Tiny vapor bubbles form at the bottom of a boiling liquid and rise, carrying heat with them. We knew very little about how these little bubbles develop and eventually separate. The Helmholtz-Zentrum Dresden-Rossendorf (HZDR)-led German-Chinese research team has now succeeded in significantly broadening this understanding.
The scientists were able to precisely predict the behavior of molecules at the liquid-gas boundary at the nanoscale scale using computer simulation, which allowed them to explain the boiling process. Future microprocessor cooling technologies or the creation of carbon-neutral hydrogen, often known as “green hydrogen,” could benefit from the research, the team said in the Journal of Colloid and Interface Science.
The kind and makeup of the surface material determines how droplets or vapor bubbles moisten it. For instance, spherical drops with a small base contact area occur on hydrophobic materials. However, with hydrophilic materials, the liquid tends to form flat deposits, increasing the size of the solid-liquid contact. The Young-Laplace equation can potentially explain such phenomena. A contact angle is produced by this equation that describes how droplets behave on a surface: big angles denote poor wetting, whereas small angles denote good wetting.
In a boiling liquid, a very thin coating of liquid that is undetectable to the eye remains underneath a vapor bubble that has formed on the wall. This movie shows how the bubble expands and separates from the wall. Another important factor in this regard is the contact angle.
The fundamental hypothesis is founded on a comparatively straightforward method. Professor Uwe Hampel, Head of Experimental Thermal Fluid Dynamics at the HZDR, noted that the model “takes into consideration both the pressure exerted externally by the liquid and the vapor pressure inside the bubble.” Then there is capillary pressure, which is brought on by the bubble surface’s curvature.
The failure of this well-established theory for very small droplets and bubbles, however, has recently been shown by a variety of studies utilizing laser measurement: at the nanoscale, recorded contact angles occasionally differed dramatically from theoretical expectations.
A complex interaction of molecules in nanoscale
The German-Chinese research team began by modifying the theory to address this issue. They focused on the procedures that take place when a liquid boils in order to do this. HZDR researcher Dr. Wei Ding said, “We considered the interfacial behavior of molecules in depth.” Then, we simulated the interaction between these molecules using a computer.
The study team made a major discovery that set them apart from other methods: the forces interacting between molecules do not simply add up linearly. The interaction is substantially more complicated and produces various nonlinear consequences as a result. The experts’ new, extended hypothesis specifically takes these consequences into account.
Ding exclaimed enthusiastically, “Our hypothesis offers a fair explanation for the outcomes reported in previous experiments.” The behavior of minute droplets and vapor bubbles is now better understood.
The results not only complete our understanding of the theoretical underpinnings but also hold promise for advancement in a number of technological fields, including microelectronics. These days’ powerful CPUs produce more heat than ever before, which cooling systems are required to remove.
Uwe Hampel said, “There are ideas to get rid of this heat by boiling a liquid.” According to our new theory, we should be able to identify the circumstances in which rising vapor bubbles can dissipate thermal energy most effectively. The equations might also make it possible for nuclear reactor fuel elements to be cooled more efficiently than in the past.
More efficient hydrogen production
Another potential use is the electrolysis of water to create “green hydrogen,” or hydrogen that is carbon-neutral. During the water splitting process, countless gas bubbles develop on the membrane surfaces of an electrolyzer. This new hypothesis makes it possible for these bubbles to be targeted more precisely than before, leading to future improvements in electrolysis efficiency. The choice and structure of suitable materials are the keys to all of these potential nanoscale uses.
Wei Ding stated, “For example, adding nanogrooves to a surface can dramatically speed up the separation of gas bubbles during boiling.” “Such structuring can now be more precisely tuned according to our new theory; we are now working on this topic.”
