The research objective of this award is to study condensation, surface wetting transitions and ice formation at superhydrophobic surfaces through a synergetic combination of experimental and theoretical approaches. Superhydrophobic surfaces are promising for a wide variety of interesting applications. Up to date, most published research on superhydrophobicity is focused on the effects of surface topology on water contact angle by trapping macroscopic air pockets at geometrically heterogeneous surfaces. While the extraordinary water repellency of a superhydrophobic surface can be successfully described by various modifications of Young?s equations, the macroscopic approach is insufficient to describe surface phase transitions affiliated with broader applications of superhydrophobic surfaces including anti-fogging and/or anti-icing. Such broader applications hinge on a better understanding of the surface hydrophobicity that depends on the microscopic details of the substrate and different states of water at inhomogeneous conditions. By bringing together complementary expertise, the research team will (i) establish a theoretical framework to describe vapor deposition, surface wetting transition and ice nucleation at superhydrophobic surfaces, (ii) develop practical schemes for fabrication of superhydrophobic surfaces with systematically tailored surface morphologies and chemical properties, (iii) conduct real-time study of condensation and icing on superhydrophobic surfaces using custom-designed experimental systems.
This research grant will contribute to the fundamental knowledge base of superhydrophobic surfaces in contact with different states of water. The project will provide graduate and undergraduate students, particularly the underrepresented minorities, with opportunities to get first-hand experience on cutting-edge research. The students will particularly benefit from exposure to both theory and experiments. The research team will promote outreach to high school and exposure of underrepresented minority pre-college students to science and innovations through (i) development of high-school course unit and "Sustainability Innovation" hands-on science workshops at Pitt and (ii) the Mathematics, Engineering, and Science Achievement Schools Program at UCR.
(i) We have used nanoparticle-polymer composites to demonstrate the anti-icing capability of superhydrophobic surfaces and report direct experimental evidence that such surfaces are able to prevent ice formation upon impact of supercooled water both in laboratory conditions and in natural environments. We find that the anti-icing capability of these composites depends not only on their superhydrophobicity but also on the size of the particles exposed on the surface. The critical particle sizes that determine the superhydrophobicity and the anti-icing property, respectively, are in two different length scales. The effect of particle size on ice formation is explained by using a classical heterogeneous nucleation theory. This result implies that the anti-icing property of a surface is not directly correlated with the superhydrophobicity and thus it is uncertain whether a superhydrophobic surface is anti-icing without detailed knowledge of the surface morphology. The result also opens up possibilities for rational design of anti-icing superhydrophobic surfaces by tuning surface textures in multiple length scales. (ii) We have studied interaction of vapor water with superhydrophobic surfaces, and found that although super repellency to liquid water may be obtained through many approaches, most superhydrophobic surfaces become hydrophilic in an environment where condensation of water vapor occurs. We fabricated a series of superhydrophobic surfaces made of aligned nanowires and studied state of condensed water droplets on these surfaces. We found that the roughness factor of the nanowire arrays plays an important role in determining whether the condensed water droplets may slip off the surface. A much higher roughness factor is required for condensed water droplets to slip off the surface than for dropped water droplets. (iii) We have studied icing on superhydrophobic surfaces when condensation occurs before or with icing, and found that the superhydrophobicity of most superhydrophobic surfaces was lost and ice adheres strongly to these surfaces. However, superhydrophobic surfaces made of high aspect ratio straight pores with their diameter in the range of 200-400 nm presents a very low ice adhesion strength even when condensation occurs, which provides us with an opportunity to make ice-release coatings for practical applications.
