- Researchers discover a method that allows water-repelling surfaces to boil water effectively under certain conditions.
- It makes these surfaces stay cooler than water-attracting surfaces.
- Hydrophobic surfaces can be used to cool high power electronics and nuclear reactors.
Many industrial applications like high-density cooling, water distillation and power generation depend on efficient heat transfer processes that involve boiling. The performance and energy efficiency across several industries could be enhanced by improving these boiling process.
In fact, consumer electronics, including some smartphone and laptop devices, contain water within ‘heat pipe’ materials that dissipate the heat produced by devices and cools them off. Large-scale electronics like those used in aircraft, supercomputers and electric vehicles require even more effective means to deal with overheating issues.
Now, researchers at the Perdue University have found a method that allows water-repelling (hydrophobic) surfaces to boil water effectively under certain conditions. Also, it makes these surfaces stay cooler as compared to water-attracting (hydrophilic) surfaces.
Today, we use hydrophilic surfaces to cool high power electronics and nuclear reactors. No one has considered using hydrophobic surface to enhance to the process of boiling. Since they cannot stay wet, they were previously thought to be inadequate for boiling liquids.
However, the studies done in a few past couple of years show that the hydrophobic surfaces are capable of boiling water at lower temperatures than hydrophilic surfaces. But scientists had not yet achieved any practical model. So how did they make it this time? Let’s find out.
The Key Step
Researchers realized that if they could remove all air and vapor from the hydrophobic surface before boiling the liquid, they could achieve the best of both hydrophilic and hydrophobic behaviors simultaneously.
Courtesy of researchers
Boiling from hydrophobic surface at initial phase [called Wenzel state] — where texture of the surface is invaded with liquid — leads to significantly low surface heat. They showed 2 different boiling behaviors on micro- as well as nano-structured surfaces based on its initial state.
For a specific initial surface condition [called Cassie-Baxter state] — where gaps of surface texture are occupied by vapor — premature film boiling is observed.
A significantly enhanced thermal performance is recorded when the texture of the surface is invaded with liquid [Wenzel state] before boiling. In this state, 3-phase contact line is pinned during the growth of vapor bubbles. This prevents the vapor film formation over the hydrophobic surface, maintaining effective nucleate boiling behavior.
More specifically, this enables the micro-structured hydrophobic surface to function in the nucleate boiling regime up to a critical heat flux of 115 Watts per centimeter square, which is comparable to typical hydrophilic wetting surfaces.
Bottom line: boiling is astonishingly efficient on hydrophobic surfaces functioning in this mode. A nucleate boiling mode is maintained on a surface up to an extreme critical heat flux on the order of the classical Zuber Limit. It can be fully utilized in practical applications that require high heat transfer coefficients.