The three-dimensional deformation of an isolated drop in an immiscible liquid phase undergoing simple shear flow was investigated by using a parallel-plate apparatus. The drop was observed by video-enhanced contrast optical microscopy, either along the vorticity direction or along the velocity gradient direction of the shear flow. An experimental methodology based on image analysis was especially developed to study in a quantitative way the three-dimensional shape of the deformed drop, both under steady-state flow and during transients. Up to moderate deformations, the steady-state drop shape was well described within experimental error by an ellipsoid having three different axes. The deviation of drop shape from an ellipsoid at higher deformations was also characterized in a quantitative way. Good agreement was found between the experimental results of this work and numerical simulations reported in the literature.

The deformation and breakup of a drop in an immiscible equiviscous liquid undergoing unbounded shear flow has been extensively investigated in the literature, starting from the pioneering work of Taylor. In this Letter, we address the case of microconfined shear flow, a problem which is relevant for microfluidics and emulsion processing applications. The main effects of confinement include complex oscillating transients and drop stabilization against breakup. In particular, very elongated drop shapes are observed, which would be unstable in the unbounded case and can be explained in terms of wall-induced distortion of the shear flow field. We show that wall effects can be exploited to obtain nearly monodisperse emulsions in microconfined shear flow.

The collision of two equi-sized drops immersed in an immiscible liquid phase undergoing a shear flow in a parallel plate apparatus has been investigated over a range of capillary numbers. The drops were observed along the vorticity direction of shear flow by video enhanced contrast optical microscopy. Images of the colliding drops were processed by image analysis techniques. The distance Δy between the drop centres along the velocity gradient direction was measured as a function of time during approach, collision and separation of the two drops. It was found that Δy increases irreversibly after collision, thus providing a mechanism for drop dispersion in a concentrated system. Drop shape evolution during collision was characterized by measuring a deformation parameter and the angle made by the drop major axis with respect to the velocity gradient direction. The extent of the near-contact region when the drops are sliding on each other was also estimated. Coalescence was a rare event and was observed in the extensional quadrant of the shear flow. The experimental results show good agreement with numerical simulations recently reported in the literature.