EAGER: Shear Transformational Zones in Amorphous Granular Packings

[09:38 . 21]富兰克林，斯科特直到最近才发展出一种理论来研究非晶材料（如聚合物固体、金属合金或多分散颗粒混合物）的塑性变形。虽然晶体材料被理解为通过位错的产生和传播而变形，但当粒子之间没有长程顺序时，这种理论结构并不适用。目前已经提出，在缺乏长程序的样品中，塑性变形是通过局部剪切转变区（STZs）引起的，并且在分子动力学模拟中已经观察到STZ理论的预测。这项EAGER提案对“环形平面Couette”几何结构中颗粒混合物的剪切变形进行了探索性研究，该几何结构允许任意大的近似纯剪切值。粒子被限制在一个直径比单个粒子大得多的环形中运动。因此，粒子的运动很好地近似于平面，因此有了这个别扭的名字。这一方案的一个显著特点是使用双折射来精确计算作用在单个粒子上的力，这与以前在这一几何领域的工作不同。这样的分析已经在具有旋转成分的Couette剪切和简单的“纯”剪切到低应变中进行，但这些实验将是第一个具有非旋转变形和极大应变的好处的实验。除了测量力外，还将跟踪单个粒子的运动，并定义塑性变形区域。这项探索性工作将确定视频和双折射分析在该几何结构中的有效性，同时为后续研究确定适当的应力和应变率极限。剪切转换带理论提出了几个可检验的预测。最重要的是，它预测了粒子的弹性势能（由与周围粒子的相互作用引起）与粒子的塑性（不可逆）运动之间的关系。拟议中的实验将测试我们同时测量弹性力、能量和粒子运动的能力，从而首次直接测试这一预测。STZ理论还预测了全球应变率、STZ的产生和应用的全球应力场之间的关系。环形平面几何结构允许控制外部施加的应力场，因此这些预测也可以进行测试。由于许多预测都集中在剪切带的行为上，这在以前的实验中没有观察到，我们的初步调查将集中在这种几何形状对这种研究的适用性上。这一建议的智力价值在于发展了第一个实验，允许任意的、无旋流，同时跟踪粒子和它们所受的力。将测量粒子迁移率，并将其与局部势能场相关联，并确定不可逆运动区域（剪切转换区）。这些区域的大小、频率和速度如何取决于颗粒大小、填料分数、外部施加的应力和应变率。所有提议的研究都将涉及本科生。一般来说，本科生研究的价值已经得到了广泛的共识，其好处包括增加了女性和少数民族的保留率和参与度。该项目更广泛的影响在于培养1-2名本科研究人员，为未来软凝聚态物质的研究生工作做准备。长期以来，PI一直与本科生合作，尤其是女性和代表性不足的群体。

0938211Franklin, ScottOnly recently has a theory been developed for the plastic deformation of amorphous materials (e.g. polymer solids, metallic alloys, or polydisperse granular mixtures). While crystalline materials are understood to deform through the creation and propagation of dislocations, such a theoretical construct does not apply when there is no long-range order between particles. It has now been proposed that plastic deformation in samples lacking long-range order is brought about through localized shear transformational zones (STZs), and predictions of STZ theory have been observed in molecular dynamics simulations.This EAGER proposal conducts an exploratory investigation of shear deformation in granular mixtures in an 'annular planar Couette' geometry that allows arbitrarily large values of approximately pure shear. Particles are constrained to move in an annular ring whose diameter is much larger than the size of an individual particle. As a result, the particle motion is to good approximation planar, hence the awkward name. A distinguishing feature of this proposal, separating it from previous work in this geometry, is the use of birefringence to accurately calculate forces on individual particles. Such analysis has been carried out in Couette shear, which has a rotational component, and simple 'pure' shear to low strains, but these experiments will be the first to have the benefits of both irrotational deformation as well as extremely large strains. In addition to measuring the forces, the individual particle motion will be tracked, and regions of plastic deformation defined. This exploratory work will establish the efficacy of video and birefringent analysis in this geometry, while determining appropriate stress and strain-rate limits for subsequent investigations. The theory of shear transformational zones makes several testable predictions. Most importantly, it predicts a relation between a particle's elastic potential energy, caused by interactions with surrounding particles, and that particle's plastic (irreversible) motion. The proposed experiments will test our ability to measure simultaneously the elastic forces, and hence energies, and particle motion, enabling the first direct test this prediction. STZ theory also predicts a relation between the global strain-rate, the creation of STZs, and the applied global stress field. The annular planar geometry allows for control of the externally imposed stress field, so these predictions can also be tested. Because many of the predictions center on the behavior of the shear band, which has not been observed in previous experiments, our preliminary investigation will focus on the suitability of this geometry for such a study. The intellectual merit of this proposal lies in the development of the first experiments to allow for arbitrary, irrotational flow while simultaneously tracking particles and the forces on them. Particle mobility will be measured and correlated with the local potential energy field, and regions of irreversible motion (shear transformational zones) identified. How the size, frequency, and speed of these zones depend on particle size, packing fraction, and externally imposed stress and strain-rate will be measured. All proposed research will involve undergraduates. There is a broad consensus of the value of undergraduate research in general, and benefits include increased retention and participation of women and minorities. The broader impact of this project lies in the training of 1-2 undergraduate researchers for future graduate work in soft condensed matter.The PI has a long track record of working with undergraduates in general, and women and under-represented groups in particular.