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

剪切增稠流体具有在剧烈流动期间增加的粘度。 这种特性使许多迷人的自然和工业现象，从人们走过充满玉米淀粉的水池到混凝土的加工性能。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，悬浮颗粒之间接触的形成起到了建立相互作用颗粒的力支撑网络的作用，该网络与流动对齐。以前的工作表明，当垂直于流动方向推挤时，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种现象的方程。这些方程将预测悬浮液在任意流动几何形状下的行为，并实现控制粘度的新策略。由于控制胶体悬浮液的增稠对于牙膏、油漆和水泥等产品的加工非常重要，因此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，造福美国经济和社会。 了解和控制剪切增稠流体的非牛顿行为对于提高从控制钻井泥浆的流动到混凝土处理的许多工业现象的效率至关重要。最近的研究表明，在稠密的胶体悬浮液中，剪切增稠是由于悬浮颗粒之间形成接触而引起的，这种接触的作用是建立一个与流动对齐的力支撑网络。先前关于连续剪切增稠的工作已经表明，当与流动方向正交地扰动时，这些网络可以被分开，从而允许悬浮液完全去稠。该奖项将支持旨在解决知识差距的实验，模拟和理论，通过解决剪切增稠领域的三个突出问题，帮助开发更有效和广泛适用的减稠策略。 首先，建议使用实验和模拟来确定正交剪切技术是否可以扩展到不连续剪切增稠和剪切堵塞制度。其次，它建议，新的剪切协议被开发，以确定张量性质的粘度。这些测量沿着模拟将被用于构建粘度的张量模型。这种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焦流变技术来对引起增稠的接触网络进行成像。这些数据将能够测试接触网的理论预测。总的来说，这些项目将产生一个更深入的了解剪切增稠，旨在提高效率的应用正交剪切和扩大其应用范围，以其他流动。这一奖项将促进科学进步，最终造福美国经济和社会。此外，还提出了一些教育活动，从围绕增稠流体的外展活动到继续为研究生和博士后开发科学传播课程。该奖项反映了NSF的法定使命，并被认为值得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来支持。

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.