Spatiotemporal Dynamics of Stresses in Shear Thickening Suspensions

由大量悬浮在流体中的固体颗粒组成的致密悬浮液表现出各种令人惊讶的机械行为，包括在增加的流速下粘度的急剧增加。粘度可以增加100倍或更多，这种效应对各种工业应用和自然过程具有重要意义，但在基本层面上仍然没有得到很好的理解。在这个项目中，研究人员采用了一种新的测量技术来直接测量增稠过程中流动悬浮液边界处的力。这些测量对于确定控制增稠过渡的因素以及确定如何通过改变悬浮颗粒的性质来控制过渡是必不可少的。这项工作支持预测模型的开发，可以显着推进材料设计和工艺工程的广泛应用。该项目还支持一项试点活动，让材料科学研究生参与科学政策的跨学科小组项目，并通过双手帮助经济困难的高中生-在乔治城大学学院沉浸计划中使用的软材料模块上。稠密悬浮液的机械响应表现出广泛的非线性现象，包括粘度随着剪切力的增加而急剧增加应力或应变率。粘度可以增加几个数量级，这种效应对各种工业应用和自然过程具有重要意义，但在基本层面上仍然没有得到很好的理解。该项目采用了一种新的技术，边界应力显微镜，由PI开发，直接测量高空间和时间分辨率的剪切悬浮液的边界处的应力，以阐明剪切增稠过渡的性质及其对材料和系统特性的依赖性。剪切增稠过程中的边界应力的测量表明，增稠是由于戏剧性的，动态的不均匀性，表明一个局部的不连续过渡到一个高粘度状态。该项目利用这些见解来（a）确定控制高粘度相的成核、生长、演化和衰减的因素;（B）量化动态高边界应力对悬浮液流场的影响;（c）将这些结果与纳米级颗粒间力的直接测量相关联;（d）将这些结果与纳米级颗粒间力的直接测量相关联。（d）采用振荡剪切来探测与向高粘度相转变相关的应变幅度和时间尺度;（e）确定负责确定非均匀应力的特征大小的机制。总的来说，这些测量支持连续统理论的发展，该理论可以通过准确描述中尺度时空动力学，将微观尺度下的颗粒和流体相互作用与宏观流变学联系起来，该奖项反映了NSF的法定使命，并通过利用基金会的智力价值进行评估而被认为值得支持和更广泛的影响审查标准。

Dense suspensions, comprised of a large quantity of solid particles suspended in a fluid, exhibit a wide range of surprising mechanical behavior, including a dramatic increase in viscosity under increasing flow rates. The viscosity can increase by a factor of a hundred or more, an effect that has important implications for a variety of industrial applications and natural processes, but is still not well understood on a fundamental level. In this project the researchers employ a new measurement technique to directly measure the forces at the boundary of flowing suspensions during the thickening process. These measurements are essential to identify the factors that control the thickening transition, and to determine how to control the transition by modifying the properties of the suspended particles. This work supports the development of predictive models that can significantly advance materials design and process engineering for a wide range of applications. The project also supports a pilot activity to involve a materials science graduate student in an interdisciplinary group project in science policy, and outreach to economically disadvantaged high school students through hands-on soft materials modules for use in the Georgetown University College Immersion Program.The mechanical response of dense suspensions exhibits a wide range of nonlinear phenomena including a dramatic increase in the viscosity with increasing shear stress or strain rate. The viscosity can increase by several orders of magnitude, an effect that has important implications for a variety of industrial applications and natural processes, but is still not well understood on a fundamental level. This project employs a novel technique, Boundary Stress Microscopy, developed by the PIs to directly measure with high spatial and temporal resolution the stresses at the boundary of sheared suspensions, to elucidate the nature of the shear thickening transition and its dependence on material and system properties. Measurements of boundary stresses during shear thickening have revealed that the thickening is due to dramatic, dynamic heterogeneities indicating a localized discontinuous transition to a high viscosity state. This project leverages those insights to (a) identify the factors that control the nucleation, growth, evolution, and decay of the high viscosity phase; (b) quantify the effect of the dynamic high boundary stresses on the flow field of the suspension; (c) relate these results to direct measurement of interparticle forces at the nanoscale; (d) employ oscillatory shear to probe the strain amplitudes and timescales associated with the transition to the high viscosity phase; (e) identify the mechanisms responsible for determining the characteristic size of the heterogeneous stresses. Collectively, these measurements support the development of continuum theories that can connect particle and fluid interactions at the microscale to macroscopic rheology by accurately describing the mesoscale spatiotemporal dynamics, thereby significantly advancing materials design and process engineering for a wide range of applications.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.