Flow, Memory and Aging of Soft Particle Pastes

该奖项由2009年《美国复苏和再投资法案》(公法111-5)资助。0854420 Bonecazazine智能优点：许多浓缩材料由软颗粒(如乳液、弹性颗粒、微凝胶、星形聚合物或聚合物涂层颗粒)填充并变形为无定形状态。这些浆体作为食品、材料和涂层工艺的流变剂、水泥泵送和水力压裂中的减摩剂非常重要。然而，他们的行为有几个方面还没有完全被理解。软颗粒浆料(SPP)表现出复杂的剪切变稀、正应力和屈服应力、涂层不稳定性、滑移、记忆和老化。它们的老化行为类似于结构玻璃、自旋玻璃和聚合物玻璃。我们建议开发模拟和模型来描述这些软颗粒浆体的行为，以便：1)建立具有任意粒间势的软颗粒浆体的完整流变模型，以描述其弹塑性流动；2)识别流动过程中发生的、引起软颗粒浆体中老化和记忆的微观结构事件，以加速、停止或以其他方式缓解这些影响；3)测试现有的老化理论，并开发专门考虑弹流相互作用的关于这些材料的老化和记忆的新理论。为了实现这些目标，我们将建立在我们对这些材料的弹流相互作用的重要性的发现以及我们最近对微凝胶悬浮液的粘弹性性质进行剪切的模拟和实验的基础上。具体地说，我们将：1)发展一种理论，用松弛的、无应变的浆体在任意粒间力作用下的成对颗粒分布函数来描述微结构，并预测SPP的弹性性质；2)发展一种确定流动扰动分布函数的理论，从而预测SPP的屈服强度和一般粘弹性性质；3)修改我们现有的基于粒子的软颗粒浆体的三维动态模拟，以模拟老化和记忆；4)使用模拟来对与宏观行为和微观结构重排相关的软颗粒浆体的老化和记忆进行参数研究；5)对现有模型进行评价，建立新的软颗粒浆体流变学、老化和记忆模型。对于这项工作的所有阶段，我们将继续将我们的理论预测与文献中的实验和ESPCI的合作者进行的实验进行比较。这些模拟将允许宏观特性(例如屈服应力、模数、有效粘度、法向应力、老化和记忆)与软颗粒的微观相互作用之间的直接联系，并从根本上了解其流变性的起源。更广泛的影响：表面活性物质的流变性似乎在尺寸从纳米到数百微米的一系列可变形颗粒中普遍存在。从这项工作中获得的知识将为工程师和胶体科学家提供一种手段，使软颗粒浆料的配方具有所需的流变性。从这项工作中预测非平衡微观结构的理论也将为模拟高度浓缩的软颗粒和其他复杂流体的无定形悬浮液的性质提供一种新的方法。在浆料中看到的老化和记忆现象也在许多其他系统中看到，包括向列相聚合物和自旋和结构玻璃。这项拟议的工作将提供对这些行为的基本见解。浆料中的老化现象是悬浮剂配方保质期的更广泛问题的一部分，也是一个具有重要工业意义的问题。这项工作的结果将为控制老化和保质期提供指导。在整个项目过程中，这项建议将涉及一名研究生和两名本科生的教育。他们将熟练掌握复杂流体的流变学、计算模拟和建模。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).0854420BonnecazeIntellectual Merit: Many concentrated materials consist of soft particles (e.g., emulsions, elastic particles, microgels, star polymers or polymer-coated particles) packed and deformed into an amorphous state. These pastes are important as rheological modifiers for food, materials and coating processes, friction reduction in cement pumping and hydrofracturing. There are, however, several aspects of their behavior that are not fully understood. Soft particle pastes (SPPs) display complex shear thinning, normal and yield stresses, coating instabilities, slip, memory and aging. Their aging behavior is similar to that in structural, spin and polymer glasses. We propose to develop simulations and models to describe the behavior of these soft-particle pastes in order to: 1) develop a complete rheological model for soft particle pastes with arbitrary interparticle potentials to describe their elasto-plastic flow; 2) identify the microstructural events that occur during flow and that give rise to aging and memory in soft particle pastes in order to accelerate, stop or otherwise mitigate these effects; 3) test existing theories of aging and develop a new theory for aging and memory of these materials specifically accounting for the elastohydrodynamic interactions. To achieve these goals we will build on our discovery of the importance of elastohydrodynamic interactions for these materials and our recent simulations and experiments on the viscoelastic properties of microgel suspensions undergoing shear. Specifically, we will: 1) develop a theory to describe the microstructure in terms of the pairwise, particle distribution function of the relaxed, unstrained paste for arbitrary interparticle forces and predict elastic properties of SPPs; 2) develop a theory that determines the perturbed distribution function under flow and thus predict the yield strength and the general viscoelastic properties of SPPs; 3) modify our existing 3D particle-based dynamic simulation of soft particle pastes to simulate aging and memory; 4) use the simulations for parametric studies of aging and memory of soft particle pastes correlating macroscopic behavior and microstructural rearrangements; 5) evaluate existing models and develop new models for the bulk rheology, aging and memory of soft particle pastes. For all stages of this work, we will continually compare our theoretical predictions to experiments in the literature and those conducted by a collaborator at the ESPCI. The simulations will allow the direct connection between the macroscopic properties (e.g., yield stress, modulus, effective viscosity, normal stresses, aging and memory) to the microscopic interactions of the soft particles and a fundamental understanding of the origins of their rheology. Broader Impacts: The rheology of SPPs appears to be universal across a broad array of deformable particles ranging in size from nanometers to hundreds of microns. The knowledge gained from this work will provide engineers and colloid scientists a means to tailor the formulation of soft particle pastes to have desired rheological properties. Theories from this proposed work for predicting the non-equilibrium microstructure will also provide a new methodology for modeling the properties of highly concentrated, amorphous suspensions of soft particles and other complex fluids. The aging and memory phenomena seen in pastes are also seen in many other systems, including nematic polymers and spin and structural glasses. The proposed work will provide fundamental insights into these behaviors. The aging phenomenon in pastes is part of a broader issue of and an industrially important concern about shelf life of formulations of suspensions. The results of this work will provide guidance into controlling aging and shelf-life. This proposed will involve the education of one graduate student and two undergraduate students over the course of the project. They will become skilled in rheology of complex fluids, computational simulation and modeling.