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

当在悬浮液的连续相中加入少量（小于1%）第二种不混溶液体时，混合物的流变性能发生显著变化，从类流体状态变为类凝胶状态。我们对这一现象的研究是由DFG资助的。我们发现，随着第二种流体体积分数的增加，屈服应力和粘度增加了几个数量级，我们已经证明这种转变归因于两种流体对固体颗粒的毛细力。在与湿颗粒材料的类比中，定义了两种不同的状态：“钟摆状态”，其中二次流体优先润湿颗粒；在“毛细状态”中，二次流体对颗粒的润湿不如一次流体好。我们发现这两种状态都与悬浮液从流体到凝胶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，毛细管悬浮液是一种新型材料，可用于制造可调流体，稳定混合物，否则将相分离，并创造新材料，如低脂食品或微孔泡沫。最近，我们已经证明了毛细管悬浮液为制造多孔陶瓷提供了一种新的、经济高效的、通用的加工路线，在小孔径下具有前所未有的高孔隙率。本研究计划旨在回答有关毛细管悬浮液的基本问题，特别是关于在钟摆和毛细管状态悬浮液中形成的样本跨越网络的性质。本文拟从微观和宏观两方面研究毛细悬浮液的老化和网络结构的变化。这些网络的微观结构包括颗粒-液滴结构、絮团（或簇）大小、对分布函数和假设的分形网络结构的维度将使用成像技术进行表征。宏观样品的强度将用流变学技术表征。这两种技术的结合使得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.