Text 649, 39 rader
Skriven 2005-11-05 17:44:42 av DAVID WILLIAMS (1:250/514)
     Kommentar till en text av Paul Rogers
Ärende: Interesting phenomenon
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-> This afternoon I decided to make a cup of tea.  I filled my small 
-> teakettle and turned on my electric stove, then went back to work on the 
-> computer.  Unfortunately I twisted the wrong knob, and when I smelled 
-> the hot metal, the element under a 3qt, flat bottomed pot was red hot. 
 
-> Hmmm.  I poured a couple ounces of cold water from the teakettle into 
-> the pot, and tipped it to swirl the water.  After a little initial 
-> spitting, it circled around, and around, and around the pot in a long 
-> thin puddle--more than a dozen times.  Almost like it had propulsion. 
-> Very gradually it slowed down, got shorter and fatter, and then broke up 
-> and disappeared in fit of spitting and sputtering. 
 
-> Water droplets on a hot smooth plate will bounce around on a cushion of 
-> steam.  But this was one big puddle of an ounce or more, 5" long and 
-> 3/4" wide.  It was fascinating! 
  
I suspect that the motion of the glob of water stabilized it. If it had 
been stationary, the film-boiling under it would have broken through it 
and made it break up. But since it was moving, the glob moved over and 
away from the steam before it got thick enough to break through. But it 
was thicker under the trailing edge than the leading one, so the glob 
"felt" like it was sliding downhill, which kept it moving. 
  
This brings me to something that has long puzzled me. If you put a few 
drops of water onto a greasy surface, so they don't wet it, and tilt 
the surface so they roll around, they often bounce off each other 
without joining together. What keeps them apart? Similarly, drops of 
water often persist for a while when "floating" on top of water. What 
holds them up? I have often seen this when using a filter-type 
coffee-maker, when the fresh coffee is dribbling into the flask when it 
is already part-full. I've done a few experiments and found that adding 
detergent to the water, to reduce the surface tension, *increases* the 
time that these floating drops persist. 
  
Liquid surfaces are odd. 
  
                             dow 
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