Coffe Stains
NOTHING LIKE A CUP OF COFFEE TO START OUT THE MORNING. . . AND
NOTHING LIKE A COFFEE STAIN TO RUIN THE REST OF YOUR DAY.  COFFEE
STAINS ARE SO DISTINCT THAT YOU CAN EASILY CLOSE YOUR EYES AND
IMAGINE WHAT ONE LOOKS LIKE. PALE, ALMOST NON-EXISTENT IN THE
MIDDLE, WITH A DARK RING ALONG THE OUTSIDE. 

Nagel: "That's actually quite a surprising thing when you start to hink about it because
when the drops started it was thicker in the center and very thin at the edge, so why did. .
.the liquid migrate to the very edge of the drop by the time it dried?"

SIDNEY NAGEL IS A PHYSICIST AT THE UNIVERSITY OF CHICAGO WHO GOT
CURIOUS ABOUT THESE SURPRISING STAINS.  HE TRIED MAKING STAINS
WITH DIFFERENT LIQUIDS.  HE TRIED WITH DIFFERENT SURFACES.  HE
TRIED THEM UPSIDE DOWN.  AND ALWAYS THE SAME DARK RING
PATTERN FORMED.  FINALLY ONE OF HIS STUDENTS WAS ABLE TO MAKE A
PERFECTLY EVEN STAIN BY COVERING THE LIQUID WITH A LITTLE PLATE
THAT KEPT AWAY THE AIR.  

Nagel: "That told us that you could vary the shape and thickness and the properties of this
stain by changing how the drop evaporated. . . that finally told us what kind of phenomena
were important for the ring formation."

IT TURNS OUT THAT A ONCE A DROP OF COFFEE--OR REALLY ANY
LIQUID--HITS A SURFACE IT MUST ALWAYS MAINTAIN THE SAME
PERIMETER.   

Nagel: "What you might think would happen is that the whole drop just gets smaller and
smaller so it all disapperas. . .but that doesn't happen. . . what happens is the outside edge
of the drop gets pinned in place and it cannot move inwards. . .  It's like a rut in a road.  It's
hard to move out of that rut."

SO AS THE DROP EVAPORATES, THE LIQUID FROM THE MIDDLE KEEPS
FLOWING OUT TO THE OUTSKIRTS TO MAINTAIN THAT PERIMETER.  A
SLOW CURRENT GETS SET UP FROM THE MIDDLE TO THE EDGE, UNTIL
THAT PILE ON THE EDGE FINALLY DRIES THERE.   NAGEL'S CURIOUSITY
WON'T DO MUCH FOR REMOVING THE BLACK RINGS FROM THE COFFEE
BLUES, BUT IT MIGHT HELP IN BUILDING MICROELECTRONIC CIRCUITS
THAT OFTEN REQUIRE PERFECTLY ROUND AND EVEN LIQUID DROPS.