Using capillary forces to control suspension rheology: Network structure, formation, and aging in capillary suspensions

当将少量（小于1%）的第二不混溶液体添加到悬浮液的连续相中时，混合物的流变性质从流体状显著改变为凝胶状状态。 我们对这一现象的研究是通过DFG赠款资助的。我们发现，屈服应力和粘度增加了几个数量级的第二流体的体积分数的增加，我们已经表明，这种转变是由于对固体颗粒的两种流体的毛细管力。 在与湿颗粒材料的类比中，定义了两种不同的状态："摆动状态"，其中次级流体优先润湿颗粒;和"毛细状态"，其中次级流体润湿颗粒不如初级流体。 我们已经发现，这两种状态都与悬浮液中从流体状到凝胶状状态或从弱凝胶到强凝胶的转变有关。我们的研究已经证实，毛细管悬浮液是一类新的材料，可用于创建可调流体，稳定混合物，否则将相分离，并创建新的材料，如低脂食品或微孔泡沫。最近，我们已经证明，毛细管悬浮液提供了一种新的，具有成本效益的和通用的加工路线，用于制造多孔陶瓷，具有前所未有的高孔隙率在小孔径。这项研究计划的目的是回答有关毛细悬浮液，特别是关于摆动和毛细状态悬浮液中形成的样品跨越网络的性质的基本问题。 该提案旨在研究毛细管悬浮液的老化以及微观和宏观尺度上的网络结构变化。这些网络的微观结构，包括颗粒液滴配置，絮凝体（或集群）的大小，对分布函数，和假设的分形网络结构的尺寸将使用成像技术的特点。将使用流变技术表征宏观样品的强度。这两种技术的组合允许网络形态的变化与网络强度和流动行为的变化直接相关。

When a small amount (less than 1%) of a second immiscible liquid is added to the continuous phase of a suspension, the rheological properties of the mixture are dramatically altered from a fluid-like to a gel-like state. Our research on this phenomenon was funded through DFG grant. We found that the yield stress and viscosity increase by several orders of magnitude as the volume fraction of the second fluid increases and we have shown that this transition is attributed to the capillary forces of the two fluids on the solid particles. In an analogy to wet granular materials, two distinct states are defined: the "pendular state" where the secondary fluid preferentially wets the particles; and the "capillary state" where the secondary fluid wets the particles less well than the primary fluid. We have found that both states are associated with a transition in the suspension from a fluid-like to gel-like state or from a weak gel to a strong gel. Our investigations have confirmed that capillary suspensions are a new class of materials that can be used to create tunable fluids, stabilize mixtures that would otherwise phase separate, and to create new materials such as low-fat foods or microporous foams. Recently, we have demonstrated that capillary suspensions offer a new, cost-efficient and versatile processing route for manufacturing porous ceramics with unprecedentedly high porosity at small pore sizes. This research proposal aims to answer fundamental questions about capillary suspensions and particularly about the nature of the sample-spanning networks formed in both the pendular and capillary state suspensions. This proposal intends to investigate the aging of capillary suspensions and changes to network structure on both the micro- and macroscopic scale. The microstructure of these networks including the particle-droplet configuration, floc (or cluster) size, pair distribution functions, and the dimension of the supposed fractal network structure will be characterized using imaging techniques. The strength of the macroscopic sample will be characterized using rheological techniques. The combination of these two techniques allows the direct correlation of changes in network morphology to the changes in network strength and flow behavior.