Direct visualization of strain-induced yielding in colloidal gels

0853648米。由胶体颗粒组成的SolomonGels系统具有缓慢、受限的动力学和不寻常的粘弹性流变性。它们对陶瓷的化学加工、微过滤膜的形成以及油漆、饰面、涂料和消费品的质量至关重要。下一代技术，如直写组装和微流控阀也依赖于凝胶化转变和胶体颗粒凝胶的流变特性。这些技术的一个共同特点是，如果施加足够大的应力或应变，凝胶就会屈服。这种屈服是一种鲜为人知的胶体对相互作用和凝胶微观结构的卷积。屈服具有机械失效的共同特征：剧烈的流变转变导致先前刚性材料的流化。近年来，对凝胶起源的实验描述和理论解释都取得了长足的进展。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一种在临界应力下打开的微流体阀，还是一种在其产品寿命内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？操纵凝胶结构如何影响这个顺序？这些过渡如何反馈到悬架微动力学中以确定局部屈服率？施加的应变如何诱导凝胶网络的应力承载主干的演化？为了解决这些科学问题，我们将执行一项研究计划，直接可视化凝胶中应变诱导的屈服和内部破坏。我们的研究计划的智力价值来自于我们在追求这些目标的共聚焦光学显微镜的综合应用，以及我们开发的新颖的、合理的方法来诱导胶体凝胶的屈服并研究其影响。首先，共聚焦显微镜的强大之处在于它能够在三维（3D）和纳米级分辨率下直接可视化局部、胶体级结构和动力学。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，之前试图可视化凝胶的内部破坏和破裂已经失败，因为遇到了剪切带的非理想性。由于剪切带是所研究的流动几何所特有的，它不能直接表征屈服，而屈服是一种具有广泛基本兴趣的固有材料性质。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。文献和我们之前的工作表明，这种流动通过产生均匀的屈服和破裂的胶体凝胶来避免剪切带。在这个项目中，我们试图通过在折射率和密度匹配的溶剂中探测由微米级立体稳定胶体聚（甲基丙烯酸甲酯）组成的凝胶的阶梯应变引起的破裂，将基本理解扩展到微观尺度。由于该系统的对势相互作用是可调的，并且具有很好的特征，因此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。项目成果将包括对胶体凝胶局部产率的首次实验评估，这是成功建立凝胶流变学软玻璃模型和模型耦合模型的关键输入。这项研究将通过发现凝胶生成和微观结构之间关系的基本特征，广泛影响陶瓷、膜、消费产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)培训研究生使用最先进的共聚焦显微镜、胶体科学和流变学方法；（ii）为暑期拓展计划开发新的工程设计部分，通过复杂流体的动手实验活动和实验，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.