Structural Heterogeneity, Microhydrodynamics and the Non-Linear Viscoelasticity of Colloidal Gels

胶体颗粒是一种具有缓慢、受限动力学和不寻常的粘弹性流变学的系统。它们对陶瓷的化学加工、微过滤膜的形成以及油漆、饰面和涂料的质量至关重要。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。因此，可以应用于预测凝胶化转变，控制凝胶的微观结构和动力学以及利用其不寻常的非线性流变学的基本理解将广泛影响这些领域的技术发展。在科学上，有必要进行实验来区分基于模式耦合框架的凝胶化转变理论和基于动态平衡聚类方法的凝胶化转变理论。此外，还应发现结构非均质性与凝胶微流体力学之间的关系，以理解凝胶中动力非均质性的作用。最后，即使是非线性变形作用下局部结构演化的定性特征也尚未通过实验解决。为了解决这些技术需求背后的科学问题，我们将执行一项实验研究计划，观察胶体凝胶的结构、动力学和非线性粘弹性之间的局部相关性。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。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。我们将主要研究微米尺度立体稳定胶体聚甲基丙烯酸甲酯在折射率和密度匹配溶剂中的悬浮液。非吸附聚合物会引起引起凝胶化的短程吸引相互作用。控制短程吸引强度的控制参数是聚合物的浓度。三维图像体积的共聚焦显微镜将用于量化凝胶诱导的团簇和弦的大小和异质性。局部结构将通过测量凝胶中颗粒接触数的分布来表征。这些结构测量将用于测试模式耦合和凝胶的热力学理论的具体预测。为了研究凝胶的动力学非均质性和其他微流体动力学特征，结构将与单个和集体粒子动力学相关联，通过三维粒子跟踪量化。最后，通过原位共聚焦显微成像，监测稳态剪切流启动和阶梯应变实验后的瞬态结构演变。流动实验的新方面将是他们对非线性现象的关注，他们通过表面拓扑工程控制壁面滑动，以及他们使用的材料将产生以前从未有过的凝胶中胶体的局部旋转动力学图像。这些直接可视化实验不同于以前的光、中子和x射线散射研究，因为它们专门探测局部现象和结构和动力学分布，而这些通常不能从散射的总体平均结果中获得。这项研究阐明了凝胶化转变、微观凝胶结构和动力学以及宏观流动之间关系的新科学认识，将广泛影响陶瓷、膜和直接写入组装等领域的技术和工程。具有更广泛影响的其他成果包括培训一名研究生在共聚焦显微镜、胶体科学和流变学方面的最先进方法，以及暑期拓展计划的新发展，该计划通过重点的、动手的实验室活动和复杂流体、化学工程和材料科学的实验，向中学女生介绍科学和工程。

ABSTRACT - 0522340University of MichiganGels 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 and coatings. 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. Thus, fundamental understanding that could be applied to predict the gelation transition, to control the microscopic structure and dynamics of gels as well as to exploit their unusual non-linear rheology would broadly impact technology development in these areas. Scientifically, there is a need for experiments that can discriminate between theories of the gelation transition that are based on the mode-coupling framework and on the dynamic equilibrium clustering approach. In addition, the relationship between structural heterogeneity and gel microhydrodynamics should be discovered to understand the role of dynamic heterogeneity in gels. Finally, even qualitative features of local structural evolution upon the application of non-linear deformation have yet to be resolved through experiment. To address the scientific questions that underlie these technological needs, we will execute an experimental research program to observe local correlations among structure, dynamics and non-linear viscoelasticity of colloidal gels.The intellectual merit of our research plan arises from our comprehensive application of confocal optical microscopy in pursuit of these aims. 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. We will principally study suspensions of micron-scale stericallystabilized colloidal poly(methyl methacrylate) in refractive-index and density-matched solvents. Short-range attractive interactions leading to gelation will be induced by non-adsorbing polymer. The control parameter governing the strength of the short-range attraction will be the concentration of polymer. Confocal microscopy of 3D image volumes will be used to quantify the size and heterogeneity of clusters and strings induced by gelation. The local structure will be characterized by measurement of the distribution of contact numbers of particles in the gel. These structural measures will be used to test specific predictions of mode coupling and thermodynamic theories of gelation. To study dynamical heterogeneity and other microhydrodynamical features of gels, structure will be correlated with single and collective particle dynamics quantified by 3D particle tracking. Finally, transient structural evolution will be monitored in start-up of steady-shear flow and after step-strain experiments through in situ confocal microcopy. Novel aspects of the flow experiments will be their attention to non-linear phenomena, their control of wall slip through surface topology engineering, and their execution with materials that will yield a never before available picture of the local, rotational dynamics of colloids in the gel. These direct visualization experiments are distinct from previous light, neutron and X-ray scattering studies because they specifically probe local phenomena and distributions of structure and dynamics that cannot commonly be obtained from the ensemble-averaged results of scattering. This study will broadly impact technology and engineering in areas as diverse as ceramic, membranes and direct write assembly by its elucidation of new scientific understanding of the relationship among the gelation transition, microscopic gel structure and dynamics as well as macroscopic flow. Additional outcomes with broader impact include the training of one graduate student in state-of-the-art methods in confocal microscopy, colloidal science and rheology as well as new development of a summer outreach program that introduces middle school girls to science and engineering through focused, hands on lab activities and experiments in complex fluids, chemical engineering and materials science.