X-rays indicate that water can behave like a liquid crystal

Date:
August 11, 2020
Source:
Stockholm University
Summary:
Scientists have discovered that water can exhibit a similar behavior
like a liquid crystal when illuminated with laser light. This effect
originates by the alignment of water molecules, which exhibit a
mixture of low- and high-density domains that are more or less
prone to alignment. The results are based on a combination of
experimental studies using X-ray lasers and molecular simulations.



FULL STORY ========================================================================== Scientists at Stockholm University have discovered that water can
exhibit a similar behavior like a liquid crystal when illuminated with
laser light. This effect originates by the alignment of water molecules,
which exhibit a mixture of low- and high-density domains that are more or
less prone to alignment. The results, reported in Physical Review Letters
on the 11th of August 2020, are based on a combination of experimental
studies using X-ray lasers and molecular simulations.


========================================================================== Liquid crystals were considered a mere scientific curiosity when they
were first discovered in 1888. Over 100 years later, they are one of
the most widely used technologies, present in digital displays (LCDs)
of watches, TVs and computer screens. Liquid crystals work by applying an electric field, which makes the neighboring molecules of a liquid align,
in a way that resembles a crystal. Water too can be distorted towards a
liquid crystal, when illuminated with laser light. It is known that the electric field of the laser can align the water molecules for less than
a billionth of a second. Can this discovery have future technological applications? An international team of researchers at the Physics
Department of Stockholm University carried out experiments at Japan's
X-ray Free-electron laser SACLA and probed for the first time the dynamics
of transiently oriented molecules using X-ray pulses. This technique,
relies on aligning the molecules with a laser pulse (with wavelength l =
800nm) and probing the alignment with X-ray pulses, which allow to see in
real time the changes in the structure on a molecular level. By varying
the time between the laser and the X-ray pulses, the researchers were
able to resolve the aligned state, which lives only for 160 fs.

"It is known that the water molecules are aligned due to the polarization
of the laser pulse" explains Kyung Hwan Kim, former researcher at
Stockholm University and currently assistant professor at POSTECH
University in Korea, "it is a unique capability however to be able
to use X-ray lasers to see the molecular alignment in real time."
"X-rays are perfect for probing molecules because their wavelength matches
the molecular lengthscales" says Dr. Alexander Späh, former PhD
student in Physics at Stockholm University, and currently being a postdoc
at Stanford University. "I really enjoy having the opportunity to use state-of-the-art X- ray facilities to investigate fundamental questions
that could have future technological applications." The experiments
were well reproduced by molecular simulations, which gave an insight
to the underlying alignment mechanism. By assuming that water behaves
like a two-state liquid, consisting of high- and low-density liquid
(HDL and LDL) domains, the researchers discovered that each domain shows
a different tendency to align.

"Water molecules in the LDL regions have stronger hydrogen bond network,
which makes the molecules easier to respond to the strong laser field"
explains Anders Nilsson, professor in Chemical Physics at Stockholm
University. "It would be fascinating to measure the lifetime of the
molecular alignment in the supercooled regime, where everything is
expected to slow down dramatically." "Being able to understand water on a molecular level by watching the changes of the hydrogen-bond network, can
play a major role in biological activity" says Fivos Perakis, assistant professor in Physics at Stockholm University. "I am curious to see
whether the observed alignment can lead to technological applications in
the future, for example in connection to water cleaning and desalination."

========================================================================== Story Source: Materials provided by Stockholm_University. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. K.H.Kim et al. Anisotropic x-ray scattering of transiently oriented
water. Physical Review Letters, 2020 DOI:
10.1103/PhysRevLett.125.076002 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200811120114.htm

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