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Hydrodynamic Drag Force Measurement of a Functionalized Surface Exhibiting Superhydrophobic Properties

Comparing the skin friction drag effects of a superhydrophobic flat plate to an untreated flat plate of the same material and geometry.

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

Topics:
Aerodynamics Drag Education and training Emissions Exhaust emissions Materials Particulate matter (PM) People and personalities Water
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Hydrodynamic Drag Force Measurement of a Functionalized Surface Exhibiting Superhydrophobic Properties

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    Aerospace & Defense Technology Magazine

    This article first appeared in the September, 2017 issue of Aerospace & Defense Technology Magazine.

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    Overview

    The document is a thesis authored by James R. Ley at the Naval Postgraduate School, completed in December 2016. It focuses on the development and analysis of superhydrophobic surfaces created through femto-second laser processing. The primary objective of the research is to investigate how these specially engineered surfaces can significantly reduce hydrodynamic drag forces when compared to untreated surfaces.

    Superhydrophobic surfaces are characterized by their ability to repel water, which is achieved through specific micro- and nano-structuring that enhances their water-repellent properties. The thesis explores the mechanisms behind the creation of these surfaces using advanced laser techniques, which allow for precise control over the surface texture and chemical properties.

    The research includes experimental studies that measure the drag reduction capabilities of the superhydrophobic surfaces in various fluid dynamics scenarios. The findings indicate that these surfaces can lead to substantial reductions in drag, which has significant implications for various applications, including marine vessels, pipelines, and other fluid transport systems. By minimizing drag, these surfaces can enhance energy efficiency and performance, leading to potential cost savings and improved operational effectiveness.

    In addition to drag reduction, the thesis also addresses the durability of the superhydrophobic surfaces. It examines how these surfaces withstand environmental factors and mechanical wear over time, which is crucial for their practical application in real-world scenarios. The study provides insights into the longevity and reliability of the laser-processed surfaces, ensuring that they maintain their hydrophobic properties under operational conditions.

    Overall, Ley's thesis contributes to the understanding of superhydrophobic technology and its potential applications in reducing drag in fluid systems. The research highlights the innovative use of femto-second laser processing as a viable method for creating durable and effective superhydrophobic surfaces, paving the way for future advancements in material science and engineering. The findings underscore the importance of continued exploration in this field to harness the benefits of superhydrophobicity for various industrial applications.

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