Why doesn’t paint dry more slowly in a humid environment

Bright picture of the polarization layer at the water-air interface at the end (t103min) in a typical experiment at RH=50%. Credit: Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.248102

A group of physicists at The University of Edinburgh working in an infection and immunity specialist at the university’s Roslin Institute, through experiment, confirmed a theory to explain why paint dries at the same rate regardless still the humidity level. The study was published in Physical Review Letters.

Common sense suggests that paint should dry faster on an exterior fence on a dry day than when it is humid because evaporation is faster when the air around a liquid source is drier. But anecdotal evidence suggests this is not the case with paint and other liquids. Six years ago, chemist Jean-Baptiste Salmon and his colleagues developed a theory to explain why this is the case. They suggest that this is because large liquid molecules are pulled to the surface during evaporation, forming a ‘polarization layer’ that prevents evaporation, and by extension, drying. In this new effort, the research team conducted an experiment to test this theory.

The researchers drilled five holes into a squat, round cylinder and inserted glass capillary tubes in a horizontal position each then sealed in place. Then they added a quantity of polyvinyl alcohol to the cylinder, which they placed on the scale. They pour a thin layer of oil on top of the liquid to prevent surface evaporation.

The final touch involves placing an RH-controlled air flow box on top of the cylinder to allow control of humidity levels. The team then ran several 17-hour tests to determine evaporation rates (using a scale to measure how much liquid has evaporated) from the tubes at different humidity levels, from by 25% to 90%.

The researchers found that as expected, evaporation rates remained constant for about three hours. But then, rates drop, as Salmon theorized, regardless of moisture levels. The evaporation rate did not decrease as the humidity increased during the first three hours. However, the theory only shows that holding humidity levels up to 80% at rates higher than that, evaporation slows down, which the team suggests is likely due to other forces.

The researchers suggest that their work may have medical applications because recent research efforts have shown that respiratory droplets tend to form skin lesions similar to those seen in experimental devices.

More information:
Max Huisman et al, Evaporation of Concentrated Polymer Solutions Insensitive to Relative Humidity, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.248102

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Citation: Why paint doesn’t dry slower in a humid environment (2023, December 20) retrieved 21 December 2023 from https://phys.org/news/2023-12-dry-slower-humid-environment .html

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