Motion of viscous drops in tubes filled with yield stress fluid

The dynamics of deformable viscous drops in yield stress medium is investigated experimentally. Tetrachloroethylene drops settle in a cylindrical tube filled with neutralized Carbopol solution under the action of gravity. For a single fluid particle, stopping conditions, established velocities and deformation patterns are examined for a variety of the physical parameters of the continuous medium determined by Carbopol concentration and the geometry of the system. For a pair of drops, we present time evolution of separation distance and of the shapes of the drops. It was found that if the initial separation between equal size drops does not exceed some critical value, the trailing particle moves with a considerably higher velocity than that of the leading one and eventually the two drops coalesce. Afterwards, a single large fluid particle is formed and it sometimes breaks up forming one large leading drop and one or several small trailing satellites. This behavior is qualitatively similar to that obtained previously in numerical simulations. (C) 2009 Elsevier Ltd. All rights reserved.