Hello,
breathing on a cold window pane results in a thin film. The observation is: the diameter gets smaller with time and the edge is very dynamic! The center seems unchanged until the "shrinking edge reaches the center".
It seems the thickness of the film stays unaltered. The only explanation I have is the edge effect: vapour pressure depends on radius of curvature. Right?
But why is this effect so massive? Can you maybe give me some background, some quantification.
Best Regards
Lothar
Dear Schuh,
I would suggest it is available heat that the other factor here. As the water evaporates it take 2.441 Kj/ml to vapourise, cooling the glass. If he have your circle and apply a point heat source in the center (hot coin from your pocket) then it will go the other way. The cold neighbour effect is well known in freeze drying / parallel evaporation studies, and to overcome this and enable even drying you have to effectively conduct heat energy across the whole area.
Kind regards,
Rob
In the example given, it is also unlikely that the film is uniform. Looking at the conditions as the film was applied, you blew in the center, depositing moisture there first and the film grew out from there. As it grew, you're still applying the heat and moisture to the center, growing the size of any droplets in the center, as well as heating the surface at that point more than the edge points, etc. Thus, even if the rate of evaporation were constant across the surface (no localized gradient in temperature or humidity) the evaporation process would occur in reverse of the application due to the larger droplets grown in the center of the film (which isn't really a film or it wouldn't be foggy, would it?)
Hello
thank you for your 2 answers. I made a variation of this simple experiment: I tool an adhesive freshfilm foil (from the kitchen, about 10 my thick), cut ~5 cm diameter hole in a bigger piece and attached it on the cold window pane surface.
I breathed onto the area of the hole in the foil (adhesively attached to the surface), so that half of the "hole" got a moisturefilm. Then I observed: the "front" the moisure film moves towards the foil-boundary, i.e.(now shrinkage is assymmetric). There, where at the start of the experiment moisture film touches the fresh film foil, l nothing happens.
Then I repeated the experiment with the fresh film foil and removed it after breathing a mousture film on the window pane.
Simple experiment you can do and let me know, what you observed.
Did you expect this? Have you an explanation??
Lothar
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