Climate change models also benefit from understanding fundamental thermodynamics of water droplets
When you see how difficult it is to get even next week's weather forecast right, you can imagine the challenges researchers are faced with when it comes to predicting the weather decades from now.
An image of two coalescing water droplets. The figure to the left shows a snapshot from a molecular dynamics simulation, where the small dots represent water molecules. The figure to the right illustrates the interface transfer coefficient for heat transfer, where the blue color means that the resistance to transfer is large. It is difficult to transport heat into or out of the region where the two droplets merge.
Illustration: Øivind Wilhelmsen, SINTEF Energi
Climatologists need good models to predict climate change over time, but to predict how the Earth's climate is changing, they also need to understand how water droplets behave.
"Now, we are capable of describing the transfer of heat and mass across both planar (flat) and curved water interfaces," says Øivind Wilhelmsen, a research scientist at SINTEF Energy Research. SINTEF is Scandinavia's largest independent research institute.
Wilhelmsen's research relates to nonequilibrium thermodynamics, and deals with an extension of the theories of the Norwegian Nobel Prize winner Lars Onsager. Most of us will find his research quite difficult to understand. However, it is of fundamental importance, not just for predicting tomorrow's weather, but also for understanding how weather and climate are going to change in the years to come.
It turns out that the water cycle and precipitation are among the largest uncertainties in current climate models.
How water droplets grow
"Water evaporates all the time from oceans, rivers and lakes. Then, clouds form in the atmosphere. Tiny droplets form in the clouds and eventually fall down as rain when they have grown large enough. How quickly these processes occur, how large the clouds become and when the rain falls all depends on how fast mass and energy are transported across water interfaces," says Wilhelmsen.
Some of the uncertainty in current weather forecasts and climate models lies in our fundamental lack of understanding of these transport processes.
"How water droplets grow depends on their interface transfer coefficients, which Wilhelmsen has calculated," says Professor Signe Kjelstrup from the Department of Chemistry at the Norwegian University of Science and Technology (NTNU).
The research was part of Wilhelmsen's doctoral thesis, for which Kjelstrup was a supervisor, along with Professor Dick Bedeaux.
Many scientific fields can benefit from the research.
"This research is very general, and the results allow us to describe a wide range of processes across many scales, from evaporation from large lakes to the growth of water droplets that are only a few nanometres in size," says Bedeaux.
Scientists can use the results to better understand natural processes, through weather forecasts and climate models. However, the findings also have industrial relevance, and are useful in industrial processes that involve evaporation or condensation of water. One important example is steam turbines, which are the most widely used equipment to generate electricity on a worldwide basis.
"For many years, this has been a missing piece of the puzzle for several important processes, both in nature and in industry," explains Wilhelmsen.
"The findings are useful in a large number of applications, and we would like to see the results put to use," says Kjelstrup.
Started with little
The researchers began with only scattered fragments of a description of how droplets behave. Their first task was to connect the right pieces. There was no satisfactory description even for completely flat water interfaces.
"Now we are even able to describe curved interfaces," Kjelstrup says.
Water is special, and its peculiar properties are one of the reasons why life was able to evolve on Earth. These same properties posed huge challenges, however.
"We had to use all the tools at our disposal--experiments at low temperatures, molecular dynamics simulations at high temperatures, and advanced new theory to make everything fit together. This would not have been possible ten years ago," says Wilhelmsen.
Part of the challenge lies in fact that the relevant experiments can only be conducted at low temperatures. At high temperatures, researchers were able to use nonequilibrium molecular dynamics simulations, where they could mimic on a computer how real water molecules interact with each other in a simulated volume. These simulations allow researchers to capture the anomalous properties of water quite precisely.
At lower temperatures however, the simulations became so computationally demanding that they were impossible to carry out, even on the most powerful supercomputers available today. The scientists then had to use advanced theory to pull the pieces together.
How the curvature of water droplets influences weather and climate
When water droplets first form in the atmosphere, they are very small. They then grow almost a million times in size before they eventually fall as raindrops. If we can determine exactly how fast water droplets grow under certain conditions, we can more easily predict when and how much it is going to rain.
"Since the water droplets are very small when they first form, the curvature of their interfaces affects how fast they grow," says Wilhelmsen.
The water droplets first grow by absorbing water from the atmosphere, and they are almost spherical in shape during this process. When they have become big enough, two water droplets can also collide and merge, or coalesce, into a bigger drop.
"We are now, for the first time, capable of describing how the transport of heat and mass occurs across water interfaces as two water droplets are coalescing. Here, the geometry and the curvature become more complicated," says Professor Bedeaux.
"Water droplet coalescence is an important mechanism in precipitation (as rain) in the tropics," says Wilhelmsen.
The scientists want to emphasize that their work is fundamental research that provides a new understanding of processes that we have known about for a long time. But at the same time, the consequences of the findings can be substantial.
"We have now figured out a new piece of the puzzle. This piece can be used in climate models and weather forecasts to improve our understanding--not only of how the weather will be like tomorrow, but also of how weather and climate will evolve in the future. It is important to reduce the uncertainty in current climate models because this will allow us to convince more people that it is super-important to act as quickly as possible to do something about the global warming," says Wilhelmsen.
The research has been published in Physical Review E and was undertaken in collaboration with the Institute of Fluid Mechanics at the Friedrich-Alexander University Erlangen-Nuremberg in Germany. Wilhemsen's co-authors are D Thuat T. Trinh, Anders Lervik and Vijay Kumar Badam, along with Kjestrup and Bedeaux.
Øivind Wilhelmsen | EurekAlert!
Ten-year anniversary of the Neumayer Station III
18.01.2019 | Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung
The pace at which the world’s permafrost soils are warming
16.01.2019 | Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung
The scientific and political community alike stress the importance of German Antarctic research
Joint Press Release from the BMBF and AWI
The Antarctic is a frigid continent south of the Antarctic Circle, where researchers are the only inhabitants. Despite the hostile conditions, here the Alfred...
World first experiments on sensor that may revolutionise everything from medical devices to unmanned vehicles
The new sensor - capable of detecting vibrations of living cells - may revolutionise everything from medical devices to unmanned vehicles.
Dead and alive at the same time? Researchers at the Max Planck Institute of Quantum Optics have implemented Erwin Schrödinger’s paradoxical gedanken experiment employing an entangled atom-light state.
In 1935 Erwin Schrödinger formulated a thought experiment designed to capture the paradoxical nature of quantum physics. The crucial element of this gedanken...
Cellulose obtained from wood has amazing material properties. Empa researchers are now equipping the biodegradable material with additional functionalities to produce implants for cartilage diseases using 3D printing.
It all starts with an ear. Empa researcher Michael Hausmann removes the object shaped like a human ear from the 3D printer and explains:
The phenomenon of so-called superlubricity is known, but so far the explanation at the atomic level has been missing: for example, how does extremely low friction occur in bearings? Researchers from the Fraunhofer Institutes IWM and IWS jointly deciphered a universal mechanism of superlubricity for certain diamond-like carbon layers in combination with organic lubricants. Based on this knowledge, it is now possible to formulate design rules for supra lubricating layer-lubricant combinations. The results are presented in an article in Nature Communications, volume 10.
One of the most important prerequisites for sustainable and environmentally friendly mobility is minimizing friction. Research and industry have been dedicated...
16.01.2019 | Event News
14.01.2019 | Event News
12.12.2018 | Event News
18.01.2019 | Materials Sciences
18.01.2019 | Life Sciences
18.01.2019 | Health and Medicine