Influence of shear and elongation on drop deformation in convergent-divergent flows

The effects of shear and elongation on drop deformation are examined throug h numerical simulation and: experiment. A two-dimensional formulation withi n the scope of the boundary element method (BEM) is proposed for a drop mov ing under the influence of an ambient flow inside a channel of a general sh ape, with emphasis on a convergent-divergent channel. Both the drop and the suspending fluid can be either Newtonian or viscoelastic of the Maxwell ty pe. The predicted planar deformation is found-to provide accurate descripti on of the physical reality, For example, small drops, flowing on the axis, elongate in the convergent part of the channel, then regain their circular form in the divergent part, confirming the experimental observations. Drops placed off-axis are found to rotate during the flow. These drops thus have longer residence time as well as larger and irreversible deformation than those moving on the axis. Both theory and experiment show a difference in d eformability for Newtonian and viscoelastic drops in a slit flow. Initially , a Newtonian drop is reluctant to deform, but then deformation is rapid. A viscoelastic drop initially deforms readily, but then the deformation slow s down. The slit flow does not flatten drops whose diameter is at least 10 times smaller than the slit gap. The effects of shear and elongation stress , the viscosity ratio, the drop diameter-to-channel-gap ratio, the initial drop position, the interfacial tension, and elasticity of the dispersed and ambient phases were examined using the BEM. (C) 1999 Elsevier Science Ltd. All rights reserved.