Observe the speedy water strider, a tiny insect with
jointed stilt- like legs that darts to and fro across the
unruffled surfaces of ponds, rivers and even the open ocean --
so quickly that human hands can rarely catch one.
What curious skill or evolutionary development keeps their
bodies from sinking into the water? What allows them to propel
themselves so swiftly atop the water's surface that each
strider can skitter forward 100 times its own body length in a
single second?
Even eight Olympic crewmen would barely thrust their
feather-light shell a single boat-length forward in that same
second.
Two mathematicians and an engineer at MIT have just
answered the puzzle of the water strider, and although they
have built a robot strider that mimics the insects, their
machine can't begin to keep pace with nature.
Their study is the subject of a report on hydrodynamics
appearing today in the scientific journal Nature, whose cover
features an image of a water strider in action.
Leader of the team at the Massachusetts Institute of
Technology is John W.M.
Bush, an associate professor of mathematics whose studies
include fluid dynamics, the phenomenon of surface tension,
even the mathematical performance of balls in sports like
soccer and tennis. His colleagues are David L. Hu, an MIT
graduate student in math, and Brian Chan, a mechanical
engineering student.
"The world of water striders is dominated by surface
tension," Bush said Wednesday in a phone interview from
Cambridge University in England, where he is continuing his
research and avidly playing tennis.
Water striders -- known as the Gerridae in entomological
terminology -- have evolved their skills for more than 100
million years, fossil record shows.
Bush's species are typically less than half an inch long
and weigh scarcely .003 of an ounce. Since they reproduce
rapidly -- every few weeks or so --
the MIT team was able to study the newborns in detail under
the microscope and catch their images with a high-speed video
camera at 500 frames a second as they skimmed on the surface
of the lab's aquarium.
Thousands of microscopic hairs on the bodies and legs of
water striders keep the water away as they move. During each
"rowing" stroke of their middle legs, Bush and his graduate
student discovered, the insects reach peak speeds of 150
centimeters a second, or precisely 3.3554044 miles an hour.
In the past, entomologists believed that the striders
propel themselves by pushing their legs into the water as they
jump, thrusting forward on ripples known as "capillary waves"
that they create.
Not so, say the MIT researchers.
From hours of analyzing the movements of infant striders on
video, they conclude from experiments in blue-dyed water that
the insects can't create such ripples but perform just as
human rowers do: They use their spindly jointed legs as oars
and the skinny lower ends of their legs as oar blades.
The water's surface tension -- its thin interface with the
air -- serves as a supporting membrane. The lightweight "oars"
of the striders create tiny dimples in the water without
breaking the surface tension, Bush and his two colleagues
conclude.
It's the thrust of a strider's legs backward against those
dimples that pushes the insects forward. Each thrust creates a
tiny swirling vortex inside each dimple -- a spin whose
physical characteristics require enough equations to satisfy
any mathematician.
Since MIT, like UC Berkeley, is home to major makers of
robots, Bush's team was inspired to try creating an imitation
water strider. They succeeded, and "Robostrider" is the
result. Modelled after the real insects, its lightweight body
is made of aluminum and it has four thin wire legs for support
and two middle legs to do the rowing, powered by a windup
elastic thread and a set of miniature pulleys.
The robot version is 10 times longer than the real insects,
and although it too rests on the surface tension of the water,
it can't go nearly as fast -- less than half a mile an hour.
"Robostrider travels (only) half a body length per stroke,"
the team concedes, "in a style less elegant than its natural
counterpart."
E-mail David Perlman at dperlman@sfchronicle.com.
