EXPLORATORY: Microrheology of Particulate Gels

了解相互作用、微观结构和微观力学如何决定复杂流体的宏观性质和响应，是关系到材料加工与发展、生物学和医学的基本问题。对于陶瓷部件制造、涂层、矿物回收和润滑油降解中出现的颗粒凝胶，我们试图了解粘弹性、屈服和非线性行为的微观根源，这些行为最终影响加工和最终性能，如热和质量传输特性。这项研究的目的是探索使用直接的显微操作和成像来了解颗粒凝胶中微观和宏观性质之间的关系。PI将开发和使用基于光学微操作和可视化的新实验工具，包括结合视频、荧光和共焦显微镜的光学捕获。由于这些技术可以通过在原位量化结构重排、应力和相互作用来研究流变学和力学行为的基本微观机制，因此它们是其他组使用的散射法和流变学方法的有力补充。具体研究目的是：1.直接测量凝胶背骨模型的微观力学性能。为了了解颗粒凝胶中凝胶弹性、剪切和压缩屈服行为的基本机制，PI将开发光学捕获和显微技术来测量凝胶骨架模型集合体的弯曲刚度、频率响应和松弛时间。利用视频显微镜，等电点将表征与蠕变、屈服和非线性行为有关的颗粒重排机制。等电点将系统地改变颗粒间的相互作用、颗粒浓度和多分散性，并表征表面的非均质性。测量絮体和絮体-絮体界面的力学性能。利用为测量凝胶骨架特性而设计的实验技术，PI将测量凝聚过程中形成的絮体的频率响应和破裂机理。同时进行的视频显微镜和光学捕捉将使PITO能够表征和关联絮体中的力学和结构重排，这些重排导致分形胶体凝胶中的应变硬化。等电点将测量絮体之间的力学，以区分内部和絮体力学在凝胶流变学中的贡献。开发用于致密凝胶悬浮液的光学捕获和显微镜。要建立颗粒凝胶的单个集合体力学、微观流变学和流变学之间的关系，将需要对散装悬浮液进行微观力学测量。PI将开发适当的核壳粒子和实验技术，以利用在稠密悬浮液中同时使用光学捕获和共聚焦显微镜的优势。这将使PI能够直接显示结构，同时在浓缩的块状凝胶中诱导局部变形和应力，最终提供一系列表征应力松弛的机制。

Understanding how interactions, microstructure and microscopic mechanics determinethe macroscopic properties and responses of complex fluids is a fundamental problem thatimpacts materials processing and development, biology and medicine. For particulate gelsthat occur in the manufacture of ceramic parts, coatings, mineral recovery and lubricantdegradation, we seek to understand the microscopic origins of viscoelastic, yield, and non-linear behavior that ultimately affects processing and final properties, such as the thermaland mass transport characteristics. The goal of this research is to explore the use of direct microscopic manipulationand imaging to understand the relationship between microscopic and macroscopic propertiesin particulate gels. The PI will develop and employ new experimental tools based on opticalmicromanipulation and visualization, including optical trapping in combination with video,fluorescence, and confocal microscopies. Because the techniques will allow investigation of thebasic, microscopic mechanisms of rheological and mechanical behavior by quantifying struc-tural rearrangements, stresses and interactions in situ, they are powerful complements toscattering and rheological methods used by other groups.Specific research aims are:1. Directly measure the micromechanical properties of models of the gel back-bone. To understand fundamental mechanisms of gel elasticity, shear and compressiveyield behavior in particulate gels, the PI will develop optical trapping and microscopy tech-niques to measure the bending stiffness, frequency response, and relaxation timescalesof model aggregates of the gel backbone. With videomicroscopy, the PI will characterizeparticle rearrangement mechanisms that are relevant to creep, yield and non-linear be-havior. The PI will systematically vary interparticle interactions, particle concentration,and polydispersity, and characterize surface heterogeneity.2. Measure the mechanical properties of flocs and floc- floc interfaces. Usingthe experimental techniques designed for measuring gel backbone properties, the PI willmeasure the frequency response and rupturing mechanics of flocs formed during theaggregation process. Concurrent videomicroscopy and optical trapping will enable the PIto characterize and correlate mechanics and structural rearrangements in flocs that giverise to strain-hardening in fractal colloidal gels. The PI will measure mechanics betweenflocs to distinguish the contribution of internal and floc- floc mechanics in the rheologyof gels.3. Develop optical trapping and microscopy for dense, gelling suspensions.Establishing the relationship between individual aggregate mechanics, microrheologyand rheology of particulate gels will require micromechanical measurements in bulksuspensions. The PI will develop the appropriate core-shell particles and experimentaltechniques to take advantage of simultaneous optical trapping and confocal microscopyin dense suspensions. This will enable the PI to directly visualize structure while inducinglocal deformation and stresses in the concentrated, bulk gel, ultimately providing ameans of characterizing mechanisms of stress relaxation.