Implications of Contact Line Dynamics on Taylor Bubble Flow Morphology

Film deposition experiments are performed in circular glass capillaries of 500 μm diameter. Two surface wettabilities are considered, contact angle of 30° for water on glass and of 105° when a hydrophobic coating is applied. It was observed that the liquid film deposited as the meniscus translates with a velocity U presents a ridge that also moves in the direction of the flow. The ridge is bounded by a contact line moving at a velocity U_CL as well as a front of velocity U_F, and it translates over the deposited stagnant film. The behavior of the ridge presents striking dissimilarities when the wettability is changed. Both U_CL and U_F are approximately twice as large for the non-wetting case at the same capillary number Ca. The Taylor bubbles forming due to the growth of the ridge are also differentiated by wettability, being much shorter in the non-wetting case. The dynamics of the contact line is studied experimentally and a criterion is proposed to explain the occurrence of a shock at the advancing front of the ridge. The hydraulic jump cannot be explained by the Froude condition of shock formation in shallow waters, or by an inertial dewetting of the deposited film. For a dynamic contact angle of θ_d = 6° and according to the proposed criterion, a hydraulic jump forms at the front of the ridge when a critical velocity is reached.