Department of Chemical Engineering & Materials Science, University of California, Irvine, CA 92697, USA.
Soft Matter. 2017 Mar 29;13(13):2513-2522. doi: 10.1039/c6sm02858f.
The microstructure of Pickering emulsion gels features a tenuous network of faceted droplets, bridged together by shared monolayers of particles. In this investigation, we use standard oscillatory rheometry in conjunction with confocal microscopy to gain a more comprehensive understanding of the role particle bridged interfaces have on the rheology of Pickering emulsion gels. The zero-shear elastic modulus of Pickering emulsion gels shows a non-monotonic dependence on particle loading, with three separate regimes of power-law and linear gel strengthening, and subsequent gel weakening. The transition from power-law to linear scaling is found to coincide with a peak in the volume fraction of particles that participate in bridging, which we indirectly calculate using measureable quantities, and the transition to gel weakening is shown to result from a loss in network connectivity at high particle loadings. These observations are explained via a simple representation of how Pickering emulsion gels arise from an initial population of partially-covered droplets. Based on these considerations, we propose a combined variable related to the initial droplet coverage, to be used in reporting and rationalizing the rheology of Pickering emulsion gels. We demonstrate the applicability of this variable with Pickering emulsions prepared at variable fluid ratios and with different-sized colloidal particles. The results of our investigation have important implications for many technological applications that utilize solid stabilized multi-phase emulsions and require a priori knowledge or engineering of their flow characteristics.
Pickering 乳液凝胶的微观结构具有由颗粒共享的单层连接的面状液滴的脆弱网络。在这项研究中，我们使用标准的振荡流变仪结合共聚焦显微镜，更全面地了解颗粒桥接界面对 Pickering 乳液凝胶流变性的作用。Pickering 乳液凝胶的零剪切弹性模量表现出对颗粒负载的非单调依赖性，具有三个单独的幂律和线性凝胶增强区，以及随后的凝胶弱化区。发现从幂律到线性比例的转变与参与桥接的颗粒体积分数的峰值相吻合，我们使用可测量的量间接计算了这一峰值，并且在高颗粒负载下网络连接的丧失导致了凝胶的弱化。这些观察结果通过 Pickering 乳液凝胶如何从部分覆盖液滴的初始群体中产生的简单表示来解释。基于这些考虑，我们提出了一个与初始液滴覆盖率相关的组合变量，用于报告和合理化 Pickering 乳液凝胶的流变性。我们使用在不同流体比和不同尺寸胶体颗粒下制备的 Pickering 乳液来证明该变量的适用性。我们研究的结果对许多利用固体稳定多相乳液的技术应用具有重要意义，并且需要事先了解或设计其流动特性。
