By Laurent Chiron, Ing. Dr. Numéricien

Following the  series of articles on surface tension and wettability, this article focuses on super-hydrophobic surfaces :

Non-wettable surfaces with high water contact angles and sliding of drops are called superhydrophobic. On such material, the contact angles of a droplet exceed 150°. Thus, only 3 % of the surface of a droplet of typical size is in contact with the surface. In everyday life, such a phenomenon can be observed on lotus leaves, which later gave the name “lotus effect” to designate such phenomena. The lotus effect gives the surface self-cleaning capabilities: as the water drops run off, it carries away the dust.

In the industry, the self-cleaning capability has been successfully used for sensors of traffic control units on German autobahns. Swiss companies have developed stain-resistant textiles, allowing tomato sauce, coffee and red wine to be easily washed away even after a few washes.

In addition to its self-cleaning capabilities, superhydrophobic surfaces are important in other areas such as aircraft anti-icing (reduce the amount of ice on the wings), spray cooling and ink-jet printing.

In practice, to industrially manufacture a hydrophobic material, its surface is engraved with nano or micrometric patterns by a powerful femtosecond pulse laser. Such process suffers from the manufacturing time: etching one squared centimeter of metal surface may take hours. During a design optimization process, when dozens of surfaces must be etched and compared, Computational Fluid Dynamics (CFD) becomes an asset to reduce costs and delay.

The Lagrangian and multi-physics features of Smoothed Particle Hydrodynamics (SPH) make this method suitable to handle such use-case. Hereafter, the computation of a drop impacting a superhydrophobic elastic beam is presented. Such fluid-structure interaction problem has been solved through a strong coupling between the SPH-flow fluid solver and the ABAQUS FEM solver. Thanks to the high-fidelity surface tension model of the SPH software, only the drop liquid is modeled and discretized with particles: unlike most meshed methods (VOF…), the surrounding air is not modeled, which reduces the computation cost.

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Fig. : Droplet impact on superhydrophobic elastic beam: experiments from [1] (left) and SPH-flow/ABAQUS coupled simulation (right)

[1] Weisensee, P., Tian, J., Miljkovic, N. et al. Water droplet impact on elastic superhydrophobic surfaces. Sci Rep 6, 30328 (2016). !