Bubbly drag reduction using a superhydrophobic inner cylinder in Taylor-Couette turbulence

We investigate the drag of a highly turbulent flow over a non-wetting surface of micro-scale roughness. The Taylor-Couette geometry is used, allowing accurate drag and flow measurements. The inner cylinder is coated with a rough, hydrophobic material, whereas the outer cylinder is kept smooth. We vary the void fraction of air α present in the working fluid to introduce bubbles to the flow. For smaller volume fractions of air, up to α ≤ 2%, we observe that the increased surface roughness from the coating increases the drag. For larger fractions of air, α > 2%, the drag decreases compared to a smooth hydrophilic, uncoated cylinder using the same volume fraction of air. This suggests that two mechanisms play a role: the roughness invokes a shift in the log-layer – resulting in an increase in drag – and the more effective drag-reducing mechanism of the superhydrophobic surface. The balance between these two effects determines whether bubble drag reduction is more effective when using a superhydrophobic surface compared to using a smooth hydrophilic surface.