This question popped up in my mind when I was going through the explanations of coffee ring effect. Almost all of the explanations state that “the outer rim of the drop evaporates faster”, but unfortunately I couldn’t find any explanation WHY it dries faster than the inner part. Is it something to do with height? area? surface tension being stronger/weaker on the edge? temperature?
Edit:
Sorry for not adding any image, but what I actually meant was this:
[uneven evaporation of drop](https://www.ds.mpg.de/3208834/original-1559025408.jpg?t=eyJ3aWR0aCI6ODAwLCJoZWlnaHQiOjYwMCwib2JqX2lkIjozMjA4ODM0fQ==–01a65b684acafba312604a11b22490739a711d36)
As you can see from the image, the arrows indicating the magnitude of evaporation are stronger on the edges in 2D cross section. I was wondering why it is stronger there, rather than being even on crest and on the flanks of the drop
In: Chemistry
* Think about that it means for the droplet to “dry”.
* The water has to evaporate.
* In order for the molecules to evaporate they need to gain energy from the air.
* They energy is going to get to the molecules on the outer-most layer first.
The inner portion has water molecules all around him to keep him insulated.
The portions on the outside edges would have half of their body exposed to the outside temperature.
It’s like if you were chilling on your living room couch as opposed to chilling at your front door with it open and half of your body sticking outside of the door. The outside temperature would affect you more.
If you are refering to the observation that the drop seems to shrink in size instead of flattening, it has more to do with surface tension than different evaporation speeds.
Surface tension is what creates the drop shape in the first place. Regardless of where the water is lost to evaporation, the remaning water will continue to group in a drop.
On top of that, unclean surfaces promote drop formation since the water doesn’t adhere that much to the surface. If you do a professional cleaning (way beyond what people understand for cleaning) water can indeed form a thin film over the surface. On this topic I recommend you check this video from Applied Scienc (a true goldmine of a channel) on cleaning, with practical demonstration:
[https://www.youtube.com/watch?v=HiL6uPNlqRw](https://www.youtube.com/watch?v=HiL6uPNlqRw)
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