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

0955241 MohrazColloidal凝胶是一种具有受限动力学和显著粘弹性的颗粒材料，在陶瓷、催化剂载体、膜、食品和复合材料等众多技术领域有着广泛的应用。静态微观结构和动态，线性粘弹性，稳态剪切流变这些材料已受到相当大的关注，在理论和实验。 然而，在瞬态大应变变形，是典型的他们的加工条件，以及它如何支配他们的非线性流变过程中，他们的微观结构演变的定量图片，仍然难以捉摸。 拟议的CAREER计划将使用一个结合的实验/计算方案，以解决如何粒子间相互作用的性质和胶体凝胶的局部结构介导其非线性流变学和瞬态变形下定义良好的宏观流场。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，其中凝胶化将通过添加非吸附聚合物来诱导。有了这个模型系统，吸引力的粒子间相互作用的强度和范围可以控制和系统地改变。弱聚集胶体凝胶宏观施加的非线性变形的微观结构响应将直接解决在3D中，并与单颗粒分辨率，通过定量共聚焦激光扫描显微镜（QCLSM）。局部现象，如成对的相对角位移，支持建议的作用，单连接的路口软支点，和微观结构的起源，非线性断裂和应力过冲的胶体凝胶，将直接解决的第一次。通过QCLSM解决的微观结构的演变将被用来作为输入到一个计算模型，以量化的热力学贡献从粒子间键的应力张量，和模型预测将进行比较，实验测量的瞬态剪切应力。这一创新方案将利用QCLSM在解决单粒子分辨率的微观结构中的独特能力，以更好地理解凝胶非线性流变学，特别是其热力学决定因素。粒子间相互作用的几何形状和局部微观结构的作用将通过系统的实验与不等轴胶体和致密系统与刚性局部亚基，分别进行评估。 从拟议的研究活动的技术和结果将直接纳入胶体科学与工程的研究生课程。CAREER计划还将培训研究生和本科生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的最先进的方法。此外，它还将启动一个夏季外联计划，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，外联活动和单元将通过PI的研究网站在全世界传播。

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.