Paint Drying Might Actually Be "Exciting" After All

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Watching paint dry isn’t known for its thrill. However, according to a recent study, the science of paint drying is actually pretty interesting if you look really, really closely.

Paints and inks are typically made up of microscopic colloids – tiny particles that move around in a liquid. When they dry out, the liquid evaporates and the particles spontaneously organize themselves into a two-layered structure according to their size. Past studies have shown that certain evaporation rates have resulted in larger particles not being able to move as fast as smaller ones, so they remain at the surface of a drying film, and it was assumed this was true for all evaporation rates.

But according to a new study published in Physical Review Letters, researchers found in water-based colloids, as the liquid evaporated, the smaller particles moved upwards towards the top-layer while the larger particles moved down. We promise this is actually quite exciting.

Since the net force is downwards (as there are more particles near the top and fewer particles below), the particles were all jostling to move downwards. The larger colloids experienced more particles bouncing off them, therefore they were pushed down at a greater velocity than the smaller ones. The researchers estimated larger particles move downwards up to 50 times faster than the smaller ones because they "catch more wind" from the force of other particles.

“The stratification that we found was neither predicted nor expected, and we spent a few months verifying the results,” said Dr. Andrea Fortini, from the University of Surrey, in a press release.

The spontaneous size stratification of particles in a paint. Andrea Fortini/University of Surrey

Of course, coatings of paint are 1,000 times thinner than the width of a human hair, so you need a microscope to see this. The physicists labeled the larger spheres with red fluorescent dye. They were then able to track their movements by measuring the color intensity of the dried paint. Using their microscopic findings, the researchers created computer simulations to verify and experiment with the results. 

The scientists hope that their findings could have some interesting applications out of the lab. The researchers believe that knowledge of this mechanism could be used to control the properties at the top and bottom of films and liquids. Some of their ideas include a sunscreen that has an ultraviolet-reflecting top layer and a moisturizing base layer, as well as a protective film for smartphones that would be scratch-resistant on top while being soft and sticky on the bottom.

“When coatings such as paint, ink or even outer layers on tablets are made, they work by spreading a liquid containing solid particles onto a surface, and allowing the liquid to evaporate,” said Dr. Fortini in a separate statement. “This is nothing new, but what is exciting is that we’ve shown that during evaporation, the small particles push away the larger ones, remaining at the top surface whilst the larger are pushed to bottom. This happens naturally.”

So, there you have it. Watching paint dry can apparently be exciting. If microscopic particle interactions are your cup of tea.

Particles stratify by size. Andrea Fortini/University of Surrey
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