Formation and Propagation of Cracks in Colloidal Packings

CBET-0754078，Russel 我们最近的工作提供了一个坚实的理解的力学控制固结和开裂的固定，即紧密包装，胶体分散体，变形非线性和粘弹性由于接触或界面力。我们的模型确定，在适当的无量纲组，空气-水，聚合物-水，或聚合物-空气界面能足以形成均匀的无空隙或多孔膜的水分散体薄层干燥的刚性基板上的条件下。该模型还预测了毛细管压力，当变形太慢而不能跟上蒸发时，高于该毛细管压力，破裂变得有利，因为当裂纹打开时恢复的弹性能超过所消耗的额外表面能。补充实验与聚合物胶乳和无机氧化物胶体在压力过滤单元允许直接测量的负毛细管压力所需的开裂，表明许多填料不开裂，直到毛细管压力超过显着的最小预测的无限裂纹。进一步的实验和理论确定了缺陷对裂纹形核的重要性，就像受拉的线弹性固体一样。这项工作为解决剩余的难题和两个相关的挑战提供了基础。 首先，在薄膜中的干燥前沿之后的裂纹尖端形成特征间距，并且通常以粘滑方式在毛细管压力梯度的方向上前进。这表明了一个额外的动力学过程，这部分归因于毛细管压力梯度驱动的水流通过颗粒堆积。为了阐明图案选择过程，我们提出了两个实验几何形状与控制传播的梯度，补充与分析的动态过程，通过扩展现有的模型。薄的矩形通道将蒸发限制在开口端，并允许通过显微镜通过平面观察从端部传播的裂纹。或者，上述压力过滤单元可以稍微倾斜以产生具有自由表面但没有由不均匀蒸发引起的横向流动的薄层的厚度梯度。裂缝形核的缺口在厚边，然后应向薄边扩展，增加毛细管压力。 第二，一些技术需要高度多孔的膜，其不能在所需厚度下干燥而不开裂，这就提出了如何将用于开裂的毛细管压力保持在膜表面处的毛细管压力可达到的毛细管压力之上的问题。为了这个目的，我们建议研究与胶体棒形成的膜，随机包装产生的孔大于棒的直径，从而降低最大毛细管压力相对于随机紧密包装的相同直径的球体。成功与否将取决于控制开裂的填料的有效模量是否下降到足以抵消其益处的程度。在压滤池中用勃姆石胶体棒（裸露或用二氧化硅涂层稳定）进行的实验将确定临界毛细管压力作为膜厚度的函数，以评估潜力。 第三，我们打算探索膜的形成和破裂的压力过滤单元的硬和软球的二元混合物的相互作用调整到尽可能高的体积分数，以延迟渗滤的硬相。这项工作，使用相同的实验工具，如上所述，将指导理论正在开发，以了解最近发现的？光环？由于小的高电荷聚合物胶乳和较大的电中性无机球体之间的静电吸引力而产生的效应。知识价值在于创造一个坚实的理解基础，并利用它来设计新的技术途径。成功地理解这些现象应该有利于干燥过程的重要技术，从传统的（但总是改善）建筑涂料，通过流延工艺制造陶瓷基板和多层设备，精心定制的颗粒涂料喷墨纸。这项研究提供了更广泛的影响，提出了一个刺激的车辆，教育研究生和本科生的职业生涯在化学和相关行业。

CBET-0754078, Russel Our recent work offers a solid understanding of the mechanics controlling consolidation and cracking of immobilized, i.e. close packed, colloidal dispersions that deform nonlinearly and viscoelastically due to contact or interfacial forces. Our model identifies, in terms of appropriate dimensionless groups, the conditions under which air-water, polymer-water, or polymer-air interfacial energies suffice to form homogeneous void-free or porous films as thin layers of aqueous dispersions dry on a rigid substrate. The model also predicts the capillary pressure above which cracking becomes favorable, when deformation is too slow to keep up with evaporation, as the elastic energy recovered when a crack opens exceeds the additional surface energy expended. Complementary experiments with polymer latices and inorganic oxide colloids in a pressure filtration cell permit direct measurement of the negative capillary pressures required for cracking, demonstrating that many packings do not crack until the capillary pressure exceeds significantly the minimum predicted for an infinite crack. Further experiments and theory establish the importance of flaws to nucleate cracks, as for linearly elastic solids under tension. This work provides a foundation for addressing a remaining puzzle and two related challenges. First, crack tips following a drying front in a thin film develop a characteristic spacing and often advance in the direction of the gradient in capillary pressure in a stick-slip fashion. This suggests an additional dynamical process, which some attribute to the flow of water driven through the particle packing by gradients in the capillary pressure. To elucidate the pattern selection process we propose two experimental geometries with controlled propagation of gradients, complemented with analysis of the dynamic process through an extension of the existing model. A thin rectangular channel confines evaporation to the open ends and allows cracks propagating in from the ends to be viewed through the flat faces via a microscope. Alternatively, the pressure filtration cell mentioned above can be tilted slightly to create a gradient in thickness of a thin layer with a free surface but without the lateral flows caused by nonuniform evaporation. Cracks nucleated by notches at the thick edge then should propagate toward the thin edge with increasing capillary pressure. Second, some technologies require highly porous films that cannot be dried at the desired thickness without cracking, raising the question of how to maintain capillary pressures for cracking above those attainable with menisci at the surface of the film. For this purpose we propose to study films formed with colloidal rods for which random packing creates pores larger than the rod diameter, thereby reducing the maximum capillary pressure relative to random close packing of spheres of the same diameter. Success will depend on whether the effective modulus of the packing, which controls cracking, does not fall enough to negate the benefit. Experiments with colloidal rods of boehmite, either bare or stabilized with a silica coating, in the pressure filtration cells will determine the critical capillary pressures as a function of film thickness to assess the potential. Third, we intend to explore film formation and cracking in the pressure filtration cell for binary mixtures of hard and soft spheres with interactions tuned to delay percolation of the hard phase to as high a volume fraction as possible. This effort, using the same experimental tools as above, will be guided by theory being developed to understand the recently discovered ?halo? effect due to electrostatic attractions between small highly charged polymer latices and larger electrically neutral inorganic spheres. The intellectual merit lies in the creation of a solid fundamental basis of understanding and using that to devise new avenues for the technology. Success in understanding these phenomena should benefit drying processes important to technologies ranging from conventional (but always improving) architectural coatings, through tape casting processes for fabricating ceramic substrates and multilayer devices, to carefully tailored particulate coatings for inkjet papers. The research provides broader impact by posing a stimulating vehicle for educating graduate students and undergraduates for careers in the chemical and related industries.