GUY DIMONTE is preparing to turn gravity upside down. This feat will be
achieved inside a transparent acrylic container the size of a small fish tank.
And even though it will all be over in a blink of an eye, 40 cameras will be
ready to record the event for posterity.
Dimonte, who works at the Lawrence Livermore National Laboratory in
California, is studying the interface between fluids of different densities. In
particular, he is interested in the way this interface breaks down as
irregularities grow. These irregularities are called Rayleigh-Taylor
instabilities and they crop up in all kinds of situations. They are one reason
why a fusion reaction tends to fizzle out before it gets started. They play an
important role in the way supernovas behave. And they are even responsible for
the sudden rush of ketchup out of an upturned bottle. The US Department of
Energy is funding Dimonte鈥檚 research to the tune of $1 million a year
because of the potential it has to help make fusion reactions more
efficient.
Nobody fully understands Rayleigh-Taylor instabilities but researchers the
world over would dearly like to know what physical laws govern the way they grow
and spread. Computer simulations are of little help鈥攅ven the world鈥檚 most
powerful supercomputers cannot make sense of the extraordinary complexity that
the instabilities create. But Dimonte believes he can study them using his
ingenious device for turning gravity upside down. By creating and filming the
instabilities between these fluids, he hopes to tease out the strange laws that
govern their behaviour.
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Physicists well understand the conditions in which Rayleigh-Taylor
instabilities occur. Many of the most common examples occur when a fluid
supports a denser one against gravity. Oil supporting a layer of vinegar, for
instance, or air supporting the ketchup in an upturned bottle. In these
circumstances, the less dense fluid should rise to the top because it will be
displaced by the denser, heavier component.
But if the interface between the two fluids were perfectly smooth, the denser
fluid could stay on top forever, almost as if it were unaware of the less dense
fluid below. In reality, however, the smoothest surfaces have microscopic
irregularities that can quickly grow into big distortions. These are
Rayleigh-Taylor instabilities and they are extraordinarily complex.
Raging maelstrom
Physicists have often observed the characteristic growth of these
instabilities. The denser fluid sends 鈥渇ingers鈥� down into the lighter one, while
the lighter fluid pushes upwards. These fingers grow extremely quickly, creating
complex, convoluted designs that are the hallmark of turbulence. Once a
turbulent mixing zone is established, it grows until the entire interface is a
raging maelstrom of chaos. But many of the details of this process remain
unresolved. How fast does the mixing zone grow? What is the range of sizes of
the penetrating fingers? What are the effects of changing various properties of
the liquids, for example, their viscosity and their ability to diffuse into each
other鈥攖heir 鈥渕iscibility鈥�?
Even though physicists have been studying Rayleigh-Taylor instabilities for
over a century, they are almost as hard to control in the lab as they are in the
dining room. Turn a bottle of vinegar and oil upside down, so that the less
dense oil is underneath, and they will start mixing long before the turn is
completed. So even creating the conditions in which the instabilities can be
studied is difficult. Some physicists have tried to solve the problem by
inserting a divider between the two fluids and then removing it鈥攂ut even
that leaves a turbulent wake that gives any instability a head start. To watch
the instability from its very inception, it seems, physicists need a magic
switch that instantaneously reverses the direction of gravity.
That鈥檚 where Dimonte and his transparent container come in. His experiment is
like an elevator accelerating downwards. As the elevator picks up speed, the
occupants feel lighter. If the elevator were to move downwards with an
acceleration equal to gravity (g), the occupants would feel weightless.
(Astronauts get used to zero-g conditions by flying in airplanes that
accelerate in this fashion.) And if the elevator were to move down at twice the
acceleration of gravity, the occupants could walk on the ceiling as if gravity
had been turned on its head.
The analogue of the elevator in Dimonte鈥檚 lab is a linear electric motor.
This consists of two vertical rails about 2 metres high, along which the 鈥渇ish
tank鈥� runs. The rails house a series of electromagnets that push against magnets
on the tank. By coordinating the way these electromagnets switch on and off, the
tank can be accelerated rapidly downwards. Over a distance of only 2 metres, the
device can achieve accelerations of up to 800 g in a fraction of a
second. At the end of its wild run, brakes bring the container to a halt through
friction.
Along the length of the track, Dimonte is installing a bank of 40 electronic
still cameras鈥攁 big improvement over his old setup, which had only eight.
As the tank accelerates by, each camera records the action at the interface as
it is frozen by a flash of laser light. Played back in sequence, these images
form an action movie of the way Rayleigh-Taylor instabilities grow when gravity
has been turned upside down.
Chain reaction
Despite the huge accelerations, Dimonte says the tank never reaches speeds of
more than 160 kilometres per hour鈥攕imilar to a fast pitch in baseball.
鈥淭he only difference is that we鈥檙e about 10 times more massive than a baseball.鈥�
Unlike baseball, Dimonte has to wait ten minutes between shots to allow the bank
of capacitors that power the machine to recharge.
Dimonte begins by filling the aquarium with two immiscible fluids of
different densities, then lets them settle into two perfectly separated layers.
Normally, he uses freon and water, but he has also resorted to more unusual
combinations such as compressed air and yoghurt. Like ketchup, yoghurt has
internal strength that slows down the formation of instabilities and even
prevents them forming at all, if the initial irregularities are too small.
Far-fetched as it may seem, yoghurt has the same properties as exotic fluids
that exist at many millions of degrees, such as plasmas. Hence the Department of
Energy鈥檚 interest in high-speed yoghurt.
Not far from Dimonte鈥檚 lab on the Lawrence Livermore campus, the Department
of Energy is building a rather more expensive experiment. The
$1.2-billion National Ignition Facility is a giant laser that will be
used to study nuclear fusion. The goal is to determine whether America鈥檚 ageing
stockpile of nuclear weapons will work if they are ever needed. But on the way,
physicists hope to show how they can control nuclear fusion on a tiny scale,
using a process called inertial confinement fusion. The idea is to focus a set
of powerful laser beams onto a tiny pellet the size of an apple seed that
contains deuterium and tritium, two isotopes of hydrogen. The lasers vaporise
the surface of the pellet creating a plasma that expands rapidly, compressing
the contents and generating huge temperatures and densities. If all goes well, a
chain reaction will occur in which the deuterium and tritium fuse, releasing
energy.
The problem is that the chain reaction tends to fizzle out before it gets
going, so physicists have to pump in far more energy than they get out. 鈥淭he old
joke is, fusion is the energy source of the future鈥攁nd always will be,鈥�
says Paul Dimotakis, a physicist at Caltech in Pasadena who studies turbulence
and combustion.
Rayleigh-Taylor instabilities are one of the main problems. The laser turns
the surface of the pellet into a dense plasma that rushes towards the mixture of
deuterium and tritium. The interface between these two fluids is unstable. The
outer plasma sends fingers into the mixture of tritium and hydrogen. This cools
it and destroys the conditions necessary for sustained fusion.
Though the stakes are higher, the physics involved are the same as those
governing the behaviour of air and yoghurt at 100 g. 鈥淭he beautiful
thing is that if you take the fluid equations and multiply them by a million, it
doesn鈥檛 really change the solution at all,鈥� Dimonte says. 鈥淚t only changes the
scale of things.鈥� So yoghurt accelerating at 100 g for a twentieth of a
second generates the same pattern of instabilities as a plasma accelerating at a
trillion g for a few billionths of a second.
The reason yoghurt is important is that its internal strength can prevent the
spread of instabilities. Dimonte hopes to understand how this process works.
Armed with this information, physicists may be able to predict how similar
stable conditions can be set up in high-temperature plasmas.
So far, Dimonte says he has an 鈥渆normous amount of data鈥� from four years of
experiments. And the new 40-camera accelerator should allow him to study
instabilities in more detail. He hopes to have it working by the end of the
year.
Already, he has learnt that the way acceleration changes over time is an
important factor governing the spread of instabilities. A rapid increase in
acceleration to 800 g followed by a more gentle acceleration causes two
fluids to mix less rapidly than a steady acceleration. With this information,
physicists may be able to design fusion pellets that reproduce the acceleration
profile they want. The information he has collected on fluids of various
densities will help to verify the computer simulations that are used to design
fusion pellets.
According to Dimotakis, experiments like Dimonte鈥檚 help to explore an area
where computer simulations are especially weak. While computer programs can
simulate some aspects of laser fusion that laboratory experiments
cannot鈥攕uch as temperatures of 100 million kelvin鈥攖hey are
notoriously bad at simulating turbulent fluids. 鈥淭hat鈥檚 what makes all of this
difficult and challenging,鈥� says Dimotakis. 鈥淚t鈥檚 out of computer reach, and out
of the range of experiments. Yet we have to understand it.鈥�
For his part, Dimonte is not fazed by the difficulty of making fusion work.
鈥淔usion is for the ages,鈥� he says. 鈥淚 don鈥檛 see it being economically viable or
necessary for another 100 years. As a scientist, that doesn鈥檛 bother
me鈥攊t鈥檚 an intellectual problem.鈥� Also, he admits, it can be a lot of fun
to shoot yoghurt at speeds no yoghurt has gone before.
