Re: Why does toothpaste in the bottom of a sink repel water away?

Well Rich, I tried your observation and here's what I found:

In my bathroom sink I could not notice the dry spot. This could bebecause 
the sink is white, but I'm not sure. However, in my kitchen sink which is 
not white, I could notice the dry spot. Both of the sinks I tried this on 
have an enamel coating. This may or may not be relevant, but your teeth 
also have an enamel coating. 

This is what I believe to be happening. It is not that the tooth paste 
residue repels the water away, it is that the tooth pase does not allow 
water to adhere to it as well as the enamel does. As an example, If we look 
at a car's windshield when it's raining out and the wipers are not on, you 
can distinguish where the wipers would go because the glass has a different 
texture where the dirt is. There is not much dirt on the areas where the 
wipers normally go because they clear it away. If you made a sink out of 
glass and part of it was dirty and part of it was smooth, the water would 
appear on the smooth parts. The enamel in your sink is very smooth, but the 
toothpaste residue is very rough, like the dirt on your windshield, and 
this is why it appears that the water is not where the toothpaste residue 
is. 

One more note, if you use enough water, on the dirt or the toothpaste, you 
will wash it away and that spot will appear clean and wet again.