Braiding patterns in flowing streams have been explained by a University
of New Mexico team (Vakhtang Putkaradze, 505-277-2234, firstname.lastname@example.org).
Attention poets: researchers have figured out the secrets behind a
beautiful fluid pattern that sharp-eyed observers can occasionally witness
in thin, narrow streams of water flowing down a hill.
Ordinarily, a stream of water meanders, or goes side to side, when
it flows down an inclined plane that is "partially wetting," or not
perfectly water-repelling. Some researchers considered such meandering
to be inevitable, even for water flowing down a perfectly smooth plane.
But the New Mexico team discovered, first of all, that meandering can
be eliminated (the centerline of the stream can be straight) if water
flows down the plane at a constant rate, a somewhat rare but possible
occurrence. Moreover, such non-meandering streams often have visually
striking "braids," a fixed pattern of wide and narrow water regions
that goes all the way down the plane.
Using a simple laboratory setup, the researchers discovered an easy
way to duplicate this braiding pattern (see Physics
News Graphics). They sent a fluid (a mix of water, glycerol and
some food coloring) down a narrow cylindrical nozzle. As it exited the
nozzle the fluid struck a slanted acrylic plane, where it formed a braiding
pattern as it ran downstream into a lower reservoir.
Describing the lab fluid's behavior with equations, the researchers
found that braiding occurs as a competition between the fluid's inertia
and surface tension: As the fluid strikes the acrylic plane, it tends
to keep moving, causing it to spread out. However, surface tension limits
the spreading and quickly manages to pull the fluid back together to
a narrow waist. Nonetheless, in the process of forming this waist, the
outer edges (which carry most of the fluid) "bounce" on impact and push
the fluid apart. This process repeats to create several braids.
The researchers found it easy to tweak the braid's properties; for
example, they could decrease the length of the braids by making the
plane less steep and they could eliminate the braids altogether by increasing
the viscosity of the fluid. It is possible these observations have geophysical
implications, but more research is needed to say that with certainty.
(K. Mertens, V. Putkaradze, and P. Vorobieff, Nature,
8 July 2004.)