Quantifying the Durability of a Friction-Reducing Surface with Recoverable Superhydrophobicity: Fid-Bau Portal

Quantifying the Durability of a Friction-Reducing Surface with Recoverable Superhydrophobicity

Auwerter, Liliane C. C. / Cheeseman, Christopher / Templeton, Michael R. / van Reeuwijk, Maarten

The durability of superhydrophobic surfaces in fully immersed conditions is a major obstacle to their application in engineering applications. We perform an experimental study to measure the friction factor $f_{d}$ as a function of time for a new superhydrophobic surface that is capable of recovering the Cassie-Baxter wetting state. Values of $f_{d}$ were obtained by measuring the pressure drop and volume flux of a turbulent water flow in a 1.5 m long duct containing one superhydrophobic wall. The Reynolds number of the flow was approximately $4.5 \times 10^{4}$ for all experiments. Reductions in $f_{d}$ were 29%–36% relative to a hydraulically smooth surface. The Cassie-Baxter state could be recovered by blowing air through the porous surface for 10 min. The durability of the drag-reduction, as quantified by the relaxation time $T$ in which the surface loses its superhydrophobic characteristics, were measured to be between 10 and 60 min depending on the initial head. The relaxation time $T$ was highly dependent on the pressure difference across the surface. In contrast to models based on Darcy flow through a porous medium, the study indicates that there seems to be a critical pressure difference beyond which the Cassie-Baxter state cannot be sustained for the material under consideration.