Spatiotemporal Dynamics of Stresses in Shear Thickening Suspensions

剪切增稠悬浮液中应力的时空动力学

基本信息

批准号：
1809890
负责人：
Jeffrey Urbach
金额：
$ 49.61万
依托单位：
Georgetown University
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2018
资助国家：
美国
起止时间：
2018-08-01 至 2023-07-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1809890&HistoricalAwards=false
关键词：
Spatiotemporal Dynamics Stresses Shear Thickening

项目摘要

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.

稠密悬浮液由悬浮在流体中的大量固体颗粒组成，表现出各种令人惊讶的机械行为，包括在流速增加的情况下粘度急剧增加。粘度可以增加一百倍或更多，这种效应对各种工业应用和自然过程具有重要影响，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转变的因素以及确定如何通过改变悬浮颗粒的性质来控制转变至关重要。这项工作支持预测模型的开发，可以显着推进各种应用的材料设计和工艺工程。该项目还支持一项试点活动，让一名材料科学研究生参与科学政策的跨学科小组项目，并通过在乔治城大学学院浸入式项目中使用的软材料实践模块向经济困难的高中生进行推广。稠密悬浮液的响应表现出广泛的非线性现象，包括粘度随着剪切应力或应变率的增加而急剧增加。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影响，但在基本层面上仍然没有得到很好的理解。该项目采用了由 PI 开发的新技术“边界应力显微镜”，以高空间和时间分辨率直接测量剪切悬浮液边界处的应力，以阐明剪切增稠转变的性质及其对材料和系统特性的依赖性。剪切增稠过程中边界应力的测量表明，增稠是由于剧烈的动态不均匀性造成的，表明局部不连续过渡到高粘度状态。该项目利用这些见解来 (a) 确定控制高粘度相的成核、生长、演化和衰变的因素； (b) 量化动态高边界应力对悬浮液流场的影响； (c) 将这些结果与纳米尺度颗粒间力的直接测量联系起来； (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) 确定负责确定异质应力特征大小的机制。总的来说，这些测量支持连续介质理论的发展，该理论可以通过准确描述中尺度时空动力学，将微观尺度上的颗粒和流体相互作用与宏观流变学联系起来，从而显着推进材料设计和工艺工程的广泛应用。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。