Scientists have used a
powerful X-ray laser to heat water from room temperature to 100,000 degrees
Celsius in less than a tenth of a picosecond (millionth of a millionth of a
second) Or 75 femtoseconds, or 75 millionths of a billionth of a second. The experimental set-up that can be seen as the world’s fastest water
heater produced an exotic state of water.
From this exotic state of
water researchers hope to learn more about the peculiar characteristics of
Earth's most important liquid. The observations also have practical use for the
probing biological and many other samples with X-ray lasers. The team of Carl
Caleman from the Center for Free-Electron Laser Science (CFEL) at DESY and
Uppsala University (Sweden) reports its findings in the journal Proceedings of
the National Academy of Sciences (PNAS).
The researchers used the X-ray free-electron laser Linac Coherent Light Source LCLS at the SLAC National Accelerator Laboratory in the U.S. to shoot extremely intense and ultra-short flashes of X-rays at a jet of water.
"It is not the usual way to boil your water," said Caleman. "Normally, when you heat water, the molecules will just be shaken stronger and stronger."
On the molecular level, heat
is motion -- the hotter, the faster the motion of the molecules. This can be
achieved, for example, via heat transfer from a stove, or more directly with
microwaves that make the water molecules swing back and forth ever faster in
step with the electromagnetic field.
"Our heating is
fundamentally different," explained Caleman. "The energetic X-rays punch
electrons out of the water molecules, thereby destroying the balance of
electric charges. So, suddenly the atoms feel a strong repulsive force and
start to move violently."
In less than 75
femtoseconds, that's 75 millionths of a billionth of a second or 0.000 000 000
000 075 seconds, the water goes through a phase transition from liquid to
plasma. A plasma is a state of matter where the electrons have been removed
from the atoms, leading to a sort of electrically charged gas.
"But while the water
transforms from liquid to plasma, it still remains at the density of liquid
water, as the atoms didn't have time to move significantly yet," said
co-author Olof Jönsson from Uppsala University. This exotic state of matter is
nothing that can be found naturally on Earth. "It has similar
characteristics as some plasmas in the sun and the gas giant Jupiter, but has a
lower density. Meanwhile, it is hotter than Earth's core."
The scientists used their measurements to validate simulations of the process. Together, the measurements and simulations allow to study this exotic state of water in order to learn more about water's general properties.
"Water really is an odd liquid, and if it weren't for its peculiar characteristics, many things on Earth wouldn't be as they are, particularly life," Jönsson emphasised.
Water displays many
anomalies, including its density, heat capacity and thermal conductivity. It it
these anomalies that will be investigated within the future Centre for Water
Science (CWS) planned at DESY, and the obtained results are of great importance
for the acivities there.
Apart from its fundamental
significance, the study also has immediate practical significance. X-ray lasers
are often used to investigate the atomic structure of tiny samples. "It is
important for any experiment involving liquids at X-ray lasers," said
co-author Kenneth Beyerlein from CFEL. "In fact, any sample that you put
into the X-ray beam will be destroyed in the way that we observed. If you
analyse anything that is not a crystal, you have to consider this."
But at 75 femtoseconds,
changes are already evident. "The study gives us a better understanding of
what we do to different samples," explained co-author Nicusor Timneanu
from Uppsala University, one of the key scientist developing the theoretical
model used. "Its observations are also important to consider for the
development of techniques to image single molecules or other tiny particles
with X-ray lasers."
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