| 摘要(英) |
Tiny bubbles readily stick onto substrates due to contact angle hysteresis (CAH). A tiny bubble can slide slowly on a surface with ultralow CAH once buoyancy overcomes pinning force. In this study, the sliding motion of bubbles at Re~O(1) is investigated by many-body dissipative particle dynamics simulation. The influence of the bubble shape is studied by varying the fluid-solid wettability. As the wettability is reduced, the bubble shape becomes more flat and the sliding velocity grows accordingly. The sliding bubble on a CAH-free surface can rise faster than a freely rising one owing to the decrease of both the frontal area and drag coefficient. Moreover, less than 10% reduction of the sliding velocity is obtained if the slip boundary condition is replaced with the no-slip boundary condition. The CAH induced by surface roughness results in the adhered bubble. As a result, the velocity of the bubble on the rough surface is much slower than that on the CAH-free smooth surface. Actually, the effectively driving force for the bubble motion is the difference between the buoyancy and the pinning force originated from the CAH.
In addition, similar phenomena are observed for the motion of the sliding drop motion on the vertical surface. It shows that the sliding velocity for the drop on the hydrophobic surface is faster than that on the hydrophilic surface due to the small contact area between solid and liquid interface. Comparing the bubble and liquid drop of the same shape, it is found that when both contact angle are small, the sliding velocity of the bubble is faster than that of the drop due to the hydrophobicity of the bubble. In contrast, as both contact angle are large, the sliding velocity of the drop is faster than that of the bubble because of the hydrophobicity of the drop. Moreover, the CAH induced by surface roughness has a strong effect on the drop motion. However, as liquid wettability becomes poorer, liquid beads tend not to impregnate the surface grooves. Thus, CAH induced by surface roughness is absent. |
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