Browsing by Author "Goveas, Jacqueline L."

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    Drop breakup in dilute Newtonian emulsions under steady shear
    (2004) Zhao, Xinyu; Goveas, Jacqueline L.
    High-speed video microscopy has been used to study drop breakup in dilute Newtonian emulsions under steady shear. Fundamental experimental studies on drop breakup have been limited to breakup in quiescent matrix or under pseudo-equilibrium conditions. This thesis represents the first direct visualization of drop breakup under steady shear at high capillary numbers (Ca). The mechanisms of drop breakup depend on Ca and the viscosity ratio (lambda). At Ca ∼ Cac , drops are broken up via necking. At Ca < 2Cac, drop breakup is caused by end pinching. At Ca > 2Cac, the capillary instability is the dominant breakup mechanism. For Ca > 2Cac, breakup dynamics are strongly controlled by lambda. For 0.1 < lambda < 1, drops with different initial sizes deform into threads with the same radius at breakup. The wavelength of the capillary instability is uniform along the length of a thread and from thread to thread. Fairly monodisperse dilute emulsions are obtained due to this size selection mechanism, with the average drop size being inversely proportional to the shear rate. For 1 < lambda < 3.5, the breakup mechanism is similar to that for 0.1 < lambda < 1.0, except that the satellite drops are substantially larger, resulting in polydisperse emulsions. For lambda < 0.1, the daughter drops are formed from long wavelength capillary instability and may break again. This induces collisions between drops, which in turn results in irregular drop re-breaking and coalescence, producing polydisperse emulsions. This re-breaking mechanism has not been observed in previous studies in the literature. Drops reach a pseudo-steady state before the capillary instability starts to grow. At this pseudo-steady state, the shear stress and the capillary pressure almost balance each other, determining a definite thread radius, which is independent of the initial drop size. We define a dimensionless thread number as the ratio of the two forces. The thread number is only a function of lambda, and shows a minimum in lambda. The measured thread number is in agreement with the slender body theory of Hinch and Acrivos (1980). Drops deform pseudo-affinely for 0.1 < lambda < 1.0, but deformation deviates from being pseudo-affine otherwise.
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    The role of dispersed phase concentration in the deformation and breakup of oil-in-water emulsions under flow
    (2004) Marcu, Cosmin Gavril; Goveas, Jacqueline L.
    This thesis investigates the role of dispersed phase concentration on the drop deformation and breakup process of oil-in-water emulsions under flow. This work is motivated by two earlier studies: one by Mason & Bibette [4], on concentrated viscoelastic emulsions, which became monodisperse under the application of a simple shear flow, and a second, by Aronson [5], where a similar emulsion became monodisperse in a complex mixing flow. In the first part of the thesis, we study the effect of changing surfactant concentration, dispersed phase concentration and flow rate on the Aronson emulsion and two other oil-in-water emulsions. We find that emulsions in a mixing flow only become monodisperse around the close packing oil concentration of 70%. Increasing mixer speed and surfactant concentration decreases the polydispersity. Final mean drop sizes follow the same trend as the polydispersity. Qualitatively similar results are obtained by subjecting the emulsions to a simple shear flow. In the second part of the thesis, we directly visualize drop breakup in a concentrated emulsion. We used a model emulsion of castor oil in aqueous maltose in a simple shear flow, over an oil concentration range of 2--75%. Above 20% oil concentration, we observe qualitative changes in the deformation process. Instead of deforming into well-defined cylindrical threads, drops deform into convoluted shapes, with fluid accumulating unevenly along the drop length. The end-pinching instability is also suppressed with increasing concentration. The time to breakup for the drops becomes increasingly independent of initial drop size with increasing concentration until at around the close packing volume fraction, all the drops in the emulsion break together.