With superhydrophobic properties being extended to a variety of metallic substrates through the process of ablation due to femto-second laser surface processing (FLSP), it is important to understand the hydrodynamic benefits of such a material, as well as its resiliency. This research focuses on the skin friction drag effects of a superhydrophobic flat plate compared to an untreated flat plate of the same material and geometry. The resiliency of this material will also be tested through the use of an accelerated corrosion fog chamber using both treated and untreated aluminum samples.

Figure 1. Self-Cleaning Action on a Superhydrophobic Surface.

A material is said to be superhydrophobic if the equilibrium contact angle of a water droplet is greater than 150 degrees and the contact hysteresis angle is less than 10 degrees. The idea of extending superhydrophobic properties to a range of materials was first inspired by observing the water repelling and self-cleaning effects of the lotus leaf and a number of other leaves found in nature. Due to the large contact angle, the water droplets center of mass is moved further above the surface causing the droplets to have a rolling action rather than a sliding action. This, combined with the more uniform surface tension of spherical geometry, allows particles to become trapped in the droplet and carried away as seen in Figure 1.

Figure 2. Cassie-Baxter State

Another important parameter of the superhydrophobic condition, and the primary area of focus for this study, comes from understanding how, in the Cassie state, a material can attain large enough contact angles to be considered superhydrophobic. When looking at the microscale roughness of a surface, if the distance between peaks is such that the static pressure of the water is not capable of overcoming the surface tension of the droplet, the valley will not become wetted. This results in an air-water interface at the material surface.

Figure 3. Hierarchical Surface Structure

It can be seen from Figure 2, that in the Cassie state the equilibrium contact angle is a result of the proportion to the air-water interface. Revisiting the Lotus leaf effect, when the microscale peaks are combined with nanoscale features, a hierarchical structure is created and the Cassie state is improved by increasing the proportion of the air-water interface and thus increasing the equilibrium contact angle, thereby creating a surface that is near perfectly superhydrophobic, as seen in Figure 3. It is this air-water interface that is of particular interest to researchers and engineers, because of the potential to change fluid-surface interaction by the addition of an air film separation between the surface and the water, therefore altering the hydrodynamic properties.

This work was done by James R. Ley for the Naval Postgraduate School. NPS-0002

This Brief includes a Technical Support Package (TSP).
Hydrodynamic Drag Force Measurement of a Functionalized Surface Exhibiting Superhydrophobic Properties

(reference NPS-0002) is currently available for download from the TSP library.

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