不同壁面取向下超疏水平面直轨道上的气泡滑移

利用特定几何分布的超疏水表面实现气泡定向输运在矿物浮选和生物孵化等领域具有广阔的应用前景, 对平面直线超疏水轨道而言, 其壁面取向是相关工程结构的关键参数, 但超疏水壁面取向对倾斜壁面气泡滑移的影响尚不明确. 本文采用高速阴影成像系统研究了不同壁面取向($-90^\circ\leqslant \beta \leqslant 90^\circ$)及轨道倾角($45^\circ\leqslant \alpha \leqslant 75^\circ$)下, 气泡($D_{eq}=2.4$ mm, $Re=500$ $\sim$ 700, $We=7$ $\sim$ 13)在轨道宽度为2 mm的超疏水直线轨道上的运动特性. 气泡在轨道上的滑移近似为匀速, 形状为具有多脊的半子弹型. 根据气液界面波动程度的不同, 滑移气泡可分为波动型和稳定型, 稳定型气泡只在较小倾角且较大方位角时出现($45^\circ\leqslant \alpha < 70^\circ$, $| \beta | \geqslant 45^\circ$). 根据倾角不同, 滑移速度关于$\beta $有2种变化规律: 当$\alpha \leqslant 65^\circ$, 气泡滑移速度近似为关于$\beta =0^\circ$ 的单峰分布($\beta =0^\circ$时, 气泡滑移速度最大); 当$\alpha \geqslant 70^\circ$, 气泡滑移速度在不同的方位角下基本保持稳定. 气泡的最大滑移速度可达0.66 m/s ($\beta =0^\circ$, $\alpha =70^\circ$), 远大于相同尺度的自由上升气泡($\approx0.25$ m/s), 这主要是壁面浸润性分布和惯性力的耦合效应所致. 轨道取向(方位角$\beta )$及轨道倾角($\alpha )$通过改变气泡沿轨道方向的驱动力和气泡迎风面积影响气泡的滑移速度和气液界面稳定性. 

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 ($\sim$0.25 m/s), mainly as a combined effect of the wall-wettability and the inertial force. Surface orientation ($\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.