Classical study of motion of a single liquid plug (water) inside a micro/mini capillary tube is revisited to understand the contribution of meniscus friction and effect of contact angle hysteresis on the pressure required to initiate the motion of liquid plug of different L/D ratio. Experiments are carried out by injecting a known mass flow rate of air to push the liquid plug from rest. While the plug is at rest, as the air pressure increases linearly, the menisci deform till a limiting value, at which the plug starts moving. The initial phase of plug motion is dominated by its acceleration, which is then opposed by wall shear, and eventually leads to a steady motion of liquid plug at constant pressure and dynamic contact angle hysteresis.
Experimental results show that, the pressure required to initiate the motion of the liquid is independent of its length, indicating the dominance of static menisci friction at initial stages. Also, steady state pressure drop is observed to be much greater than that predicted by fully developed tube flow.
The flow is simulated in COMSOL using level set method. Simulation results agree well with the experimental result. Contribution of menisci towards static and dynamic friction and velocity development near menisci are obtained from simulations to model the friction factor for the flow.
