Muriel Diego F, Katz Joseph
Department of Mechanical Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States.
Langmuir. 2021 Feb 9;37(5):1725-1742. doi: 10.1021/acs.langmuir.0c02986. Epub 2021 Jan 26.
This study examines the time evolution and effects of adding dispersant (Corexit 9500A) at varying concentrations on the microscopic morphology and bulk viscosity of saltwater-in-crude-oil (Louisiana) mechanically mixed emulsions. Rheology is used for measuring the viscoelastic properties and viscosity, the latter at varying shear rates. Microscopy, followed by machine-learning-based analysis, is used for characterizing the size and spatial distribution of the water droplets in the emulsions. Initially, the water droplets appear as a multiscale lattice with a Sauter diameter of 5.3 μm and a polydispersity of 0.43, with small droplets aggregating around large ones. The corresponding bulk viscosity decreases with increasing shear rate from 2 orders of magnitude to 5 times higher than that of the weathered crude oil. After 7 days, the number of submicron droplets increases, the nearest-neighbor distance decreases, indicating preferential aggregation, and the viscosity increases by 56-112% at high shear rates (5-100 s). After 14 and 21 days, some droplets coalesce resulting in loss of clusters and a decrease in viscosity. These trends suggest that changes in the aggregation contribute to the variations in viscosity. Subsequent analysis applies previously developed models for the effect of aggregation on the properties of the emulsion. While the reduction in viscosity is predicted by this model, matching of rates requires modification to the assumed relationship between yield stress and interdroplet forces. Adding dispersant without mixing generates Marangoni-driven flows as the water droplets coalesce. In time, part of the water separates, a fraction forms clouds of submicron droplets, and the rest remains unchanged. Mixing dispersant at low concentration with the emulsion accelerates the coalescence and phase separation. The removed water fraction increases with dispersant concentration, reaching 99.6% for a dispersant-to-emulsion concentration of 10. The remaining emulsion consists of fine droplets with Newtonian viscosity that is still 4 times higher than that of the fresh crude oil but only 14% higher than that of the weathered oil.
本研究考察了在不同浓度下添加分散剂（Corexit 9500A）对原油（路易斯安那州）与盐水机械混合乳液的微观形态和整体粘度随时间的演变及影响。流变学用于测量粘弹性性质和粘度，后者是在不同剪切速率下进行测量。显微镜检查，随后基于机器学习的分析，用于表征乳液中水滴的大小和空间分布。最初，水滴呈现为具有索特直径5.3μm和多分散性0.43的多尺度晶格，小水滴聚集在大水滴周围。相应的整体粘度随着剪切速率从2个数量级增加到比风化原油高5倍而降低。7天后，亚微米级水滴数量增加，最近邻距离减小，表明优先聚集，并且在高剪切速率（5 - 100 s）下粘度增加56 - 112%。14天和21天后，一些水滴合并导致聚集体损失和粘度降低。这些趋势表明聚集的变化导致了粘度的变化。后续分析应用先前开发的关于聚集对乳液性质影响的模型。虽然该模型预测了粘度的降低，但速率匹配需要修改屈服应力与液滴间力之间的假定关系。不混合而添加分散剂会在水滴合并时产生马兰戈尼驱动的流动。随着时间的推移，一部分水分离，一部分形成亚微米级水滴云，其余部分保持不变。将低浓度分散剂与乳液混合会加速合并和相分离。去除的水部分随着分散剂浓度增加，对于分散剂与乳液浓度比为10时达到99.6%。剩余的乳液由具有牛顿粘度的细水滴组成，其粘度仍比新鲜原油高4倍，但仅比风化油高14%。
