Flow Field Measurements Near a Moving Meniscus of a Capillary Flow With Micro Particle Image Velocimetry (µPIV)

Due to the relevance of capillary flows for industrial applications, the dynamics of menisci have been extensively investigated. Contrary to the meniscus velocity, the flow pattern near the meniscus is rather unexplored. The main focus of this study is the visualization of the flow field near the fast moving meniscus with micro particle image velocimetry (μPIV). Therefore an invasive flow of ethanol in a vertical cylindrical glass tube is investigated. The entrance of the tube is located inside a reservoir with a rising liquid surface. Especially near the tube entrance the flow is high dynamical (ca. 120 mm/s) which necessitates high temporal resolution measurement techniques. Flow field measurements at several positions in flow direction have been conducted to compare the meniscus shape and the flow fields near the three phase contact line. The flow pattern near the meniscus is extremely complex. While the typical Hagen-Poiseuille profile with no radial velocity component is to be found far from the meniscus, the flow near the meniscus cannot be a convex velocity profile. As a matter of fact the meniscus movement is characterized by adsorption of the liquid near the wall and liquid transport from the middle of the flow to the three phase contact line. This special flow pattern has been examined. Furthermore it was found that the shape of the meniscus varies over the whole invasion of the tube. The curvature of the meniscus and therefore the contact angle between solid and liquid are getting smaller over the infiltration length which has influence on the capillary force. Measurements near the entrance of the tube show an acceleration of the liquid column until it reaches a certain speed and after that a deceleration. To determine the velocity of the raising meniscus, global measurements of the whole tube have been performed.