You can do worse than watching paint dry – ask physics

I live in Chennai, a city whose multifaceted identity includes its unrelenting humidity. Its summers are seldom hotter than those in Delhi but they are more unbearable because it leaves people sweaty, dehydrated, and irritated. Delhi’s heat doesn’t have the same effect because when people sweat there, the droplets evaporate into the air, whose low relative humidity allows it to ‘accommodate’ moisture. But in Chennai, the air is almost always humid, more so during the summer, and the sweat on people’s skin doesn’t evaporate. Yet their bodies continue to sweat because it’s one of the few responses they have to the heat.

Paint, fortunately, has a different story to tell. Fresh paint on a wall doesn’t dry faster or slower depending on how humid the air is. This is because pain is made of water plus some polymers whose molecules are much larger than those of water. At first, water does begin to escape the paint and evaporate from the surface. This pulls the polymer molecules to the surface in a process called advection. On the surface, the polymer molecules form a dense layer that prevents the water below from interacting directing with the air, or its humidity. So the rate of evaporation slows until it reaches a constant low value. This is why, even in dry weather, paint takes its time to dry.

Scientists have verified that this is the case in a new study, in which they also reported that their findings can be used to understand the behaviour of little respiratory droplets in which viruses travel through the air. (Some studies – like this and this – have suggested that a virus’s viability may depend on the relative humidity and how quickly the droplet dries, among other factors. Since the relative humidity varies by season, a link could explain why some viral outbreaks are more seasonal.)

Generally, the human skin – as the largest outer-organ of the human body – is responsible for making sure the body doesn’t lose too much water through evaporation. Scientists think that it can adjust how much sweat is released on the skin by modifying the mix of lipids (fatty substances) in its outermost layer. If it did, it would be an example of an active process – a dynamic response to environmental and biological conditions. Paint drying, on the other hand, is a non-active process: the rate of evaporation is limited by the polymer molecules at the surface and their properties.

In 2017, a chemical engineer at the University of Bordeaux named Jean-Baptiste Salmon predicted that an active process may not be needed at all to explain humidity-independent evaporation because it arises naturally in solutions like that of paint. The new study tested the prediction of Salmon et al. using a non-active polymer solution, i.e. one that’s incapable of developing an active response to changes in humidity.

They filled a plastic container with polyvinyl alcohol, then drilled a small hole near the bottom and fit a glass tube there with an open end. The liquid could flow through the tube and evaporate from the end. To prevent the liquid from evaporating from its surface, they coated it with an oily substance called 1-octadecene. They placed this container on a sensitive weighing scale and the whole apparatus inside a sealed box with adjustable humidity. The researchers adjusted the humidity from 25% to 90% and each time studied the evaporation rate for more than 16 hours.

They found that Salmon et al. were right: the evaporation rate was higher for around three hours before dropping to a lower value. This was because polymer molecules had accumulated at the layer where the liquid met the air. But in these three hours, the rate of evaporation didn’t drop even when the humidity was increased. In other words, humidity-independent evaporation begins earlier than Salmon et al. predicted.

The researchers also reported another divergence: the evaporation rate wasn’t affected by a relative humidity of up to 80% – but beyond that, the rate fell if the humidity increased further. So what Salmon et al. said was at play but it wasn’t the full picture; some other forces were also affecting the evaporation.

The researchers ended their paper with an idea. They took a closer look at the open end of the tube, where the polyvinyl alcohol evaporated, with a microscope. They found that the polymer layer was overlaid with a stiffer semisolid, or gel-like, layer. Such layers are known to form when there is a compressive stress, and further block evaporation. The researchers found that their equations to predict the evaporation rate roughly matched the observed value when they were modified to account for this stress. They also found that a sufficiently thick gel layer could form within one second – a short time span considering the many hours over which the rate of evaporation evolves.

“These discrepancies motivate the search for extra physics beyond Salmon et al., which may again relate to a gelled polymer skin at the air-solution interface,” they concluded in their paper, published in the journal Physical Review Letters on December 15.