Stress Formation and Relaxation in colloidal Dispersions: Transient, Nonlinear Microrheology

PI: Zia， roseanna提案号：1605836拟议研究的目标是研究作用在胶体上的力和应力发生变化时的胶体分散性。理论分析和一类被称为加速斯托克动力学模拟的计算将被用来实现这一目标。这项工作可能会对化学和食品工业产生影响，在这些工业中，对材料特性和复杂流体（如胶体）的控制非常重要。该研究的目标是通过主动微流变学的框架，为胶体分散体中粘度和应力的时间形成和松弛建立预测理论和计算模型，重点关注随着时间的推移与颗粒微观结构的联系。迄今为止，大多数非平衡微流变学工作都集中在稳态流动行为上，对稳态非牛顿行为有了深入的了解，并将它们与流体动力、布朗力和粒子间力的相对贡献引起的流动强度变化联系起来。然而，对于这些相对贡献是如何随时间变化的——在流体启动和停止过程中——理解滞后。这样的理解是解决一些重要问题的关键，比如流体启动过程中应力“超调”的起源，以及结构演化和流变演化是否同时达到稳态。拟议的研究结果具有基础科学兴趣，并将影响细胞内流动，钻井液和抗冲击材料等多种应用。PI假设流体动力、颗粒间力和布朗力对胶体分散体中的应力和粘度的相对贡献不是在流动启动时立即形成的，因此，向稳态流变的攀升可能导致或滞后稳态结构的形成。拟议的工作将结合理论和计算流变学研究。理论研究将集中在与时间相关的Smoluchowski方程的公式和解上，然后将用于计算悬浮粘度和应力随时间的变化，适用于任何范围的颗粒间排斥力（即流体动力相互作用的强度），粒径比和吸引势。该教育计划将通过指导学习，利用有指导的动手实验，提高代表性不足的学生的参与度。PI将为康奈尔大学的课堂和外展活动开发流体力学模块，为国家科学院基金会高中的少数族裔学生开发流变学会议和季度“科学日”。

PI: Zia, RoseannaProposal Number: 1605836The goal of the proposed research is to investigate colloidal dispersions when forces and stresses acting on them change. Both theoretical analysis and a class of computations known as Accelerated Stokesian Dynamics simulations will be used to accomplish this goal. The work could have an impact on the chemical and food industry, where control of material properties and of complex fluids, like colloids, is important.The goal of the proposed research is to develop predictive theory and computational models for the temporal formation and relaxation of viscosity and stress in colloidal dispersions via the framework of active microrheology, with a focus on connections to particle microstructure as it evolves in time. Most non-equilibrium microrheological work to date has focused on steady-state flow behavior, yielding insight into steady-state non-Newtonian behaviors, relating them to flow-strength induced changes in the relative contributions of hydrodynamic, Brownian, and interparticle forces. However, understanding of how such relative contributions evolve in time-during flow startup and cessation-lags behind. Such understanding is key to resolving important questions such as the origins of stress "overshoots" during flow startup, and whether structural evolution and rheological evolution reach steady state simultaneously. The results of the proposed research are of fundamental scientific interest and will impact applications as diverse as intracellular flow, drilling fluids, and impact-resistant materials. The PI hypothesizes that the relative contributions of hydrodynamic, interparticle, and Brownian forces to the stress and viscosity in colloidal dispersions are not instantaneously formed upon flow startup, and thus that the climb to steady state rheology may lead or lag formation of steady state structure. The proposed work will combine theoretical and computational rheological studies. Theoretical studies will center on formulation and solution of time-dependent Smoluchowski equations, which will then be utilized to compute suspension viscosity and stress as it evolves in time, for any range of interparticle repulsion (i.e., strength of hydrodynamic interactions), particle size ratio, and attractive potentials. The educational plan will improve the participation of underrepresented students via mentored learning utilizing guided, hands-on experiments. The PI will develop fluid mechanics modules for classrooms and outreach at Cornell University, rheology conferences, and quarterly "Science Days" at National Academy Foundation high schools for underrepresented minority students.