CAREER: Shear-induced Deformation and Nonlinear Rheology of Colloidal Gels

胶体凝胶是一种颗粒材料，具有有限的动力学和显著的粘弹性，并享有多种技术应用，包括陶瓷，催化剂载体，膜，食品和复合材料。这些材料的静态微观结构和动力学、线性粘弹性和稳态剪切流变学在理论上和实验上都得到了相当大的关注。然而，在瞬态大应变变形（典型的加工条件）过程中微观结构演变的定量图像，以及它如何控制其非线性流变，仍然难以捉摸。拟议的CAREER计划将使用实验/计算相结合的方案来解决粒子间相互作用的性质和胶体凝胶的局部结构如何在定义良好的宏观流场下调节其非线性流变和瞬态变形。模型材料将是荧光聚甲基丙烯酸甲酯微球悬浮液，悬浮在密度和指数匹配的有机溶剂中，其中通过添加非吸附聚合物来诱导凝胶化。有了这个模型系统，吸引粒子间相互作用的强度和范围可以被控制和系统地改变。通过定量共聚焦激光扫描显微镜（QCLSM），可以直接在三维单粒子分辨率下解析弱聚集的胶体凝胶对宏观施加的非线性变形的微观结构响应。局部现象，如成对相对角位移，支持单连接结点作为软枢轴点的建议作用，以及胶体凝胶中非线性破裂和应力超调的微观结构根源，将首次直接解决。QCLSM解析的微观结构演变将被用作计算模型的输入，以量化粒子间键对应力张量的热力学贡献，并将模型预测与瞬态剪切应力的实验测量结果进行比较。这一创新方案将利用QCLSM在单粒子分辨率下解析微观结构的独特能力，更好地理解凝胶的非线性流变学，特别是其热力学决定因素。粒子间相互作用几何和局部微观结构的作用将分别通过非等长胶体和具有刚性局部亚基的致密系统进行系统实验来评估。拟议研究活动的技术和结果将直接纳入胶体科学与工程研究生水平课程。CAREER项目还将培养研究生和本科生使用最先进的定量共聚焦显微镜、胶体合成、数据驱动建模和复杂流体流变学方法。此外，它还将启动一项夏季推广计划，旨在向传统上在这些学科中代表性不足的高中生灌输对科学和工程职业的兴趣，并为本科生和研究生提供指导机会。最后，外联活动和模块将通过PI的研究网站向全世界传播。

0955241MohrazColloidal gels are particulate materials with constrained dynamics and remarkable viscoelasticity, and enjoy a diverse host of technological applications including ceramics, catalyst supports, membranes, food products, and composites. The quiescent microstructure and dynamics, linear viscoelasticity, and steady shear rheology of these materials have received considerable attention, both theoretically and experimentally. However, a quantitative picture of their microstructural evolution during transient large strain deformations that are typical of their processing conditions, and how it governs their nonlinear rheology, has remained elusive. The proposed CAREER program will use a combined experimental/computational scheme to address how the nature of interparticle interactions and the local structure of colloidal gels mediate their nonlinear rheology and transient deformation under well defined macroscopic flow fields. The model materials will be suspensions of fluorescent poly(methylmethacrylate) microspheres suspended in density and index matching organic solvents, in which gelation will be induced by the addition of a non adsorbing polymer. With this model system, the strength and range of attractive interparticle interactions can be controlled and systematically varied. The microstructural response of weakly aggregating colloidal gels to macroscopically imposed nonlinear deformation will be directly resolved in 3D, and with single particle resolution, by quantitative confocal laser scanning microscopy (QCLSM). Local phenomena, such as pair wise relative angular displacements, which underpin the suggested role of singly connected junctions as soft pivot points, and the microstructural origins of nonlinear rupture and stress overshoot in colloidal gels, will be directly resolved for the first time. The microstructural evolution as resolved by QCLSM will be used as input into a computational model to quantify the thermodynamic contribution from the interparticle bonds to the stress tensor, and the model predictions will be compared to experimental measurements of the transient shear stress. This innovative scheme will exploit the unique power of QCLSM in resolving the microstructure with single particle resolution, to develop a better understanding of gel nonlinear rheology, and specifically, its thermodynamic determinants. The roles of interparticle interaction geometry and local microstructure will be assessed through systematic experiments with anisometric colloids and dense systems with rigid local subunits, respectively. The techniques and findings from the proposed research activities will be directly incorporated into a graduate level course in Colloid Science and Engineering. The CAREER program will also train graduate and undergraduate students in state of the art methods of quantitative confocal microscopy, colloid synthesis, data driven modeling, and complex fluids rheology. Moreover, it will initiate a summer outreach program, aiming to instill interest in science and engineering careers in high school students from populations that are traditionally underrepresented in these disciplines, and provide the undergraduate and graduate students with a mentoring opportunity. Finally, the outreach activities and modules will be disseminated worldwide via the PI's research website.