I have read that ceiling tile stains and coffee ring stains are darker on the edges than the center because the puddles evaporate fastest at the point of contact between the surface, air, and water and water that is evaporated leaves behind its sediments. My question is: why does water evaporate faster at this boundary than in the center or any other part of the water puddle?
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All liquids are not evenly spaced like a rectangular block, but rather like an irregular ellipsoid with a bulge in the center. Its impossible to discern this bulge with the naked eye, however, it is very visible in mercury:
Why this bulge is created in the first place because of surface tension. The liquid tries to have the least surface area possible to make surface energy minimum, and ideally, the least surface area is possible in the sphere, liquids like water don't have enough surface tension to hold themselves and create droplets like mercury, but it tries and creates very eccentric(squashed down) ellipsoid
Because of the bulge, more water molecules are exposed to air at the corners than at the bulge which facilitates evaporation.
I hope it helps.
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$\begingroup$ Can you explain this in more detail? How does the bulge cause more water molecules to be exposed to air at the boundary? It seems like the bulge and surrounding water have a larger surface area than the contact point. $\endgroup$– Oliver GCommented Nov 27, 2022 at 15:52
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1$\begingroup$ @OliverG No bulge has lesser area than surface contact point, it's hard to explain without maths. But you can think this way, bulge part has larger number of molecules/volume than at corner. Corner having less volume and greater surface area evaporates and nothing remains, and slowly it takes it way to centre. $\endgroup$ Commented Nov 27, 2022 at 16:30
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$\begingroup$ Ah that makes sense. Because the contact point has fewer water molecules, evaporation causes holes to form which are filled by the surrounding molecules which ultimately result in the movement of the larger particles towards the edges. $\endgroup$– Oliver GCommented Nov 27, 2022 at 16:48
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$\begingroup$ @OliverG Yes sort of, you are correct. You have to imagine few things on own to understand. $\endgroup$ Commented Nov 27, 2022 at 17:06