It refers to the force acting at the exact boundary where a liquid droplet, air and a solid surface all meet at the same point. Scientists call a droplet resting on a solid surface a sessile droplet, and understanding its behavior is central to how textiles are designed.
The practical stakes are real. Athletic wear has to absorb sweat, move it away from the skin, and allow it to evaporate. Protective gear worn around toxic materials has to do the opposite, keeping liquids from penetrating the fabric at all. Raincoats need to repel water entirely.
"When you sweat, to make you feel dry, a piece of athletic wear absorbs the sweat, then wicks the sweat away, and then the sweat evaporates. So, the interactions between liquids, vapors and solids are important there," Jasper said. "On the other hand, when you look at protective equipment, often it is used around toxic materials. In that case, you want the opposite — you don't want it to penetrate the fabric. You want to maintain that sessile droplet and not have it wet the surface and be absorbed."
Building better models is a central part of Jasper's work. A model that cannot accurately predict how a droplet behaves leaves engineers guessing about why their materials work or fail.
"If you don't have an accurate model, then you don't understand how that droplet is actually functioning," Jasper said. "You might be able to observe how a droplet functions on different surfaces, but you're missing the 'why' of it. Eventually you dig into practical applications, like making a better raincoat or inventing a better dyeing process that uses less energy. But if you don't understand the fundamentals, or if the fundamentals can't predict what you're seeing, then you know you're stuck. It's important that we continue to make breakthroughs in this area."
The paper focuses specifically on line tension because it is the least well understood of the forces at play in a three-phase system involving gas, liquid and solid. Getting the sign and magnitude of that force right in a mathematical model is the key step toward predicting droplet behavior accurately enough to be useful in textile design.
