BUOYANCY-DRIVEN MOTION OF DROPS AND BUBBLES IN A PERIODICALLY CONSTRICTED CAPILLARY

Buoyancy-driven motion of viscous drops and air bubbles through a vert ical capillary with periodic constrictions is studied. Experimental me asurements of the average rise velocity of buoyant drops are reported for a range of drop sizes in a variety of two-phase systems. The insta ntaneous drop shapes at various axial positions within the capillary a re also quantitatively characterized using digital image analysis. Per iodic corrugations of the capillary wall are found to have a substanti al retarding effect on the mobility of drops in comparison with previo us experimental results in a straight cylindrical capillary. For syste ms characterized by small Bond numbers, drop deformations are found to be periodic. In large Bond number systems, however, drop breakup even tually occurs as the drop size is increased beyond a critical limit. T he observed mode of breakup is a tail-pinching process similar to that observed by Olbricht and Leal (1983) for pressure-driven motion of lo w viscosity ratio drops through a sinusoidally constricted capillary. In contrast to their results, however, the same mode of breakup was al so observed for systems with O(1) viscosity ratios.