Measurements of droplet deformation in simple shear flow with zero interfacial tension

The deformation of Newtonian droplets with zero interfacial tension in a Ne wtonian matrix is studied in simple shear flow for viscosity ratios ranging from 0.095 to 10.29. The experiments use a counter-rotating Couette appara tus driven by independent servo motors, with a control scheme that keeps th e droplet centered under a video camera. Droplet deformation is measured by analyzing the video images, using the method of moments. Both the matrix a nd the droplet are silicons oils, and the droplet contains a fluorescent dy e. Low-viscosity droplets stretch monotonically, while higher viscosity dro plets may stretch, wobble, or tumble. The theory of Wetzel and Tucker (2001 ) for ellipsoidal droplets with zero interfacial tension is compared to the data, by adjusting those portions of the initial shape and orientation tha t are not observable in the experiment. The agreement is excellent in all c ases, requiring only that the initial droplet shape be close to an ellipsoi d. (C) 2001 The Society of Rheology.