Orientation and stress dynamics in shear thickening colloidal rod suspensions

由悬浮在流体中的固体颗粒组成的致密悬浮液表现出广泛的令人惊讶的力学行为，包括在增加流速下粘度的急剧增加。这种被称为剪切增厚的现象可以用于重要的应用，包括能量耗散、减震和抗冲击。然而，剪切增厚也会在材料运输和加工中产生不良影响。虽然在理解球形和其他致密颗粒悬浮液中的剪切增厚方面取得了实质性进展，但对棒状颗粒悬浮液中的剪切增厚知之甚少。这些体系可以在颗粒浓度远低于球形颗粒悬浮液中剪切增稠所需的浓度时显示出戏剧性的剪切增稠。这表明，具有相对较低浓度的超长棒的悬浮液可以在应用中利用剪切增厚行为，或者在剪切增厚没有帮助的情况下，在材料运输中提出挑战。抽油杆的流动方向动力学是抽油杆悬浮液性质的关键决定因素。这项工作旨在描述和理解剪切增稠状态下的定向动力学，从而能够预测和控制快速流动悬浮液的粘度。研究团队还将参与“科学/政策界面”（Science/Policy interface），这是一个让材料科学研究生参与科学政策跨学科小组项目的试点项目，并通过在乔治城大学浸入式课程中使用的动手软材料模块，为经济困难的高中生提供推广活动。该研究的目的是表征胶态二氧化硅棒悬浮液的剪切增厚特性，该悬浮液可以用一系列宽高比合成，并利用新技术来确定棒的方向动态，并直接测量剪切增厚过程中的应力波动。使用定制的高分辨率显微镜与商用流变仪相结合，研究人员将能够直接测量颗粒的空间和时间分辨取向动力学。同时，他们将使用他们开发的一种新技术来测量局部的边界应力。结合这两种技术将使研究小组能够确定方向波动和应力波动之间的相互作用，从而确定决定剪切增厚的动态特性。然后，研究人员将把这种方法扩展到具有可控纳米级粗糙度的硅棒上。取向在各向异性分子和粒子的行为中起着至关重要的作用，通过流动、外场或边界效应控制取向的能力是广泛的商业应用的核心。该研究将开发和测试新的工具和方法，以表征定向在抽油杆悬浮液中的作用，并提供必要的表征，以确定操纵颗粒定向以控制剪切增厚的机会。更一般地说，拟议的研究将支持开发和传播一种强大的技术来测量和分析流动悬浮液的方向和应力动力学。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Dense suspensions composed 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. This phenomenon, known as shear thickening, can be exploited for important applications, including energy dissipation, shock absorption and impact resistance. However, shear thickening can also produce undesirable effects in material transport and processing. While substantial progress has been made in understanding shear thickening in suspensions of spherical and other compact particles, much less is known about shear thickening in suspensions of rod-like particles. These systems can show dramatic shear thickening at particle concentrations much lower than the concentration required for shear thickening in suspensions of spherical particles. This suggests the possibility that suspensions with relatively low concentrations of very long rods can take advantage of shear thickening behavior in applications, or present challenges in material transport when shear thickening is not helpful. The dynamics of the orientations of the flowing rods is a critical determinant of the properties of suspensions of rods. This work seeks to characterize and understand the orientation dynamics in the shear thickening regime to enable prediction and control of the viscosity of rapidly flowing suspensions. The research team will also participate with the Science/Policy interface, a pilot project to involve a materials science graduate student in an interdisciplinary group project in science policy, and with outreach activities aimed at economically disadvantaged high school students through hands-on soft materials modules for use in the Georgetown University College Immersion Program.The objective of the proposed research is to characterize shear thickening in suspensions of colloidal silica rods, which can be synthesized with a range of aspect ratios, and to utilize novel techniques to determine the dynamics of rod orientations anddirectly measure stress fluctuations during shear thickening. Using a customized high resolution microscope coupled with commercial rheometers, the investigators will be able to directly measure the spatially and temporally resolved orientation dynamics of the particles. In parallel, they will use a new technique they have developed to measure boundary stresses locally. Combining these two techniques will enable the team to determine the interplay between orientation fluctuations and stress fluctuations, and therefore identify the dynamical characteristics that determine shear thickening. The researchers will then extend this approach to silica rods with controlled nanoscale roughness. Orientation plays a critical role in the behavior of anisotropic molecules and particles, and the ability to control orientation through flow, external fields, or boundary effects is central to a wide range of commercial applications. The proposed research will develop and test novel tools and methods for characterizing the role of orientation in rod suspensions, and provide the characterization necessary to identify opportunities to manipulate particle orientation to control shear thickening. More generally, the proposed research will support the development and dissemination of a powerful techniques for measuring and analyzing orientation and stress dynamics of flowing suspensions.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.