Low-pressure driven displacement of gas-fluid-gas plugs in a capillary tube

This talk focuses on experiments conducted to study the effects of low-pressure driven displacement of liquid plugs in a capillary tube. Experiments were performed in a capillary tube (diameter $\approx$ 800 $\mu$m) by displacing liquid plugs containing aqueous glycerol solution using pressurized air with a range of 0.02 psig $\leq$ $P$ $\leq$ 0.1 psig (at an increment of 0.01 psig). Two CCD cameras placed in front of the set-up captured the displaced and displacing fluid interfaces simultaneously. At these low pressures, we categorized the flow behavior as following: $1)$ At the lowest pressure, displacement of fluid plug becomes stationary after a certain time, $2)$ With further increase in pressure, the residual film of the displaced fluid deforms into drops along the tube walls, and $3)$ At the highest pressure, a flat and thin film was left behind by the displaced fluid. Subsequently, we measure the fluid fraction left in the tube using the expression $m=1-U_m/U_t$, where the mean ($U_m$) and tip ($U_t$) velocities were measured by analyzing the experimental data using an in-house MATLAB code. We report the $m$ versus $Ca$ trends observed in our experiments and compare them against classical results for immiscible fluid displacement.