BUBBLE SLIPPING ON A SUPERHYDROPHOBIC PLANAR STRAIGHT TRAJECTORY UNDER DIFFERENT SURFACE ORIENTATIONS

Bubble directional transportation using the superhydrophobic surfaces of different specific geometry in the water has broad application prospects in the fields of mineral flotation and biological incubation. The surface orientation of the planar straight superhydrophobic surfaces is a crucial parameter for the related engineering structures. However, it is still unclear that the effect of surface orientation on the bubble slipping along the inclined surface. The high-speed shadowgraphy is used to study the movement characteristics of the slipping bubble (D_eq=2.4 mm, Re=500 \sim 700, We=7 \sim 13) on the superhydrophobic linear trajectory with the width of 2 mm under different surface orientations (-90^\circ\leqslant \beta \leqslant 90^\circ) and inclination angles (45^\circ\leqslant \alpha \leqslant 75^\circ). The slipping velocity of the bubble (u) on the trajectory is approximately stable, and the shape like semi-bullet with multi-ridges. The slipping bubble can be divided into two shape types: the stable and the unstable according to the fluctuation level of the gas-liquid interface. Stable bubble only appear when the inclination angle is small and the azimuth angle is large (45^\circ\leqslant \alpha <70^\circ, | \beta | \geqslant 45^\circ). As \alpha changes, two kinds of u-\beta relations can be found: When \alpha \leqslant 65^\circ, the slipping velocity is approximately a unimodal distribution about \beta =0^\circ (the maximum sliding velocity at \beta =0^\circ); When \alpha \geqslant 70^\circ, the azimuth angle has no significant influence on u. The maximum sliding velocity can be upto 0.66 m/s (\beta =0^\circ, \alpha =70^\circ), which is much higher than that of the free-rising bubble of the similar size (\sim0.25 m/s), mainly as a combined effect of the wall-wettability and the inertial force. Surface orientation (\beta) and trajectory inclination angle (\alpha) affect the slipping velocity and the stability of the gas-liquid interface by changing the driving force, as a buoyance component, of the bubble along the trajectory direction and the bubble frontal area.