Using bidirectional shear protocols to determine microstructural changes responsible for thickening and dethickening in colloidal suspensions

剪切增稠流体的粘度在剧烈流动时会增加。这一特性使许多引人入胜的自然和工业现象成为可能，从人们在水混合物中充满玉米淀粉的池子中行走，到混凝土的加工特性。最近的研究已经开始确定颗粒悬浮液中剪切增稠的机理。这些研究表明，悬浮粒子之间的接触形成了一个相互作用的粒子的作用力网络，该网络与流动相一致。以前的工作已经表明，当与流动方向垂直推挤时，这些网络在某些情况下可以被打破，从而使悬浮液完全减稠。该项目将确定这种减稠方法是否取决于悬浮液的密度。新的实验、模拟和理论将被用来确定控制这一现象的方程。这样的方程将预测任意流动几何形状下的悬浮行为，并使控制粘度的新策略成为可能。由于控制胶体悬浮液的增稠对牙膏、油漆和水泥等产品的加工非常重要，这项工作将产生非常广泛的影响，有可能改变科学界和工业界的做法，使美国经济和社会受益。了解和控制剪切稠化流体的非牛顿行为对于提高许多工业现象的效率至关重要，从控制钻井泥浆的流动到混凝土的加工。最近的研究表明，稠密胶体悬浮液中的剪切增稠是由于悬浮颗粒之间形成接触而产生的，这些接触建立了一个与流动相一致的力支撑网络。以前关于连续剪切增稠的工作已经表明，当扰动与流动方向垂直时，这些网络可以被打破，从而使悬浮液完全减稠。该奖项将支持旨在解决知识空白的实验、模拟和理论，这些空白将通过解决剪切浓缩领域的三个悬而未决的问题来帮助开发更有效和更广泛适用的减厚策略。首先，建议通过实验和模拟来确定是否可以将正交剪切技术推广到不连续剪切增厚和剪切干扰区域。其次，建议开发新的剪切方案来确定粘度的张量性质。这些测量和模拟将被用来构建粘度的张量模型。这样的理论将有助于确定其他流动几何形状中的接触网络。第三，将使用最先进的成像共聚焦流变学技术来成像引起增厚的接触网络。这些数据将使对联系网络的理论预测进行测试。总而言之，这些项目将加深对剪切增稠的理解，旨在提高应用垂直剪切的效率，并将其应用范围扩大到其他流动。该奖项将推动科学进步，最终造福美国经济和社会。此外，还提出了一些教育活动，从专注于增稠液体的推广到继续为研究生和博士后开发科学交流课程。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Shear-thickening fluids have viscosities that increase during vigorous flows. This property enables many fascinating natural and industrial phenomena ranging from people walking across pools filled with cornstarch in water mixtures, to the processing properties of concrete. Recent studies have started to define the mechanism of shear thickening in particle suspensions. These studies have shown that formation of contacts between the suspended particles acts to build a force-supporting network of interacting particles that aligns with the flows. Previous work has shown that when jostled perpendicularly to the flow direction these networks can in some cases be broken apart allowing for complete dethickening of the suspension. This project will determine if this dethickening approach depends on the density of the suspension. New experiments, simulations, and theory will be used to determine the equations governing this phenomenon. Such equations will predict suspension behavior in arbitrary flow geometries and enable new strategies for controlling the viscosity. Because controlling the thickening of colloidal suspensions is important for processing products such as tooth-pastes, paints, and cement, this work will have very broad impact with the potential to change practices in both the scientific and industrial communities to the benefit of the US economy and society. Understanding and controlling the non-Newtonian behavior of shear-thickening fluids is crucial for increasing the efficiency of many industrial phenomena ranging from controlling the flow of drilling muds to the processing of concrete. Recent studies have shown that shear thickening in dense colloidal suspensions arises from the formation of contacts between the suspended particles, which act to build a force supporting network that aligns with the flows. Previous work on continuous shear thickening has shown that when perturbed orthogonally to the flow direction these networks can be broken apart allowing for complete dethickening of the suspension. This award will support experiments, simulations, and theory aimed at addressing knowledge gaps that would aid development of more efficient and broadly applicable dethickening strategies by tackling three outstanding questions in the area of shear thickening. First, it is proposed to use experiments and simulations to determine whether the orthogonal shear technique can be extended to the discontinuous shear thickening and shear jamming regimes. Second, it is proposed that new shear protocols be developed to determine the tensorial nature of the viscosity. These measurements along with the simulations will be used to construct a tensorial model of the viscosity. Such a theory would aid in determining the contact network in other flow geometries. Third, a state of the art imaging confocal rheology technique will be used to image the contact network giving rise to thickening. These data will enable testing of theoretical predictions for the contact network. Collectively, these projects will yield a deeper understanding of shear thickening aimed at improving the efficiency of applying orthogonal shear and extending its range of application to other flows. This award will promote the progress of science and ultimately benefit the US economy and society. Additionally, a number of educational activities are proposed ranging from outreach focused around thickening fluids to continued development of a science communication course for graduate students and postdocs.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.