Interfacial Rheology of Sterically Stabilized Colloids at Liquid Interfaces and Its Effect on the Stability of Pickering Emulsions

Particle-laden
interfaces can be used to stabilize a variety of
high-interface systems, from foams over emulsions to polymer blends.
The relation between the particle interactions, the structure and
rheology of the interface, and the stability of the system remains
unclear. In the present work, we experimentally investigate how micron-sized,
near-hard-sphere-like particles affect the mechanical properties of
liquid interfaces. In particular, by comparing dried and undried samples,
we investigate the effect of aggregation state on the properties of
the particle-laden liquid interface and its relation to the stability
of the corresponding Pickering emulsions. Partially aggregated suspensions
give rise to a soft-solid-like response under shear, whereas for stable
PMMA particulate layers a liquid-like behavior is observed. For interfacial
creep-recovery measurements, we present an empirical method to correct
for the combined effect of the subphase drag and the compliance of
the double-wall ring geometry, which makes a significant contribution
to the apparent elasticity of weak interfaces. We further demonstrate
that both undried and dried PMMA particles can stabilize emulsions
for months, dispelling the notion that particle aggregation, in bulk
or at the interface, is required to create stable Pickering emulsions.
Our results indicate that shear rheology is a sensitive probe of colloidal
interactions but is not necessarily a predictor of the stability of
interfaces, e.g., in quiescent Pickering emulsions, as in the latter
the response to dilatational deformations can be of prime importance.

负载颗粒的界面可用于稳定多种高界面能体系，涵盖泡沫、乳液乃至聚合物共混物。目前，颗粒间相互作用、界面结构与流变特性，以及体系稳定性之间的关联仍未明确。本工作通过实验探究微米级类近硬球颗粒对液态界面力学性能的影响。具体而言，通过对比干燥与未干燥样品，我们分析了聚集态对负载颗粒液态界面特性的调控作用，及其与对应皮克林乳液（Pickering emulsion）稳定性的关联。部分聚集的悬浮体系在剪切作用下表现出类软固体响应，而稳定的聚甲基丙烯酸甲酯（PMMA）颗粒层则呈现类液体行为。针对界面蠕变回复测试，我们提出了一种经验校正方法，以消除亚相拖曳与双壁环几何构型柔顺性的联合影响——这两个因素会对弱界面的表观弹性产生显著贡献。我们还证实，未干燥与干燥的PMMA颗粒均可使乳液稳定数月之久，破除了“无论在本体还是界面处，颗粒聚集都是制备稳定皮克林乳液的必要条件”这一固有认知。本研究结果表明，剪切流变学是探测胶体相互作用的灵敏手段，但未必能预测界面的稳定性；例如在静态皮克林乳液中，拉伸形变响应往往是决定稳定性的关键因素。