Effect of groove orientation on drag reduction of superhydrophobic surfaces

This paper presents the experimental results of drag reduction on superhydrophobic surfaces with millimetric-scale grooves of varying orientations. Experiments were conducted by allowing torpedo-shaped test samples to free fall into a pool of glycerin under laminar flow conditions. Under experimental conditions, the Reynolds number ranged from 0.5 to 8.0, and capillary number ranged from 0.4 to 6.0. At low Reynolds numbers, all the surfaces maintained the Cassie-Baxter state during motion, trapping air within micro/nano-structures. As the Reynolds number increased, a transition to the Wenzel state occurred, in which the liquid penetrated the microgrooves. Groove orientation critically influences air-layer stability, with longitudinal grooves (parallel to flow) retaining air layers more effectively than transverse grooves owing to reduced shear stress. Experiments confirm this hypothesis by demonstrating a maximum drag reduction of 26.39% for longitudinal grooves at Re = 0.5, surpassing prior studies showing an 8% reduction. A critical threshold is identified at Re = 1.4 and Ca = 1, beyond which viscous forces become dominant, leading to the collapse of air layers.