(133d) Elastic and Surfactant Effects on Bubbles Rising in a Rectangular Capillary

Buoyancy-driven motion of an air bubble in a capillary filled with an immiscible liquid is experimentally studied. When the bulk fluid is Newtonian, the small bubbles are nearly spherical. As the bubble volume increases, the bubbles become prolate with fore and aft symmetry. The terminal velocity also increases as the bubble volume increases. As the bubble size becomes comparable to the capillary size, the terminal velocity reduces and for very large bubble volumes, the bubble velocity is independent of the bubble size. As the Bond number for the flow is increased, the bubble loses its fore and aft symmetry and develops a negative curvature at the rear of the bubble. The effect of elasticity in the bulk fluid phase under buoyancy-driven motion is also considered. In the presence of elasticity in the bulk phase, a cusp develops at the trailing end of the bubble. The increased mobility of bubbles observed in Newtonian fluid is not seen for bubbles translating in the elastic fluids. The bubble migration velocity increases as the bubble size increases and reaches a plateau for large bubble volumes. The effect of surfactants on the dynamics of bubbles translating in square capillaries is also studied. At very low surfactant concentrations, the equilibrium surface tension is not affected by the presence of surfactants but the migration speed of the bubbles is reduced.