Envision the following scenario: you are painting your garden fence in the summertime when the rain begins to fall. Simultaneously, not too far away, a buddy has just completed doing the same thing. However, their weather is much finer, so the dry circumstances surround their newly painted fence. Which fence is going to dry faster? We could naturally assume that the latter since there is less humidity, but a recent study has shown that this is not the case.
The evaporation rate of water or other solvents in a polymer solution, like paint, can be independent of the surrounding humidity, according to a notion that has been tested in a recent study.
The results of the trials indicate that the polymer molecules are forced toward the paint’s surface, where they create a thick layer, as water evaporates from the paint. This skin-like covering protects the paint from the effects of humidity and effectively slows down evaporation.
Furthermore, although this would seem like a straightforward response with little significance outside of home décor, the consequences might also apply to something quite different: viruses.
Of bacteria and barriers
In many circumstances, humidity-independent evaporation has benefits. For instance, our skin plays a role in preserving moisture by evaporating at a roughly constant pace. Our skin’s cell membranes, which contain lipid molecules that can alter how quickly perspiration evaporates, are to blame for this. However, our bodies actively respond to this when necessary. Is there another version that isn’t in use?
It was hypothesised in 2017 by a chemical engineer at the University of Bordeaux, France, that humidity-independent evaporation may not always necessitate an active reaction. The scientist in issue, Jean-Baptiste Salmon, asserts that this process ought to happen every time a solvent evaporates from a solution containing big molecules, as the solvent is previously known to attract the molecules to the drying surface.
Salmon predicted that regardless of whether the environment is completely humid or bone dry, the rate at which the solution evaporates should stay constant after the big molecules form the thick layer.
Max Huisman, a doctoral student studying soft matter physics at the University of Edinburgh in the United Kingdom, stated in a release, “We wanted to know whether this [hypothesis] might have implications for the evaporation of respiratory virus droplets, which also contain high-molecular-weight polymers.”
The part that biopolymers play in droplet evaporation is overlooked in current models of viral dissemination. This is because, unlike human skin cells, these molecules are not intended to carry out an active process.
Thus, the Edinburgh team set out to investigate the parameter ranges in which Salmon’s idea applies and test it using a simple, nonactive, and nonbiological polymer solution.
The group constructed a device that gauges evaporation rates in a typical water-polymer solution—polyvinyl alcohol, or PVA—at various humidity levels to evaluate this procedure.
The device was a plastic reservoir that was cylindrical and had five holes punched into its walls. Each hole was connected to a glass capillary tube, which was then filled with a solution (PVA). These rectangular tubes faced away from the reservoir in a horizontal direction.
They put a coating of oil on the solution’s upper surface to guarantee that evaporation took place at the tubes’ projecting ends. The reservoir was then placed within a humidity-controlled cage on a scale. After maintaining humidity levels between 25 and 90 percent, the crew kept an eye on the amount of water that was draining from the reservoir. Each of these tests required seventeen hours to complete.
As Salmon had predicted, the evaporation in each experiment peaked after roughly three hours of constant evaporation as the polymer layer thickened near the surface.
Nonetheless, the experiment expanded on the original theory in two ways. First off, rising humidity did not affect the early-stage constant evaporation rate, which happened before the formation of the protective skin. Second, as was to be predicted, the evaporation rate decreased after the first three hours, although this decline was only humidity-independent at humidity levels up to 80%. Above this point, the evaporation rate drops as humidity rises, indicating the presence of additional forces.
Microscope investigation revealed that the solution had created an extra layer of gel skin that Salmon had not anticipated, severely reducing the capacity of water molecules to reach the surface. Recently, similar gel skins have been seen at the respiratory droplet surface, suggesting that biopolymers may also exhibit this mechanism.
Paint drying, who would have guessed it was so interesting?