Superhydrophobic surfaces to boil water without bubbles

Vol 2, Issue 6:  September 2012

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In this article:

  • Interview with Prof Neelesh Patankar.
  • Water can be boiled without producing bubbles exploiting a phenomenon known as the Leidenfrost effect.
  • Applications of this research.

Prof Patankar, your work on superhydrophobic surfaces is amazing. Recently, you have demonstrated that water can be boiled without producing bubbles exploiting a phenomenon known as the Leidenfrost effect. How and when did you come up with this idea?

I want to acknowledge my collaborator Dr Ivan Vakarelski from the King Abdullah University of Science and Technology (KAUST), who led the experiments in this work. I am the theoretician in this work.

Two years ago I had proposed theoretically that if one has the right kind of surface chemistry and the right length scale of surface roughness, then it should be possible to sustain vapour pockets at liquid-solid contact (Patankar, N A Supernucleating surfaces for nucleate boiling and dropwise condensation heat transfer. Soft Matter 6, 1613–1620, 2010).

One of the goals of my research is to obtain vapour adjacent to a solid even without heating. This will have many application, which I shall describe later.

Dr Vakarelski had been independently studying drag reduction (method to increase speed of vehicles by minimising the force that causes them to decelerate) on heated objects with vapour layers formed due to the Leidenfrost effect (Vakarelski, I U et al, Drag reduction by Leidenfrost vapor layers. Physical Review Letters; 106, 214501, 2011) – it’s the same effect that causes a water drop to levitate on a vapour layer and skitter on a hot pan.

However, the Leidenfrost vapour layer is not stable typically below about 200-250°C. Dr Vakarelski was interested in obtaining these vapour layers at lower temperatures so that drag reduction is viable without too much heating.

These interests led to our collaboration, where we show that if we have superhydrophobic surfaces with the right chemistry (hydrophobic) and the right scale of surface roughness (order 1 micron and smaller) then the Leidenfrost vapour film is sustained right up to the boiling point of 100°C (at atmospheric pressure).

Boiling without bubbles, drag reduction using superhydrophobic surfaces. Courtesy of Dr. Ivan Vakarelski
Boiling without bubbles, drag reduction using superhydrophobic surfaces. Courtesy of Dr Ivan Vakarelski
About Prof Neelesh PatankarProfessor and Associate Chair
Dept. of Mechanical Engineering,
Northwestern University, USA
(Click here to know more)

Prof Neelesh Patankar received his BS (B.Tech.) in Mechanical Engineering from the Indian Institute of Technology, Bombay (1993) and his doctorate in Mechanical Engineering from the University of Pennsylvania (1997). He did his post-doctrate at the University of Minnesota until 2000. He received the NSF CAREER award and also won the International Conference on Multiphase Flow’s Junior Award.

Prof Patankar is one of the fifteen selected to the Defense Science Study Group. He is on the Editorial Boards of the Journal of Computational Physics and the ASME Journal of Fluids Engineering. He is also on the Advisory Board of the International Journal of Multiphase Flow.

His two primary areas of specialisation are fast and efficient algorithms for fully resolved simulation (FRS) of immersed bodies in fluids, and roughness-induced superhydrophoicity.

In your experiment you used tiny steel spheres. Was there any specific reason why you chose this material and this specific size and shape?

Steel is easy to heat and cool. This made it easy to do the quenching experiments where we put a hot sphere in water and observe the modes of boiling on its surface as it cools down. In addition, a cooling sphere is one of the classical configurations used historically in this field to study the destabilisation of the Leidenfrost vapour layer.

Could you explain, in brief, what results you got and what were the conclusions for this experiment?

First, let’s understand the Leidenfrost effect that was studied by the German scientist Johann Gottlob Leindenfrost in 1756.

Imagine a hot pan and you put a drop of water on it. If the pan is sufficiently hot then a water-vapour-film is formed between the surface and liquid water. As a result the drop is levitated on top of this film and it skitters. This is the ‘film-boiling’ mode.

If the temperature of the pan is reduced, then the vapour film becomes unstable and the liquid starts to touch the surface directly. In this mode, small vapour bubbles emerge from the liquid-solid contact. This is the ‘nucleate boiling’ mode that we typically see when boiling starts as we heat water in kitchen pots.

Every boiling equipment in the world is designed based on the fact that there are always these modes of boiling. You will find this in every heat transfer textbook. Our result changes this reality.

To understand our results, imagine a hot rough surface whose material is hydrophobic (water-hating) and the roughness scale is on the order of 50nm to 1 micron (that is one-thousandth to one-hundredth the thickness of a strand of hair.) Imagine this surface to be hot and you put a drop of water on it. As said above, a vapour-film will form between the surface and the drop.

As the surface is cooled, the water drop will want to touch the surface, however, this time the liquid ends up resting on top of the roughness peaks, like a fakir lying on a bed of nails. Consequently, the vapour is still maintained in the roughness valleys; the solid-liquid contact is minimal, and vapourisation predominantly continues at the liquid-vapour interface hanging between roughness peaks.

This practically eliminates the ‘nucleate boiling’ phase altogether and the ‘film-type boiling’ mode continues right up to the boiling point. For the first time the explosive transition from the film-wise mode to the nucleate boiling mode is eliminated (see video below).

The above phenomenon is evident in the adjoining image and the video. When a hot sphere with a superhydrophobic coating is immersed in water an explosive transition from a ‘film-boiling’ phase to a ‘nucleate boiling’ phase is completely eliminated. All other surfaces will transit to ‘nucleate boiling’ at some superheat.

In my view, the key outcome of this work is the fact that by tuning the surface chemistry and texture one can dramatically control what phase exists next to a surface. This concept can have fundamental implications on a number of fields.

“I completed my undergraduate degree at a time (1993) when, irrespective of the engineering major, students were migrating to Information Technology as their career path. I didn’t make that change – I could easily have, but I followed my passion for research in my field.”

You had said that this coating material can be used in many vapour-loving, water-hating applications. How do you intend to introduce your research into these applications?

Two specific cases considered in our work are: superhydrophobic (super-water-hating) coatings that stabilise the vapour next to a solid and superhydrophilic (super-water-loving) coatings on which a liquid does not like to form the vapour even when the temperatures are very high.

The thermal properties of the superhydrophobic and superhydrophilic surface coatings are directly relevant to tuning heat transfer rates in heat-exchange devices.

This work is also relevant in the development of the drag reduction technologies in which the superhydrophobic surface sustained vapour layer can reduce the drag on a solid body moving in a liquid.

Our result fits in a very broad and fundamental theme of being able to control the phase.

In line with this, the future research direction is to explore how surface chemistry and textures could be used to manipulate not only liquid-vapour phase transition but also liquid-solid phase change (eg ice formation) and vapour-solid phase change (eg frost formation).

There are many challenges, though. Here is one that is being studied in my group: How to maintain a vapour-film next to a solid at room temperature? If possible, this can have dramatic effects. For example, if you were hypothetically covered with such a coating and were to jump into a swimming pool, you would remain practically dry under water.

There are many applications of this, including robust drag reduction technologies that do not need heating.

Future applications of the ability to control phase are numerous: drag reduction, anti-icing and anti-frosting for aircrafts, heat exchangers involving boiling and condensation, desalination plants, dew collectors to gather precipitation in remote semi-arid areas in the world, environment friendly anti-fouling coatings, among others.

Courtesy of Dr Ivan Vakarelski

Prof Patankar, I can’t think of a single morning where I might not have spilt milk by over-boiling it for my tea. How long would it take for your work to be applicable in real-world uses? Can it be used to control the production of bubbles in other fluids like milk?

The research focus of my group is to enable fundamental technologies – in this case: controlling phase. Bringing this technology to bear upon engineering applications in industry or for consumer products may not be far into the future.

There are however some key challenges: robustness of coatings to wear and extreme temperatures, and the ability to apply such coatings over large surface areas. I believe researchers are on track to overcome these challenges.

You head the Patankar Group for Computation and Theoretical Fluid Dynamics at Northwestern University. Could you tell us about some of your other work?

There are two major areas in my research group at Northwestern University. One area is phase-control based on chemistry and texture. The other area is focused on understanding the neural control of movement as well as of organs (eg esophagus, heart, etc.) We are developing some of the leading high-fidelity neuro-mechanical simulation techniques in the world to enable interrogation of such problems for the first time.

Any last words for mechanical engineers, such as yourself, who are working / studying here in India?

I completed my undergraduate degree at a time (1993) when, irrespective of the engineering major, students were migrating to information technology as their career path. I didn’t make that change – I could easily have, but I followed my passion for research in my field. I hope young mechanical engineers make a similar choice. There are many groundbreaking ideas waiting to be found. Mechanical engineering is rich with interdisciplinary problems that can change the technological landscape and the way we know things.

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