 Press Release
Angewandte Chemie International Edition ,
doi: 10.1002/anie.200703987 Nr. 01/2008 Animated Movie of IceMelting ice crystals in a computer animationContact: David van der Spoel, Uppsala University (Sweden)
Registered journalists may download the original article here:
Picosecond Melting of Ice by an Infrared Laser Pulse: A Simulation Study
An animated movie shows an ordered structure dissolving little by little
into a disordered mess after a light pulse: Swedish researchers from the
University of Uppsala have used a computer to simulate ice melting after
it is heated with a short light pulse. As they report in the journal
Angewandte Chemie, the absorbed energy first causes the OH bonds to
oscillate. After a few picoseconds (10-12 s) the energy is
converted into rotational and translational energy, which causes the
crystal to melt, though crystalline domains remain visible for quite a
while.
The common form of ice crystals is known as hexagonal ice. In this form
the oxygen atoms of the water molecules are arranged in a tetrahedral
lattice. Each water molecule is bound to four neighboring molecules by
means of bridging hydrogen bonds, leading to an average of two bridges
per molecule. In water, there are, on average, only 1.75 bridging
hydrogen bonds per molecule.
What happens in the process of melting? Carl Caleman and David van der
Spoel have now successfully used a computer to simulate “snapshots” of
melting ice crystals. These molecular dynamics simulations are ideal for
gaining a better understanding of processes like melting or freezing
because they make it possible to simultaneously describe both the
structure and the dynamics of a system with atomic resolution and with a
time resolution in the femtosecond (10-15 s) range.
The simulation demonstrated that the energy of the laser pulse initially
causes the OH bonds in the water molecules to vibrate. Immediately after
the pulse, the vibrational energy reaches a maximum. After about a
picosecond, most of the vibrational energy has been transformed into
rotational energy. The molecules begin to spin out of their positions
within the crystal, breaking the bridging hydrogen bonds. After about 3
to 6 picoseconds, the rotations diminish in favor of translational
motion. The molecules are now able to move freely and the crystal
structure collapses. This process starts out locally, at individual
locations within the crystal. Once the symmetry of the structure is
broken, the likelihood of melting processes occurring in the area
immediately surrounding the crystal defect rises significantly. The
melting process thus spreads out from this point little by little. At
other locations the ice can maintain its crystalline structure a little
longer.
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A movie is available online at
http://xray.bmc.uu.se/molbiophys/images/Movies/melt.mpg
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