464703 The Effect of a Yield Stress on the Drainage of the Thin Film Between Two Colliding Newtonian Drops
The objective of this paper is to assess the validity of the yield-stress explanation for water-in-bitumen emulsion stability. A survey of the literature reveals that a theoretical understanding of the drainage of the thin film of Bingham fluid between two colliding drops is still lacking, with the exception of two studies that make ad-hoc assumptions about the film shape3,4. In this work, we examine this problem for low capillary numbers via a combination of scaling analysis and detailed numerical simulations based on the lubrication analysis5,6. One obvious trend is that the introduction of a yield stress in the suspending fluid retards the drainage process relative to a Newtonian fluid of the same viscosity. But there are at least other three notable features of the film drainage process of Bingham fluids. First, the presence of yield stress prolongs the spherical regime of the drainage process and delays transition into the dimpled regime. Second, the drainage time shows a minimum with respect to the force applied to push the drops towards each other. Third, drainage may be arrested completely below a critical height due to the yield stress. This critical height scales as the square of the yield stress, the cube of the drop radius and inversely as the square of the interfacial tension. Counterintuitively, the critical height independent of the force colliding the two drops! This and other distinguishing characteristics of the drainage process will be elucidated in the presentation. We will conclude the presentation with an evaluation of the ‘yield stress’ explanation for the stability of water-in-bitumen emulsions, outlining the regime of parameters for which this explanation may be valid.
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See more of this Group/Topical: Engineering Sciences and Fundamentals