In a first, scientists capture a 'quantum tug' between neighboring water molecules

The first direct observation of atomic movement in liquid water molecules has been made by researchers using laser light. These results could explain the unusual properties of water's microscopic origin. Credit: Greg Stewart/SLAC National Accelerator Laboratory
Water is the most common and least well-known liquid in nature. Scientists are still trying to understand its strange behavior. Water is the most dense liquid at 39 degrees Fahrenheit. This is just above its freezing point. This is why ice can float to the top of a glass, and lakes freeze from below. It allows marine life to survive the cold winters. Water has an unusually high level of surface tension which allows insects to walk on it. It also has a large heat storage capacity, which keeps the ocean temperatures stable.

Researchers from the Department of Energy's SLAC National Accelerator Laboratory and Stanford University, as well as researchers from Stockholm University in Sweden, have now made the first direct observation of hydrogen atoms in water molecules pulling and pushing each other when excited by laser light. The Nature Today publication reveals effects that could explain key aspects of water's bizarre microscopic origin and help us understand how water affects proteins in living organisms.

Anders Nilsson from Stockholm University, who collaborated on the study, said that although the so-called nuclear effect has been hypothesized as being at the root of many water's bizarre properties, this experiment marks its first direct observation." The question is whether this quantum effect might be the missing link in the theoretical models that describe water's anomalous properties.

Each water molecule has one oxygen atom, two hydrogen atoms and a web hydrogen bond between negatively charged oxygen molecules in nearby molecules. Although this intricate network is responsible for many water's mysterious properties, researchers have not been able to observe the interactions between water molecules and their neighbors until recent years.

A cartoon shows how water molecules react to laser light. This view shows an effect believed to be responsible for many of the water's strange properties. It was created using SLAC's MeV-UED electron cam, which blasts samples with high-energy electrons in short pulses to see inside. Credit: Greg Stewart/SLAC National Accelerator Laboratory

Kelly Gaffney, a Stanford Pulse Institute scientist at SLAC, said that the low mass of hydrogen atoms accentuates the quantum wave-like behavior. "This is the first study to demonstrate that the reaction of the hydrogen bond networks to an energy impulse depends on the quantum mechanical nature how hydrogen atoms are placed out. This has been long suspected to be the reason for water's unique characteristics and hydrogen bond network.

Love your neighbor

This observation was difficult until now because of the tiny and slow motions caused by the hydrogen bonds. This experiment overcomes that problem using SLAC’s MeV-UED. It is a high-speed electron camera that uses a strong beam of electrons to scatter subtle molecular movements off samples.

The researchers created 100-nanometer thick jets of liquid water, about 1,000 times thinner than a human hair. They then set the water molecules to vibrate with an infrared laser beam. They then blasted the molecules using short pulses high-energy electrons, MeV-UED.

The research team (left-to-right: Xiaozhe shen, Pedro Nunes and Jie Yang) used SLACs meV-UED, a high speed electron camera that uses a strong beam of electrons in order to detect subtle molecular movement in samples. Credit: Dawn Harmer/SLAC National Accelerator Laboratory

This produced high-resolution images of the molecules' changing atomic structure, which they stitched together to create a stop motion movie of how the water molecules responded.

These snapshots were taken of groups of three water molecules. They showed that when a water molecule vibrates, its hydrogen atom pulls oxygen atoms from nearby water molecules closer, before pushing them away with its newfound strength. This expands the space between the molecules.

Jie Yang, a former SLAC scientist who is now a professor at Tsinghua University, China, led the study. The beauty of this experiment was that we were able for the first-time to observe how these molecules move.

A window to water

Researchers hope to use this method in order to gain greater insight into the quantum nature and role of hydrogen bonds in water's bizarre properties as well as their key role in many chemical or biological processes.

"This has really opened a window to study water," said Xijie Zhang, a SLAC distinguished scientist and study collaborator. "Now that we see hydrogen bonds moving, it's time to link those movements with the larger picture. This could shed light on the origin and survival water and help us develop renewable energy methods.

MeV-UED, an instrument of LCLS user facility, is operated by SLAC for the DOE Office of Science. This research was funded by the DOE Office of Science.

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More information: Nature Direct observation of ultrafast hydrogen bonds strengthening in liquid water (2021). Journal information: Nature Direct observation of ultrafast hydrogen bond strengthening in liquid water,(2021). DOI: 10.1038/s41586-021-03793-9