Formation and Propagation of Cracks in Colloidal Packings

胶体填料裂纹的形成和扩展

• 批准号: 0754078
• 负责人: William Russel
• 金额: $ 31.5万
• 依托单位: Princeton University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-07-01 至 2013-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0754078&HistoricalAwards=false
• 关键词: Formation; Propagation; Cracks; Colloidal; Packings

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.

CBET-0754078，Russel我们最近的工作使我们对控制固定化固结和破裂的力学有了坚实的理解，即紧密堆积的胶体分散体，由于接触或界面力而非线性和粘弹性变形。我们的模型根据适当的无量纲基团确定了空气-水、聚合物-水或聚合物-空气界面能足以形成均匀的无气孔或多孔膜的条件，作为干燥在刚性衬底上的薄层水分散体。该模型还预测了毛细压力，当变形太慢，跟不上蒸发时，破裂变得有利，因为裂缝张开时回收的弹性能量超过了额外的表面能量消耗。在加压过滤单元中用聚合物乳胶和无机氧化物胶体进行补充实验，可以直接测量裂解所需的毛细负压，这表明许多填料直到毛细压力显著超过无限裂纹的最小预测值时才会破裂。进一步的实验和理论证实了缺陷对于形成裂纹的重要性，就像拉力作用下的线弹性固体一样。这项工作为解决一个剩余的谜题和两个相关的挑战提供了基础。首先，薄膜中干燥前沿后的裂纹尖端形成一个特征间距，并通常以粘滑方式沿着毛细压力梯度的方向前进。这表明了一个额外的动力学过程，一些人将其归因于水在毛细压力梯度的推动下通过颗粒堆积的流动。为了解释模式选择过程，我们提出了两个梯度传播受控的实验几何，并通过对现有模型的扩展对动态过程进行了分析。一条薄的矩形通道将蒸发限制在开放的一端，并允许通过显微镜观察从两端传播的裂纹。或者，上面提到的压滤池可以稍微倾斜，以产生具有自由表面的薄层的厚度梯度，但不会由于不均匀蒸发而引起横向流动。随着毛细压力的增加，厚边缺口形核裂纹应向薄边扩展。其次，一些技术需要高度多孔的薄膜，这些薄膜不能在不破裂的情况下以所需的厚度干燥，这就提出了一个问题，即如何保持毛细管压力，使其破裂超过薄膜表面的半月板所能达到的压力。为此，我们建议研究胶体棒形成的薄膜，其中随机堆积产生大于棒直径的孔，从而降低相对于相同直径的球体的随机紧密堆积的最大毛细压力。成功将取决于控制裂缝的填料的有效模数是否下降到足以抵消好处的程度。在加压过滤单元中使用薄水铝石胶体棒进行实验，无论是裸露的还是用二氧化硅涂层稳定的，都将确定临界毛细压力作为膜厚度的函数，以评估潜力。第三，我们打算探索压力过滤池中硬球和软球的二元混合物的膜形成和破裂，这些混合物的相互作用被调整为将硬相的渗流延迟到尽可能高的体积分数。这项工作，使用与上面相同的实验工具，将由正在开发的理论来指导，以理解最近发现的光晕？由于小的高电荷聚合物乳状液和较大的电中性无机球体之间的静电吸引而产生的影响。智力的价值在于为理解和利用这一点创造了坚实的基本基础，并为这项技术设计了新的途径。成功理解这些现象将有利于干燥工艺，这些工艺对各种技术非常重要，从传统的(但一直在改进的)建筑涂料，到用于制造陶瓷基材和多层设备的流延工艺，再到用于喷墨打印纸的精心定制的颗粒涂料。这项研究提供了更广泛的影响，为培养化学及相关行业的研究生和本科生提供了一种激励工具。