Vinod K. Sikka

ANTI-ICING SUPERHYDROPHOBIC COATINGS

We use 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 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.

Experimental study of skin friction drag reduction on superhydrophobic flat plates in high Reynolds number boundary layer flow

In this paper, we report the measurement of skin friction drag on superhydrophobic-coated flat plates in high Reynolds (Re) number boundary layer flows, using a high-speed towing tank system. Aluminum flat plates with a large area (4 feet × 2 feet, 3/8 in. thick) and sharpened leading/trailing edges (1 in. long) were prepared as a boundary layer flow model. Spray coating of hydrophobic nanoparticles was applied to make two different types of superhydrophobic coatings: one with low contact angle and high contact angle hysteresis, and the other with high contact angle and low contact angle hysteresis. Skin friction drag of the superhydrophobic plates was measured in the flow speed up to 30 ft/s to cover transition and turbulent flow regimes (105 < ReL < 107), and was compared to that of an uncoated bare aluminum plate. A significant drag reduction was observed on the superhydrophobic plate with high contact angle and low contact angle hysteresis up to ∼30% in transition regime (105 < ReL < 106), which is attributed to the shear-reducing air layer entrapped on the superhydrophobic surface. However, in fully turbulence regime (106 < ReL < 107), an increase of drag was observed, which is ascribed to the morphology of the surface air layer and its depletion by high shear flow. The texture of superhydrophobic coatings led to form a rugged morphology of the entrapped air layer, which would behave like microscale roughness to the liquid flow and offset the drag-reducing effects in the turbulent flow. Moreover, when the superhydrophobic coating became wet due to the removal of air by high shear at the boundary, it would amplify the surface roughness of solid wall and increase the drag in the turbulent flow. The results illustrate that drag reduction is not solely dependent on the superhydrophobicity of a surface (e.g., contact angle and air fraction), but the morphology and stability of the surface air layer are also critical for the effective drag reduction using superhydrophobic surfaces, especially in high Re number turbulent flow regimes.

Measurement of Hydrodynamic Frictional Drag on Superhydrophobic Flat Plates in High Reynolds Number Flows

In this paper, we report the characterization of large-scale superhydrophobic surfaces for hydrodynamic drag reduction in boundary layer flows using a high-speed towing tank system. For making superhydrophobic surfaces, flat aluminum plates (4 ft × 2 ft × 3/8 in, with sharpened leading/trailing edges) were prepared and coated with nano-structured hydrophobic particles. The static and dynamic contact angle measurements indicate that the coated surfaces correspond to a de-wetting (Cassie) state with air retained on the nano-structured surfaces. Hydrodynamic drag of the large-area superhydrophobic plates was measured to cover turbulent flows (water flow speeds up to 30 ft/s, Reynolds number in the range of 105 −107 ) and compared with that of an uncoated bare aluminum control plate. Results show that an acceptable drag reduction was obtained up to ∼30% in the early stage of the turbulent regime which is due to reduced shear forces on the plates because of the lubricating air layer on the surface. However, in a fully developed turbulent flow regime, an increase in drag was measured which is mainly attributed to the amplified surface roughness due to the protrusions of air bubbles formed on the surface. Meanwhile, a qualitative observation suggests that the air bubbles are prone to be depleted during several runs of the high shear-rate flows, as revealed by streak lines of depleted air bubbles. This suggests that the superhydrophobic coating is unstable in maintaining the de-wetted state under dynamic flow conditions and that the increased drag results from the inherent surface roughness of the coating layer where the de-wetted state collapses to a wetted (Wenzel) state due to the depletion of air bubbles. However, it was also observed that the air bubbles would reform on the surface, with the same properties as a dry surface immersed in water, while the plate was kept statically immersed in water for 12 hours, suggesting that the superhydrophobic coating retains static stability for a de-wetted state. The experimental results illustrate that drag reduction is not solely dependent on the superhydrophobicity of a surface (e.g., contact angle and air fraction), but the morphology and stability of the surface air layer are also critical for the design and use of superhydrophobic surfaces for large-scale hydrodynamic drag reduction, especially in turbulent flow regimes.Copyright

Superhydrophobic Coatings on Electronic Components

Superhydrophobic coatings provide exceptional protection to electrical circuits, switches, and other electrical devices which operate in wet environments, such as food processing plants or outdoor applications. Among various electrical device applications, electric motors and electrical switches have been successfully tested in the field at two food processors for nearly 20 months with exceptionally good results. Coated microelectronic circuit board has been in operation without any incidence for over 1 year.