Tailoring flow behavior of colloidal dispersions with short-range repulsive interactions using depletion forces

非吸附聚合物引入的弱吸引相互作用，是在加工和应用过程中根据需要在较大范围内调节胶体分散体流动性能的有力工具。然而，我们的初步结果表明，吸引强度和宏观流动行为之间并不存在微不足道的关系。对这种分散体的微观结构和相应的流变性的详细了解是进行有针对性的产品设计的关键。关于这些现象的基本知识对于理想的硬球系统是可用的。在这里，我们将重点介绍由粒子之间的短程排斥相互作用稳定的技术相关系统。我们想要了解排斥力的范围如何控制分散体的微观结构，以及当存在额外的吸引耗尽力时，相行为、结构和流动特性之间的复杂关系。我们的初步工作表明，多粒子跟踪(MPT)微观流变学是表征这种混浊的、含水的胶体分散体的微观结构和微观非均质性的独特工具，具有重要的技术意义。可以确定流体/晶体共存状态下的分散相组成，包括结晶区的大小和形状，甚至可以获得这些微晶的剪切模数。可以根据特征时间和长度尺度来分析高度集中的阻滞态中的动态和静态不均一性。可以区分不同的非均相凝胶态，具有不同的局部颗粒迁移率和整体流动行为，这在真正的硬球系统中迄今还没有观察到。初步研究还表明，与硬球体系相比，需要更强的吸引力来诱导从流体到流体/结晶或凝胶态的转变，分散体可以在远高于硬球玻璃化转变的颗粒负载下通过弱耗尽力流态化。MPT实验将与经典的定常和振荡剪切流变仪相结合，系统地研究微观结构、局部颗粒迁移率和弥散流动。此外，还将利用微流控流道与PIV数据分析相结合的方法，在微观水平上研究高浓度体系在玻璃和凝胶状态下的剪切流动。最近提出的描述通过团簇协同运动在受限几何空间中流动的非局部流变学模型将基于MPT的非均质长度尺度数据进行评估。将这些实验方法结合起来，将建立的流变模型和连续介质力学本构方程的参数与微观结构和胶体分散特性相关联。因此，当样品组成中仅存在附加的弱耗尽力时，我们将能够预测具有短程粒子斥力的分散体的流动行为。

Weak attractive interactions introduced by non-adsorbing polymer added to the fluid phase, are a powerful tool to tune flow properties of colloidal dispersions in a wide range according to the demands during processing and application. Our preliminary results, however, show that there is no trivial relationship between attraction strength and macroscopic flow behavior. Detailed knowledge of the microstructure of such dispersions and corresponding rheological properties are the key requisite for a targeted product design. Fundamental knowledge about these phenomena is available for ideal hard sphere systems. Here we will focus on technically relevant systems stabilized by short-range repulsive interactions among particles. We want to understand how the range of repulsion controls the dispersions microstructure, and finally the complex relationship between phase behavior, structure and flow properties when additional attractive depletion forces are present. Our preliminary work demonstrated that multi particle tracking (MPT) microrheology is a unique tool to characterize microstructure and microscopic heterogeneities in such turbid, aqueous colloidal dispersions which are of utmost technical relevance. The dispersions phase composition in the fluid/crystalline co-existence regime can be determined including size and shape of the crystalline regions, even the shear modulus of these micro-crystals is accessible. Dynamic and static heterogeneities in highly concentrated arrested states can be analyzed regarding characteristic time and length scales. Various heterogeneous gel states with different local particle mobility and bulk flow behavior could be distinguished, so far not observed in true hard sphere systems. Preliminary investigations also revealed that compared to hard sphere systems much stronger attraction is needed to induce transitions from fluid to fluid/crystalline or gel states and dispersions can be fluidized via weak depletion forces at particle loadings far above the hard sphere glass transition. MPT experiments will be combined with classical steady and oscillatory shear rheometry to study microstructure, local particle mobility, and dispersion flow systematically. Furthermore, a microfluidic flow channel combined with PIV data analysis will be employed to study shear flow of highly concentrated systems in glass and gel states on a microscopic level. A recently suggested non-local rheological model describing the flow in confined geometries via cooperative motion of clusters will be evaluated based on heterogeneity length scale data from MPT. Combining these experimental approaches will correlate parameters of established rheological models and continuum mechanical constitutive equations to microstructure and colloidal dispersion properties. Thus we will be able to predicting the flow behavior of dispersions with short-range particle repulsion when additional weak depletion forces are present solely from sample composition.