Direct visualization of strain-induced yielding in colloidal gels

0853648M。胶体颗粒的SolomonGels是具有缓慢的、受约束的动力学和不寻常的粘弹性流变学的系统。它们对于陶瓷的化学加工、微过滤膜的形成以及油漆、面漆、涂料和消费品的质量至关重要。 下一代技术，如直接写入组件和微流体阀也依赖于胶体颗粒凝胶的凝胶化转变和流变特性。这些技术的一个统一特征是它们依赖于这样一个事实，即如果施加足够大的应力或应变，凝胶就会屈服。这种屈服是胶体对相互作用和凝胶微观结构的一种知之甚少的卷积。屈服具有与机械故障相同的特征：戏剧性的流变转变导致先前刚性材料的流化。近年来，对凝胶化起源的实验描述和理论解释都取得了很大进展。然而，从工程设计和实践的角度来看，我们需要更多：我们还必须了解应力诱导屈服，破裂和流化。无论我们的兴趣是生产在临界应力下打开的微流体阀，还是在其产品寿命期间保持均匀和稳定的洗涤剂，我们都应该解决：在施加应变时导致凝胶破裂和内部失效的事件顺序是什么，以及操纵凝胶结构如何影响该顺序？这些转变是如何反馈到悬架的微观动力学，以确定当地的产量？施加的应变如何引起凝胶网络的应力承载骨干的演变？为了解决这些科学问题，我们将执行一项研究计划，直接可视化应变诱导的屈服和内部故障的凝胶。我们的研究计划的智力价值来自我们的共聚焦光学显微镜在追求这些目标和我们的发展的新的，良好的方法来诱导屈服的胶体凝胶和研究其影响的综合应用。首先，共聚焦显微镜的能力取决于其以纳米级分辨率直接可视化三维（3D）局部胶体水平结构和动态的能力。由于屈服是一种局部现象，因此直接可视化方法是我们方法的关键优势。其次，我们认识到，以前的尝试，可视化内部故障和破裂的凝胶已经失败，因为剪切带的非理想性遇到。由于剪切带是特定的流动几何形状的研究，它不直接表征屈服，广泛的基本利益的内在材料属性。为了解决这个问题，我们将直接可视化高速率步进应变变形的屈服。文献和我们以前的工作表明，这种流动避免剪切带产生均匀的屈服和破裂的胶体凝胶。在这个项目中，我们试图扩展基本的理解，以微观尺度的探测步骤应变引起的凝胶破裂组成的微米级空间稳定的胶体聚（甲基丙烯酸甲酯）在折射率和密度匹配的溶剂。由于该系统的对潜在的相互作用是可调的和良好的特点，该模型系统的结果适用于工程实践中遇到的广泛的材料和凝胶结构。为解决上述三个基本问题，将执行三项任务。项目成果将包括首次对胶体凝胶的局部产率进行实验评估，这是成功建立凝胶流变学的软玻璃态和模型耦合模型的关键。这项研究将通过发现凝胶屈服和微观结构之间关系的基本特征，广泛影响陶瓷、膜、消费品和直写组装等不同领域的技术和工程。具有更广泛影响的其他成果包括：（i）培训研究生掌握共聚焦显微镜、胶体科学和流变学的最新方法;（ii）为暑期外展计划开发新的工程设计部分，通过实验室活动和复杂流体实验，向中学女生介绍化学工程和材料科学。

0853648M. SolomonGels of colloidal particles are systems with slow, constrained dynamics and unusual, viscoelastic rheology. They are central to the chemical processing of ceramics, the formation of membranes for microfiltration and the quality of paints, finishes, coatings and consumer products. Next generation technologies such as direct-write assembly and microfluidic valving also rely on the gelation transition and the rheological properties of colloidal particle gels. A unifying feature of these technologies is their dependence on the fact that gels yield if a stress or strain of sufficient magnitude is applied. This yielding is a poorly understood convolution of colloid pair interactions and gel microstructure. Yielding has features common with mechanical failure: a dramatic rheological transition results in fluidization of the previously rigid material. Recently, substantial progress has been made in both experimental description and theoretical explanation of the origin ofgelation. However, from the point of view of engineering design and practice we require more: we must also understand stress-induced yielding, rupture and fluidization. Whether our interest is to produce a microfluidic valve that will open at a critical stress, or a detergent that will remain homogeneous and stable over its product life, we should address: What is the sequence of events that leads to gel rupture and internal failure upon application of strain and how does manipulating gel structure affect this sequence? How do these transitions feedback into suspension microdynamics to determine the local yield rate? How does an applied strain induce evolution of the stress bearing backbone of a gel network? To address these scientific questions, we will execute a research program to directly visualize strain-induced yielding and internal failure in gels.The intellectual merit of our research plan arises from our comprehensive application of confocal optical microscopy in pursuit of these aims and our development of novel, well posed methods to induce yielding in colloidal gels and study its implications. First, the power of confocal microscopy rests on its ability to directly visualize local, colloid-level structure and dynamics in three dimensions (3D) and with nanoscale resolution. Since yielding is a local phenomena, the direct visualization methodology is a key strength of our approach. Second, we recognize that previous attempts to visualize internal failure and rupture of gels have foundered because the nonideality of shear banding was encountered. Because shear banding is particular to the flow geometry studied, it does not directly characterize yielding, an intrinsic material property of broad fundamental interest. To address this issue, we will directly visualize yielding by high-rate stepstrain deformation. The literature and our prior work demonstrate that this flow avoids shear banding by generating homogeneous yielding and rupture of colloidal gels. In this project, we seek to extend fundamental understanding to the microscopic scale by probing the step-strain induced rupture of gels comprised of micron-scale sterically-stabilized colloidal poly(methyl methacrylate) in refractive-index and density-matched solvents. Because this system's pair potential interactions are both tunable and well characterized, results for this model system are applicable to the broad range of materials and gel structures encountered in engineering practice. Three tasks will be executed to address the three fundamental questions posed above. Project outcomes will include the first experimental assessment of the local yield rate of a colloidal gel, a key input to the successful soft glassy and model coupling models of gel rheology. This study will broadly impact technology and engineering in diverse areas such as ceramic, membranes, consumer products and direct write assembly by discovering fundamental features of the relationship between gel yielding and microstructure. Additional outcomes with broader impact include: (i) the training of a graduate student in state-of-the-art methods in confocal microscopy, colloidal science and rheology; (ii) development of a new engineering design component for a summer outreach program that introduces middle school girls to chemical engineering and materials science through hands on lab activities and experiments in complex fluids.