A new model now describes the boiling process much more accurately – ScienceDaily

When a liquid boils in a pot, tiny vapor bubbles form at the bottom and rise up, transferring heat in the process. How these tiny bubbles grow and eventually separate was not previously known in any great detail. A German-Chinese research team led by Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has now been able to fundamentally expand this understanding.

Using computer simulations, the experts have successfully modeled the behavior of molecules at the gas-liquid interface on the nanometer scale, enabling them to describe the boiling process with extreme precision. The results could be applied to future cooling systems for microprocessors, or to the production of carbon-neutral hydrogen, known as green hydrogen, the team reports in Journal of Colloid and Interface Sciences.

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

When a vapor bubble forms on a wall in a boiling liquid, a very thin layer of liquid—invisible to the eye—remains underneath. This film outlines how the bubble grows and how it separates from the wall. The contact angle also plays a major role in this regard.

The basic theory is based on a relatively simple approach. “It takes into account both the external pressure of the liquid and the vapor pressure inside the bubble,” explained Professor Uwe Hampel, Head of Experimental Thermofluid Dynamics at HZDR. “Then there is capillary pressure caused by the curvature of the bubble surface.”

Recently, however, a set of experiments using laser scaling has shown that this well-established theory fails for very small droplets and bubbles: on the nanoscale, the measured contact angles deviated significantly in some cases from theoretical expectations.

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 occur when a liquid boils. “We have studied in detail the interfacial behavior of the molecules,” explained HZDR researcher Dr. Wei Ding. We then used a computer to simulate the interaction between these molecules.

In doing so, the research group discovered a significant difference from previous approaches: the forces acting between molecules do not simply add up linearly. Instead, the interaction is much more complex, which leads to distinct nonlinear effects. These are exactly the effects that experts are considering in their new, expanded theory.

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

Besides completing our understanding of the theoretical basis, the results also herald advances in many areas of technology, such as microelectronics. In this area, processors are now so powerful that they release increasing amounts of heat, which must then be dissipated by cooling systems.

“There are ideas of removing this heat by boiling a liquid,” commented Uwe Hämpel. “With our new theory, we should be able to determine the conditions under which rising vapor bubbles can dissipate heat energy most efficiently.” The formulas can also help cool the fuel elements in a nuclear reactor more effectively than in the past.

more efficient hydrogen production

The electrolysis of water to produce carbon-neutral hydrogen, referred to as green hydrogen, is another potential application. Countless gas bubbles form on the surfaces of the electrolyser membrane during the water separation. With this new theory, it seems conceivable that these bubbles could be affected more specifically than before, allowing for more efficient electrolysis in the future. The key to all of these potential applications lies in the selection and structuring of appropriate materials.

“Adding nanoscale grooves to a surface, for example, can greatly speed up the separation of gas bubbles during boiling,” Wei Ding explained. “With our new theory, this structuring can now be much more subtle – a project we’re already working on.”

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