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

0955241 Mohraz胶体凝胶是一种具有约束动力学和显著粘弹性的颗粒材料，具有广泛的技术应用，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、线粘弹性和稳态剪切流变性在理论和实验上都受到了极大的关注。然而，它们在瞬时大应变变形过程中微观结构演变的定量图像仍然难以捉摸，这是它们加工条件的典型特征，以及它如何控制它们的非线性流变学。拟议的CARARE计划将使用实验/计算相结合的方案来研究胶体凝胶的颗粒间相互作用的性质和局部结构如何在明确定义的宏观流场下调节其非线性流变性和瞬时变形。模型材料将是悬浮在密度和折射率匹配的有机溶剂中的荧光聚甲基丙烯酸甲酯微球的悬浮液，其中通过添加不吸附的聚合物来诱导凝胶。利用这个模型系统，可以控制和系统地改变吸引粒子间相互作用的强度和范围。弱聚集胶体凝胶对宏观非线性变形的微观结构响应将通过定量共聚焦激光扫描显微镜(QCLSM)直接在三维和单粒子分辨率下进行解析。将首次直接解决局部现象，如成对相对角位移，这些现象支持单连接连接作为软枢点的作用，以及胶体凝胶中非线性破裂和应力超调的微观结构根源。由QCLSM解析的微结构演化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，并将模型预测与瞬时剪应力的实验测量进行比较。这一创新方案将利用QCLSM在以单颗粒分辨率分辨微观结构方面的独特能力，以更好地了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构型和局部微结构的作用将分别通过系统的非等距胶体和具有刚性局部亚单位的致密体系的实验来评估。拟议研究活动的技术和发现将直接纳入胶体科学和工程研究生水平课程。该职业计划还将培训研究生和本科生，掌握定量共焦显微镜、胶体合成、数据驱动建模和复杂流体流变学的最新方法。此外，它还将启动一个暑期推广计划，旨在向高中生灌输对科学和工程职业的兴趣，这些学生传统上在这些学科中的比例较低，并为本科生和研究生提供指导机会。最后，外展活动和单元将通过国际和平研究所的研究网站在世界各地传播。

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.