A new paper in the journal Nature Physics offers insights into the physics of liquid droplets – and while many people may not appreciate the mathematical accomplishment, they will benefit from the athletic wear and raincoats it makes possible.

The recent article, “Tricky Tension,” explores the intersection of physics and textiles, and how wetting is influenced by the structure of individual tiny liquid droplets.

A new paper in the journal Nature Physics offers insights into the physics of liquid droplets – and while many people may not appreciate the mathematical accomplishment, they will benefit from the athletic wear and raincoats it makes possible.

In physics, the cohesive force between two phases is called surface tension.  This allows small insects to walk on water.

In a three-phase system — where gas, liquid, and solid objects all interact – there is a less understood phenomenon called the line tension of a liquid droplet. This refers to the force acting at the boundary where the liquid droplet, the gas of the air and the solid surface the droplet sits on all collide. Learning more about the mechanics of droplets on solid surfaces, known as sessile droplets, is important for understanding the mechanics of wetting and drying textiles, especially for very small droplets. 

“To understand what is happening with textiles, we need to have a better understanding of the physics involved when all three phases interact,” says Warren Jasper, author of the Nature Physics paper and a professor in NC State’s Wilson College of Textiles.

“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 interaction between liquids, vapors and solids are important there,” he said. “On the other hand, when you look at protective equipment, often those are being 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.”

The same can be said for raincoats, which need to repel water rather than absorb it. Making a better raincoat means understanding how the geometry of a droplet interacts with the solid surface of the textile. In a sense, building a better raincoat is where mathematical modeling meets experimentation. Among Jasper’s areas of interest is building better models, in this case for correctly predicting the sign and magnitude for the line tension of a droplet.

“If you don’t have an accurate model, then you don’t understand how that droplet is actually functioning,” said Jasper. “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 dying 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.”

This is where Jasper’s review article comes in. It captures both what is currently understood about this physical phenomenon – and the things that still confound us.

“By providing an overview of what we think we know, and outlining how new tools and techniques can help us address unresolved questions, I’m optimistic this paper can help to chart a path forward for this line of research – which seems simple but continues to surprise us.”