A new model now describes the boiling process much more precisely – Zoo House News

A new model now describes the boiling process much more precisely – Zoo House News

  • Science
  • November 23, 2022
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When a liquid boils in a vessel, tiny bubbles of steam form at the bottom and rise, transferring heat. How these small bubbles grow and finally detach was not previously known in detail. A German-Chinese research team led by the Helmholtz Center Dresden-Rossendorf (HZDR) has now succeeded in fundamentally expanding this understanding.

Using computer simulation, the experts succeeded in modeling the behavior of molecules at the liquid-gas interface in the nanometer range and thus describing the boiling process extremely precisely. The findings could be applied to future cooling systems for microprocessors or to the production of CO2-neutral hydrogen, so-called green hydrogen, the team reported in the Journal of Colloid and Interface Science.

How droplets or vapor bubbles wet a surface depends on the type and nature of the surface material. For example, spherical droplets form on hydrophobic materials with minimal contact area to the base. However, with hydrophilic materials, the liquid tends to form flat deposits – the solid-liquid interface is then much larger. Such processes can theoretically be described by the Young-Laplace equation. This equation gives a contact angle that characterizes the droplet behavior on the surface: large angles mean poor wetting, while small angles indicate good wetting.

When a bubble of vapor forms on a wall of a boiling liquid, a very thin film of liquid remains underneath – invisible to the eye. This film determines how the bubble grows and how it detaches from the wall. The contact angle also plays a decisive role here.

The underlying theory is based on a relatively simple approach. “It takes into account both the pressure that the liquid exerts from the outside and the vapor pressure inside the bubble,” explains Professor Uwe Hampel, head of the Experimental Thermal Flow Dynamics department at the HZDR. “Then there’s capillary pressure, which is caused by the curvature of the bubble’s surface.”

The fact that this established theory fails in the case of very small droplets and bubbles has recently been proven by a series of experiments using laser measurement: on the nanoscale, the contact angles measured deviate significantly from the theoretical predictions in some cases.

A complex interaction of molecules

To solve this problem, the German-Chinese research team set out to revise the theory. To do this, they took a closer look at the processes that take place when a liquid boils. “We took a close look at the interface behavior of molecules,” explains HZDR researcher Dr. white thing “Then we used a computer to simulate the interaction between these molecules.”

In doing so, the research group discovered a significant difference to previous approaches: the forces acting between the molecules do not simply add up linearly. Instead, the interaction is much more complex, resulting in pronounced nonlinear effects. It is precisely these effects that the experts take into account in their new, expanded theory.

“Our hypothesis provides a good explanation for the results of the recent experiments,” said Ding. “We now have a much more precise understanding of the behavior of tiny droplets and vapor bubbles.”

The results not only complete our understanding of the theoretical foundations, but also promise advances in several areas of technology, such as microelectronics. In this area, processors are now so powerful that they emit more and more heat, which then has to be dissipated by cooling systems.

“There are ideas for dissipating this heat by boiling a liquid,” remarked Uwe Hampel. “With our new theory, we should be able to determine the conditions under which rising vapor bubbles can release heat energy most efficiently.” The equations could also help to cool fuel elements in a nuclear reactor more effectively than previously.

More efficient hydrogen production

Another potential application is the electrolysis of water to produce CO2-neutral hydrogen, the so-called green hydrogen. Countless gas bubbles form on the membrane surfaces of an electrolyser during water splitting. With this new theory, it seems conceivable that these bubbles can be influenced in a more targeted way than before, which will enable more efficient electrolysis in the future. The key to all these possible applications lies in the selection and structuring of suitable materials.

“Introducing nanogrooves into a surface can, for example, significantly accelerate the detachment of gas bubbles during cooking,” explains Wei Ding. “With our new theory, such a structuring can now be more finely tailored – a project we are already working on.”

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