Superhydrophobic, Drag Reduction & Application in Aviation

Drag is a net force in the opposite direction of flow of a fluid due to pressure and shear forces on the surface of an object. There are several types of drag namely pressure drag, viscous drag, interference drag, lift-induced drag and wave drag. Superhydrophobic is a type of material in which it has the ability to repel water. The name itself shows this property in which hydrophobic means "water hating".

In this case, we will concentrate on the second mentioned drag, the viscous drag. The viscous drag or commonly called friction drag and sometimes also called skin friction drag is a part of drag that is directly related to the shear stress on the object. It is a function of not only the magnitude of the wall shear stress, but also of the orientation of the surface on which it acts. In other words, the viscous drag occurs when there is contact or friction between the particles of the flowing fluid with the surface of the object. That is why it is called the friction drag. The viscous drag creates boundary layer with the thickness of the boundary layer depends on the viscous drag itself.

As the superhydrophobic nano-coating owns ability to repel water, the water molecules will be repelled away from any surface coated with this material. Thus this condition does not allow the water molecules to get in contact with the surface. As a result, the friction between the water molecules and the surface will never occur and friction drag is reduced. Since the superhydrophobic nano-coating just repel water molecules, the friction drag still exist as the other particle in the flowing fluid still able to get in contact with the surface to create friction but the different is, the amount will be smaller.

An experiment conducted by a team of researchers shows positive result. Two discs, one coated with superhydrophobic coatings and the other was left uncoated was allowed to rotate in a water-containing container. The current used to rotate the discs at different speed (Reynolds' number) was considered as parameter to determine the drag. The lesser current means lesser drag. The result also tells us that the percentage reduction in drag is higher for laminar flow (lower Reynolds' number) than turbulent flow.

Source: Experimental Investigation of Viscous Drag Reduction of Superhydrophobic Nano-Coating in Laminar and Turbulent Flows written by Kh.Moaven, M. Rad and M. Taeibi-Rahni.