Designing surfaces that make water boil more efficiently

The boiling of water or other fluids is an energy intensive step at the heart of a wide range of industrial processes.

It's possible to improve the efficiency of systems that evaporate water. Researchers at MIT have found a way to tailor the surface treatment for the materials used in these systems.

There are different kinds of surface modifications at different sizes. Youngsup Song is a graduate of MIT and is one of the authors of the paper. More work needs to be done to develop a practical, industrial-scale process for this initial finding.

There are two important parameters that describe the boiling process. Anything that improves one of the parameters tends to make the other worse. The team has achieved a way of improving both properties at the same time through their combination of different textures added to a material's surface.

"Enhancing both parameters together is difficult because they have an inherent trade off," Song said. If we have a lot of bubbles on the boiling surface, that means boiling is very efficient, but if we have too many bubbles on the surface, they can coalesce together, which can cause a film over the boiling surface. The film introduces resistance to the heat transfer. He says that if we have vapor in between the surface and water it will prevent the heat transfer efficiency.

Song carried out a lot of the research while he was a PhD student at MIT. The researchers say this work is the first to show that the methods can be combined to overcome the tradeoff between the two parameters.

Adding dents to a surface is a way of controlling the way bubbles form on that surface and preventing them from spreading out into a heat-resisting film. The dents were created to prevent film formation. Concentration of bubbles at the surface can be reduced by separation. The team created tiny bumps and ridges at the nanometer scale in order to compensate for that.

The pillars on the material's surface were used to make the cavities. This enhances the boiling process by providing more surface area exposed to the water by combining the pillars with nanostructures. The three tiers of the surface texture give a greatly enhanced efficiency for the boiling process.

He says that the position where bubbles come up is defined by the micro-cavities. The bubbles are separated by 2 millimeters and the coalescence of bubbles is minimized. Liquid is supplied to the bubble base by the capillary action of the pillars. That keeps a layer of water between the boiling surface and the bubbles of vapor.

Although their work has confirmed that the combination of these kinds of surface treatments can work and achieve desired effects, this work was done under small-scale laboratory conditions that could not be scaled up to practical devices. She says that the structures were used to prove that a system can work. The next step is to find alternative ways of making these kinds of surface texture so that they can be scaled up to practical dimensions.

She says showing that we can control the surface is a first step. Next, we need to think about moreScalable approaches. Though the pillars on the surface in these experiments were created using clean room methods, there are other, less demanding ways to create such structures. Some of the ways to produce the surface texture may be more easy to scale.

There may be some significant small-scale applications that could use this process in its current form, such as thermal management of electronic devices, an area that is becoming more important as Semiconductor devices get smaller and managing their heat output becomes ever more important. There is a space where this is important.

Thermal management systems for electronics use liquids other than water so it will take some time to develop. The dimensions of the surface features would have to be adjusted for the different properties of these liquids. One of the next steps is to work on the differences.

Song suggests adjusting the dimensions to account for the different properties of the liquids in order to apply the same multiscale structuring technique to different liquids. He says that the details can be changed.

More information: Youngsup Song et al, Three‐Tier Hierarchical Structures for Extreme Pool Boiling Heat Transfer Performance, Advanced Materials (2022). DOI: 10.1002/adma.202200899 Journal information: Advanced Materials

MIT News ( web.mit.edu/newsoffice/) is a popular site that covers news about MIT.