Date: Mon, 27 Oct 1997 09:56:06 -0500
From: Matteo Schmill 

Good monday morning to everyone. This week, as a special surprise, the
MMC Collective will be offering coffe, donuts, and bagels for sale in
the computer science lounge at 10am. Early birds will be forced to
stand outside the lounge door to brush up on stretching exercises for
computer scientists.

This week's Monday Morning Information is brought to you by Nature,
through NPR, and then our very own Brian Horling. Let's give a great
big round of applause to Brian, who made the Monday Morning Message
Boy's job wickedly easy this week. Then, visit the Nature web site and
read up on this fascinating phenomenon.

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from last week's Nature (www.nature.com)

Capillary flow as the cause of ring stains from dried liquid drops

      When a spilled drop of coffee dries on a solid surface, it
leaves a dense, ring-like deposit along the perimeter.  The
coffee---initially dispersed over the entire drop---becomes
concentrated into a tiny fraction of it.  Such ring deposits are
common wherever drops containing dispersed solids evaporate on a
surface, and they influence processes such as printing, washing and
coating.  Ring deposits also provide a potential means to write or
deposit a fine pattern onto a surface.  Here the authors ascribe the
characteristic pattern of the deposition to a form of capillary flow
in which pinning of the contact line of the drying drop ensures that
liquid evaporating from the edge is replenished by liquid from the
interior.  The resulting outward flow can carry virtually all the
dispersed material to the edge.  This mechanism predicts a distinctive
power-law growth of the ring mass with time---a law independent of the
particular substrate, carrier fluid or deposited solids.  They have
verified this law by microscopic observations of colloidal fluids.
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