LAUSR

Abstract The effect of imposed flow oscillations on the local hydrodynamics of a completely wetting isolated liquid plug (of silicone oil) placed inside a horizontal square glass capillary tube (1 mm × 1 mm) is studied. A known volume of liquid (plug with a given L/D ratio) is made to harmonically oscillate inside the capillary tube by the action of air pressure on one of its menisci, while the other meniscus interacts freely with the atmosphere. The effect of plug length, imposed oscillation frequency and amplitude, on the net pressure drop and menisci shapes in terms of normalized radii of curvature, is addressed. A hydrodynamic model, based on Bretherton's theory, coupled with Tanner's law is reviewed and simplified for oscillatory flows, which shows excellent correlation with the experimental data, via a single universal fitting parameter. This parameter manifests the combined effect of wall shear stress and additional dissipation due to the dynamically deforming menisci. During oscillations of the liquid plug, the contact lines of both menisci stay pinned and the plug slides over a thin film, exhibiting dynamic hysteresis at both the respective menisci. The system parameter that governs the hydrodynamic of oscillating liquid plug is the instantaneous capillary number, which depends on both, the imposed frequency and amplitude, the former having an overbearing effect on the dynamic menisci shapes. The study clearly brings forward the fact that capillary induced dissipation at the menisci is much more prominent than the wall induced shear.