91av

ONE night in 1922, on a freighter from Amsterdam bound for the Spanish port
of Alicante, the Dutch artist M.C. Escher witnessed one of the marvels of the
sea. The first mate walked him forward to the ship’s bows to show him glowing
dolphins.

The sea was dark except for an intense bioluminescence surrounding the ship
and illuminating its wake, and just ahead Escher saw the luminous blue shapes of
dolphins swimming in the bow wave. “An unforgettably beautiful spectacle,” was
how he described it in his diary, “they shot forwards leaving a tail of light
behind them.”

The following day, he sketched what he had seen, and a few months later he
turned the sketch into a woodcut called “Dolphins in a phosphorescent sea”.
Escher often said he felt closer to science than art, but he had no idea that
his woodcut would later attract the interest of marine biologists trying to
understand how dolphins swim. Nor could he have imagined that the same bright
light he saw that night would one day help scientists to develop artificial
hearts, design incubators for human cells, and even understand the weather.

What all of these challenges have in common is the movement of fluids—a
remarkably difficult thing to study. Any fluid—be it water, wine or
air—looks much the same whether it’s still or swirling. Just as sailors
look at clouds, flags and ripples on the water to judge the wind, fluid
dynamicists need tricks to help them see how fluids move. They can inject dye
into a stream of water, or track moving particles with lasers. But even the most
sophisticated methods give an incomplete picture.

On an evening cruise in the waters off San Diego, Jim Rohr, a fluid
dynamicist working for the US Navy, realised that bioluminescent organisms in
the sea had been doing for aeons what scientists had failed to do in the lab:
reveal the motion of fluids to the naked eye. He and a friend were marvelling at
the brilliant display of bioluminescence in the boat’s wake. “My friend said,
`You’re a fluid dynamicist. How thin would this boat have to be to have no
wake?'” says Rohr. “And that’s how it all started.”

Rohr realised that the glowing plankton were giving him a readout of how much
wake the boat was creating. Supposing the brightness of the wake depended on the
amount of turbulence? And what if the plankton were reacting to some very
particular property of the moving water, such as its velocity, that researchers
must bend over backwards to measure in the lab? If he could connect the
brightness to the motion of the water, he reasoned, it would be like seeing the
wind.

So Rohr got in touch with Michael Latz, a marine biologist a few miles up the
coast at the Scripps Institution of Oceanography in La Jolla. Latz is an expert
on dinoflagellates, the microscopic, single-celled plankton responsible for most
of the bioluminescence in the sea.

Poison tides

Dinoflagellates inhabit every ocean on Earth, but when conditions are just
right they multiply into a dense bloom that stains the sea red or brown. These
infamous red tides can force the closure of fisheries, because some
dinoflagellates secrete poisons.

Despite their toxic reputation, dinoflagellates put on truly a great show at
night, when the slightest disturbance of the water around them triggers their
chemical flashbulbs. This ancient burglar alarm probably evolved to light up
predators such as copepods as they swim through the plankton, making them
vulnerable to their own predators. But could the alarm be triggered in the
lab?

Rohr and Latz set out to test this, pumping seawater through clear pipes to
see how fast it had to move to make the dinoflagellates luminesce. What they
discovered was a perfect correlation between the speed of the water flow and the
intensity of the luminescence. And different types of dinoflagellate gave the
same result. “I was just amazed how repeatable this was,” says Rohr. “No one had
ever dreamt that their response could be calibrated.”

But surely plankton in a current can no more tell whether the water is moving
than we can feel the Earth flying through space? How do they do it? The answer
has to do with the mechanical strain of relative motion. Liquid flowing through
a pipe moves slower near the wall than in the middle. As the fast molecules drag
past the slower ones, they tug with a force called shear stress. Since the
velocity at the wall is always zero, the faster the liquid is pumped through the
pipe, the greater the shear stress.

Latz speculates that one layer of water shearing past the next deforms the
dinoflagellates and makes them light up. No one knows exactly how, but the
strain may force open channels in the cell membrane, bringing about a change in
pH which in turn speeds up an enzyme-driven, light-generating chemical
reaction. What makes this useful for fluid dynamics is that the brightness is
related to the shear stress: the faster the flow, the brighter the glow.

That’s because, rather than all the cells lighting up at once, a certain
amount of shear corresponds to a certain probability that any one cell will
flash. In a low-speed flow maybe only one cell in ten thousand would light up,
but ten times as many might flash in a high-speed flow. The beauty of this is
that dinoflagellates can reveal exactly what is going on in each region of a
moving liquid, in real time and in three dimensions. If you tow a model for a
new boat design through seawater awash with these creatures, the parts of the
hull with high shear stress—and therefore lots of drag—will glow
bright blue. So the designer knows immediately where to make improvements.
“Bioluminescence only lights up when the fluid is doing something,” says
Rohr.

The possibility of ultra-efficient boats is one reason why researchers have
been intrigued by the skin of the dolphin. In the 1930s, British zoologist James
Gray concluded that dolphin skin must be extraordinary. He had seen the
creatures swimming at up to 10 metres per second, yet he calculated that their
muscles weren’t powerful enough to overcome the water drag at that speed. The
only way the figures could make sense, he argued, is if dolphins somehow prevent
the water from becoming turbulent as it flows over their bodies.

Turbulence is the enemy of speed. Imagine pulling a plastic dolphin through
the water on a string. If you pull it very gently, you feel very little
resistance. That’s because the water flows smoothly over the body and causes
very little drag—a condition known as laminar flow. But as you pull
harder, the flow around the toy becomes turbulent, and the random motion of the
water increases drag. If a living dolphin can somehow trick the water into
maintaining laminar flow, Gray reasoned, it could swim faster.

Since Gray made his observations, scientists have proposed many ways dolphins
might do this. Perhaps they slough off skin cells when they swim fast, or heat
the water around them to change its properties, or shed tears of slippery
polymers?

None of these theories has panned out. Another idea is that at high speeds,
ripples run over the dolphins’ skin and absorb the turbulence
(91av, 18 January 1997, p 28).
Researchers have tried repeatedly to
confirm this, but so far they have failed. One bizarre experiment, conducted in
the Soviet Union in the 1970s, involved towing naked female swimmers through a
pool. The researchers hoped that they would see a decrease in drag when the
women were pulled fast enough for ripples to form in their skin. Instead the
drag increased.

More recently, some scientists have argued that Gray made faulty assumptions
in his original calculations, and that dolphins don’t need special skin. Terrie
Williams, a marine mammal researcher at the University of California in Santa
Cruz, points out that Gray assumed dolphins could swim fast for a long time. But
we know today that they can only sprint for a few seconds before needing a rest.
By concentrating their muscle power in a short burst, they may be able to
overcome more drag than Gray thought possible. “They are just darn good
athletes,” Williams says. Even so, she adds, dolphins are such highly evolved
swimmers their skin may well have special drag reduction properties—
albeit not as miraculous as Gray assumed.

Rohr and Latz think bioluminescence could settle the mystery of dolphin skin
once and for all. Compared with turbulent flow, laminar flow produces lower
shear stress at the same velocity—because there is no swirling or random
motion. So the intensity of the bioluminescence that dolphins trigger while
swimming should reveal the way water flows over their bodies.

As it happened, Rohr had access to some friendly dolphins—he works at
the US Navy`s Space and Naval Warfare Systems Center in San Diego, where the
animals are trained to sniff out mines. To find out whether dolphin luminescence
could be measured, he covered a row of ocean pens with a tent to block out all
light—even a new moon gives out enough light to overwhelm bioluminescence.
Then he waited for a red tide and filmed a dolphin at night as it swam at about
2 metres per second.

Turbulent wake

The effect was brilliant. Just like the Escher woodcut, most of the dolphin’s
body lit up as it swam. The brightest regions were just behind the blowhole,
which might be expected to cause turbulence, and behind the fins and flukes,
which probably leave a turbulent wake. On average, the tail half of the dolphin
was brighter than the front half, as if the flow became more disturbed as it
moved along the body. The darkest region was on the dolphin’s dome-shaped
forehead, the “melon”. This is an acoustic lens that the dolphin uses to collect
sonar chirps and locate its prey.

One way to interpret these light patterns is that, at speeds around a fifth
of what Gray had observed in the 1930s, the fluid moving over the dolphin’s body
starts out as a laminar flow. Then as it rushes along the dolphin’s body it
becomes increasingly turbulent. The melon is probably shaped specifically to
prevent turbulence, since chaotic water flow would interfere with the dolphin’s
sonar.

On the other hand, without calibrating exactly what flow causes what
intensity of light, it is impossible to tell for certain from this experiment
whether there is any laminar flow at all. The intensities Rohr measured inside
the pipe may not be relevant to this situation, because of the different
relationship between velocity and shear stress around a dolphin’s body compared
with those inside a tube.

A simple way to find out may be to look carefully at the thin layer of water
around the dolphin. In laminar flow, this region—called the boundary
layer—is only a few millimetres thick, but in turbulent flow it can be ten
times thicker. Video film is too crude to measure the complexities of the
boundary layer directly, but Rohr has an ingenious yet simple solution. He plans
to film a dolphin with a rubber band several millimetres thick placed at
different points along the length of its body. If the flow is turbulent, the
band will be well inside the boundary layer and the luminescence should barely
change. But if it is laminar at that point, the band will trip the flow into
turbulence and light it up. By filming the dolphin swimming with the band at
different points along its body, Rohr should be able to see if and where the
transition from laminar to turbulent flow takes place.

Unfortunately, Rohr must wait for more research funding to arrive, and for
the plankton to return. The red tides, which normally hit the waters off San
Diego in the spring, were unusually sparse this year because of abnormally warm
waters. But in the meantime, Rohr and Latz are putting plankton to good use.

In his lab, Latz is rearing his own private army of dinoflagellates in dozens
of large flasks. Tap one of these flasks in the dark, and you ignite a flash of
bright blue fire inside. With these bioluminescent plankton to light the way,
Latz, Rohr and Juan Lasheras, professor of fluid mechanics at the University of
California at San Diego, hope to learn what laws govern the swirls and eddies of
turbulent flow, whether they are slowing down a dolphin, diverting the Gulf
Stream, or forming a storm cloud. “Almost everything that happens in nature is
turbulent,” says Lasheras. If we could control turbulence, he adds, many
industrial processes could be streamlined, from refining oil to pumping water.
“But we haven’t had sufficient tools to study it. So this new technique could be
of great value.”

Fountain of light

In September, the team took their first steps towards this goal. They pumped
a high-pressure jet of seawater laden with dinoflagellates through a fine nozzle
into a tank of ordinary seawater. The result was spectacular—like a
fountain of light. But best of all, as the injected stream slowed down a little
way in front of the nozzle, the eddies and swirls showed up clearly.

If they can calibrate this glow, the researchers will be able to determine
the shear stress at any point in the flow, simply from the brightness of the
light. “Hopefully this will allow us to observe turbulence in much greater
detail than we are able to now,” says Lasheras. Eventually, it may even enable
fluid dynamicists to tease out the previously invisible laws of turbulent motion
that apply to everything from curling cigarette smoke to weather patterns.

For now, the plankton can be put to more prosaic uses, for example to improve
the design of bioreactors—the incubators used to culture human and animal
cells for research and medicine. These mix the cells about so that each one
receives the oxygen and nutrients it needs. The problem is that this motion
generates shear stresses that can kill the cells, and what designers need is a
way to visualise where these dangerous stresses occur.

So Latz and cell biologist John Frangos from the University of California at
San Diego put dinoflagellates in a bioreactor, switched it on and recorded where
their light was brightest. With this information, they will be able to design a
bioreactor that has no shear stress hot spots at all, and which should be better
at keeping the valuable cells alive.

John Tarbell, an engineer who designs artificial hearts at Pennsylvania State
University in State College, is also excited by the potential uses of glowing
plankton. The danger in artificial hearts is not high shear stress but areas of
low shear stress, because artery-plugging thrombi—clots—can begin to
form in the blood at such points
(91av, 6 February, p 32).
Within the next few months, Tarbell’s group hopes to begin testing
dinoflagellates in artificial heart prototypes. If they place windows in these
devices and pump bioluminescent seawater through them rather than blood, they
will be able to measure the patterns of light emission that form. “Instead of
looking for the bright spots, you could look for the dark spots,” says
Tarbell.

Standing on the bow of the freighter that night in 1922, Escher could
scarcely have dreamt of the uses to which those spectacular trails of blue light
would be put. But as a master of visual illusion and complexity in art, he would
surely have appreciated the excitement of unravelling the mysteries of fluid
dynamics.