Evolution of the Size Distribution of an Emulsion under a Simple Shear Flow

Abstract Understanding the rheology of immiscible liquids mixtures, as well as the role played by its micro-structures are important criteria for the production of new materials and processes in industry. Here, we study changes over time of the droplet size distributions of emulsions induced by slow shearing flows. We observe that the initial heterogeneous microstructure may evolve toward more complex structures (such as bimodal distribution) as a result of coalescence and rupture of droplets. These dynamic structures were produced using a flow cell made up of two parallel disks, separated by a gap of 100 μm. The steady rotation of the lower disk generates a simple shear flow of γ ˙ = 0.75 s ？ 1 , during ~ 400 s. After a brief rest time, this procedure was repeated by applying a step ramp until the maximum shear rate of 4.5 s ？ 1 was reached, using step increments of 0.75 s ？ 1 . During the last portion of the flow and during the rest time in between flows, structures of emulsions were characterized. Initially, a broad single-peak distribution of drops was observed, which evolved toward a rather narrower bimodal distribution, at first due to the coalescence of the smaller droplets and subsequently of the larger drops. The rupture of drops at higher shear rates was also observed. The observed evolutions also presented global structures such as “pearl necklaces” or “bands of particles”, the latter characterized by alternating bands of a high density of particles and regions of the continuous phase with only a few droplets. These changes may indicate complex, time-dependent rheological properties of these mixtures. View Full-Tex