Hydrophobicity and superhydrophobicity with self-cleaning properties are well-known characteristics of several natural surfaces, such as the leaves of the sacred lotus plant (Nelumbo nucifera). To achieve a superhydrophobic state, micro- and nanometer scale topography should be realized on a low surface energy material, or a low surface energy coating should be deposited on top of the micro-nano topography if the material is inherently hydrophilic. Tailoring the surface chemistry and topography to control the wetting properties between extreme wetting states enables a palette of functionalities, such as self-cleaning, antifogging, anti-biofouling etc. A variety of surface topographies have been realized in polymers, ceramics, and metals. Metallic surfaces are particularly important in several engineering applications (e.g., naval, aircrafts, buildings, automobile) and their transformation to superhydrophobic can provide additional functionalities, such as corrosion protection, drag reduction, and anti-icing properties. This review paper focuses on the recent advances on superhydrophobic metals and alloys which can be applicable in real life applications and aims to provide an overview of the most promising methods to achieve sustainable superhydrophobicity.

Superhydrophobic surfaces are extensively investigated in the literature, yet the phenomenon of drop motion on such surfaces and the corresponding friction properties of surfaces with different topography are not sufficiently analyzed. Here, drop motion on hydrophobic and superhydrophobic surfaces with different size topography is investigated for drops of largely varying viscosity (i.e., water and glycerol). The threshold force required to initiate drop movement is probed, the drop motion (velocity and acceleration) is analyzed, and the friction force on each surface is calculated. It is evident that as roughness increases, the threshold force to initiate 20 µL drop motion decreases; the lowest value for water is 17.9 ± 4.0 µN. For glycerol, the lowest threshold force value is 22.3 ± 5.9 µN. The results also indicate that this threshold force required for the initiation of the drop motion seems to be higher than that when the drop starts moving. Finally, this force (being proportional to the contact line) is expected to be about half smaller for 5 µL droplets. Water drops obtain higher velocities and accelerations by an order of magnitude compared to glycerol drops, which is attributed to the combinational effect of the higher hysteresis and the larger contact line of glycerol drops.

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