No abstract

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.

In this work, we report on a systematic fluidodynamic investigation of red blood cell (RBC) suspensions flowing in microcapillaries with diameters comparable to the cell size in vitro. By using high-speed video microscopy and image analysis, we provide the first simultaneous determination of both cell velocity and shape parameters related to RBC membrane deformability over an extended range of pressure drops, and the first quantitative comparison with theoretical results from the literature. Good agreement was found with the predicted axisymmetric shapes tending towards an apparent bullet-like asymptotic configuration at increasing cell velocity. A potential application of this work is in the design of flow-based devices to study the pathological conditions affecting cell deformability, thus allowing us to overcome the limits of classical static methods, such as micropipette aspiration, which are not suitable for handling a large number of cells.

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.