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As compared to when the coffee is just hot from brewing. I suspect it has something to do with the way the microwaves are affecting the molecules of the coffee.

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Foam heads are a chemical effect. I don't think there is any physics involved (except indirectly, through chemistry). –  David Z Apr 12 '11 at 20:43
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Question closed moments before I posted the Answer that follows here. You may be microwaving the coffee for too long. That is, you may be superheating the coffee. When you add the milk, the heat of the water in superheated coffee, which is at a temperature higher than 100C, may be enough to put bubbles into the milk. If you are superheating the coffee, I recommend you microwave it for a shorter time, because superheated liquids can boil explosively, causing very nasty scalds. You haven't given quite enough information. How long are you microwaving your cup of coffee for? –  Peter Morgan Apr 12 '11 at 20:45
    
@David If it's superheating, then it's Physics. –  Peter Morgan Apr 12 '11 at 20:46
    
@Peter Morgan: I also don't understand why this question was closed... –  Fabian Apr 12 '11 at 20:52
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""Foam heads are a chemical effect. "" Ahh, only problem is, that "chemistry" dosn't know about that. –  Georg Apr 12 '11 at 20:58
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closed as off topic by Sklivvz, David Z Apr 12 '11 at 20:42

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