當汽水倒入玻璃杯時，杯壁上那些微小的小氣泡會因為接觸角遲滯的影響而黏滯於杯壁上不動。若氣泡要能在表面上移動，則需克服接觸角遲滯所造成的毛細阻力，因此通常自由浮升的氣泡速度，會遠比在杯壁上的氣泡速度來的大很多。本研究透過多體耗散粒子動力學法，模擬於穩態層流系統中，極小氣泡在垂直表面上的運動行為(Reynolds Number，Re ~ O(1))。分別討論表面潤溼性、滑移邊界條件、以及接觸角遲滯的改變對氣泡運動的影響。結果顯示，氣泡的形狀會因流體與固體表面之間的潤濕性質不同而改變。表面潤溼性越差會使氣泡的接觸角減小，即形狀越扁，進而導致參考面積及阻力係數的顯著下降，因此其滑移速度能夠超越自由浮升之氣泡的速度。此外，在固體邊界條件有無滑移，對氣泡移動速度的影響小於10%。接觸角遲滯對氣泡移動有極大的影響，模擬結果顯示，當表面具有粗糙度而產生接觸角遲滯時，於氣泡三相接觸線的位置將會產生毛細阻力進而阻礙氣泡的移動，導致氣泡的移動速度會比其在平滑表面上時顯著變慢。實際上，當外力(即浮力)扣除毛細阻力後，所剩下的作用力才是真正使氣泡移動的有效驅動力。 另外，液滴沿垂直平板運動的結果也和氣泡相同，當液滴潤濕性質越疏水時，液滴會因為和固體接觸面積減少而速度越快。比較體積及形狀相同的液滴和氣泡，發現潤濕性質將主導其速度大小，即氣泡和液滴皆為低接觸角時，氣泡速度較快；反之，兩者皆為高接觸角時，液滴速度較快。表面粗糙度同樣會對液滴產生接觸角遲滯，且對液滴影響大於對氣泡的影響，但在潤濕性質變很差時，液體傾向不潤濕粗糙表面上的孔洞，因此接觸角遲滯趨近於0。 ;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.
