Metasurface design for controlling heterogeneous phase transitions
"We like to find elegant solutions to complex problems" - Neelesh Patankar
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Brief history of the field
In nature, raindrops deposited on superhydrophobic (water-hating) plant leaves tend to "bead up" and roll off the plant surface. In the mid 1990s it was discovered this water-hating phenomena for lotus leaves was due to a double-roughness nano-structure on the surface of the leaves. Water drops sit on top of the roughness peaks, while gas fills the roughness valleys. This allows water to easily move on the surface, removing dirt away from plant leaves in the process. This is known as the lotus effect.
Given the advances in microfabrication technology, there has been a worldwide interest in fabricating artificial surfaces that mimic the lotus effect. It was soon realized that the geometry of the nanoscale roughness greatly influences the ability of a surface to retain the low drag and self-cleaning properties ideal for numerous technological applications. Hence, there was a need to develop a theoretical understanding of this phenomenon.
Our contributions and innovations
I. Theoretical & computational analysis of the lotus effect
We have made fundamental contributions that provided novel insights into the design and synthesis of new materials with superhydrophobic properties. Specifically, we helped answer the following questions:
- Which strategies can be used to make hydrophobic surfaces from hydrophilic materials?
- How to model hysteresis, which is undesirable for low drag applications?
- What type of roughness geometry amplifies the non-wetting properties of surfaces?
- How to ensure that the liquid does not impale the roughness grooves?
The last two questions have also been addressed using molecular dynamics simulation. A cylindrically pored geometry with nanoscale roughness remained practically dry under water at temperatures and pressures below the boiling point of water.
II. Vapor-stabilizing surfaces
The design of roughness-based superhydrophobic surfaces has relied upon (or assumed) the presence of air in roughness grooves. This is problematic for applications where the surface is immersed, as the air can dissolve into the ambient liquid.
We have introduced a new paradigm that is based on designing surface roughness, such that, a liquid in contact with the surface will vaporize by way of capillary evaporation. The vapor pockets formed from the liquid itself may self-lubricate the liquid flow, thus leading to ultra-low drag substrates. The same mechanism of stabilizing the vapor phase can lead to effective nucleation sites for nucleate boiling, leading to the inception of boiling at dramatically low superheats.
In the quest to develop high slip surfaces for liquid flow, we are focusing on modulating surface roughness and also the crystal structure of the material itself. We are interested in understanding the factors that affect molecular scale slip. To that end we have shown for the first time that molecular scale slip can in fact be reproduced using continuum equations if the key mechanisms for slip are added in the continuum model (Hsu & Patankar 2010).
Uniqueness, impact on the field, and outlook
Our work has been widely referenced in industry (e.g. General Electric) and academia alike. It has significantly contributed to the development of low drag and self-cleaning surfaces relevant in numerous applications. Our future work is focused on vapor-stabilizing surfaces. Such surfaces could help overcome decades-old limitations on drag reduction and improvements in heat transfer coefficients in a variety of applications. Our calculations indicate that the vapor-stabilizing surfaces we propose can reduce drag by more than 40% in typical scenarios. In boiling heat transfer, a two-fold improvement in heat transfer coefficients is expected as a consequence of vapor-stabilizing surfaces. This can have a profound impact on energy savings in a variety of industrial applications, such as, chemical process plants, steam turbines, desalination, de-icing, etc.