"Watching paint dry" may be a term most people use to show how boring something can be, but the physics of what happens when anything changes from wet to dry is enough to grab scientists' attention. No, really -- there are still many unanswered questions about drying. Now, a new paper looks at exactly how the drying process works in tiny detail.
The research, published in the journal Physical Review Applied in February, uses a technique called nuclear magnetic resonance spectroscopy to measure the distribution of water in a material. The results could help researchers understand how moisture behaves in a variety of contexts -- from humidity in a house's insulation to drying clothes after washing.
"With all these tools, we are able to effectively determine the law of absorption that has been so unknown," said Philippe Coussot, a research engineer at Gustave Eiffel University in France. "This basic approach is just the first step to be able to more systematically determine the real kinetic equation for absorption."
In a wet piece of fabric, water can either be bound to the structure of the fabric or circulate as vapor in between the weave. Coussot and colleagues put samples of wet fabric into open-topped containers and exposed them to dry air flows. Then, they used nuclear magnetic resonance to make measurements of the quantity of water bound to the fabric at different times and determined the overall drying rate.
Research into drying is essential to create more efficient ways to clean clothes, said Saeed Moghaddam, an engineer at the University of Florida, who was not involved in the new study. When someone moves clothes from the washer to the dryer, the wet clothes are usually 50% or 60% heavier because they're saturated. And evaporating that water takes a lot of energy -- 2.2 megajoules of energy for every 1 kilogram of water, said Moghaddam. "Water is the most energy-consuming thing to evaporate."
When the dryer gets started, it slowly evaporates that water using heat and air. The clothes are considered dry when they contain 2%-3% water in the fabric. To evaluate the efficiency of any appliance, engineers look at the time involved to go from 55% saturation down to 2%-5% in the fabrics.
Moghaddam says that tinier spaces can also impact the drying. Water droplets that sit on fabric are roughly tens of microns wide -- about a quarter of the size of a human hair. The spaces between woven strands fill with water, and the surface tension keeps it there.
The new work, he said, does a good job of explaining the forces at play at the interface of the fabric and water. "It's good, basic science work."
Coussot plans to expand the research to examine cellulose fibers in insulation -- he has previously worked on exacting studies of how wood dries, but now he'd like to test wood fibers. Biobased construction materials -- like wood-based insulation -- could help the world by lowering the carbon footprint of building houses and creating less waste.