Collaborative Research: Viscoelastic Effects at the Nanoscale: Probe Rheology Theory and Simulations

该奖项支持理论和计算研究，以及开发计算机模拟方法来确定纳米尺度上材料的机械性能的教育，纳米尺度比人类头发的直径小10万倍。化学合成和制造技术的最新进展使得纳米技术的应用越来越普遍。例子包括用于运动器材的纳米复合材料和基于纳米颗粒的药物递送。这些纳米系统的有趣特性是由系统中分子之间的特定相互作用决定的，这些特性使它们具有实用性和实用性。用于测量机械性能的传统仪器无法探测重要的分子相互作用及其对系统纳米级机械性能的影响。在这个项目中，将创建一个强大的理论模拟技术，用于研究材料的分子特性与其纳米级机械性能之间的关系。纳米尺度的实验本身就很困难，而且非常昂贵。如果成功，该项目开发的技术可以帮助指导实验，从而节省时间和成本。这种形式也可以用于设计实际应用，例如使用纳米颗粒为基础的药物输送来治疗癌症。这项工作的教育方面将包括研究生的多学科培训。推广活动将通过参与一个旨在让初高中女生对科学事业感兴趣的项目来实现。该奖项支持理论和计算研究，以及开发探针颗粒微流变学的计算机模拟方法的教育，其中包括通过监测微观探针颗粒在介质中的运动来确定材料的粘弹性特性。该方法已成为测定复杂软物质粘弹性的一种成熟的实验技术。实验技术的最新进展使这项技术在纳米尺度上的应用成为可能。感兴趣的纳米级粘弹性特性是由系统在这些长度尺度上的特定相互作用和结构决定的。因此，模拟技术对于解释探针颗粒纳米流变性实验结果在系统的分子性质方面是有价值的。除了需要明确地解释分子相互作用之外，这种努力还面临着纳米尺度上的额外挑战，例如扩展频率范围的可用性，以及在分析粒子轨迹时需要考虑介质和粒子惯性。该项目将创建纳米级珠流变的颗粒理论模拟技术，该技术将允许研究复杂物质中的粘弹性。广义Stokes-Einstein关系（IGSER）的惯性扩展将用于从纳米粒子运动中提取介质粘弹性。pi将集中在三个方面：(1)由于模拟中使用的周期性边界条件导致运动探针粒子图像之间的远程流体动力相互作用，因此在分子模拟中可以研究的频率范围受到严重限制。将创建一个框架来修改粒子轨迹分析程序，以便定量地解释这些水动力相互作用。这样的形式将显著地扩展可以通过模拟探测的频率范围。(2)假定探针流变性能确定介质的局部粘弹性特性。这一假设将通过创建一个具有纳米级温度梯度的模型系统来验证，这反过来又会导致粘弹性的梯度。(3)颗粒流变学通常用来根据探测颗粒运动的知识来推断粘弹性。在给定介质的粘弹性谱的情况下，在相反方向上使用颗粒流变性-预测复杂介质中颗粒运动的程度-将在该项目中进行研究。完成这些任务将产生一种理论与模拟相结合的技术，用于测定复杂软物质的纳米级粘弹性。纳米尺度的实验本身就很困难，而且非常昂贵。如果成功，该项目开发的技术可以帮助指导实验，从而节省时间和成本。这种形式也可以用于设计实际应用，例如使用纳米颗粒为基础的药物输送来治疗癌症。这项工作的教育方面将包括研究生的多学科培训。推广活动将通过参与一个旨在让初高中女生对科学事业感兴趣的项目来实现。

NONTECHNICAL SUMMARYThis award supports theoretical and computational research, and education to develop computer simulation methods for the determination of mechanical properties of materials on the scale of nanometers - some 100,000 times smaller than the diameter of a human hair. Recent advances in chemical synthesis and fabrication technologies have made applications involving nanotechnology increasingly commonplace. Examples include the use of nanocomposites for athletic equipment and nanoparticle based drug delivery. The interesting properties of these nanosystems which make them practical and useful are determined by the specific interactions between the molecules in the system. Conventional instruments that are used for measuring mechanical properties are unable to probe the important molecular interactions and their implications for nanoscale mechanical properties of the system. In this project, a robust theory-simulation technique will be created for investigating the relationship between the molecular characteristics of a material and its nanoscale mechanical properties. Experimentation at the nanoscale is inherently difficult and very expensive. If successful, the technique developed in this project can help guide the experiments, thus leading to savings in time and cost. The formalism can also be used for designing practical applications such as cancer treatment using nanoparticle based drug delivery. Educational aspects of the work will consist of multidisciplinary training of graduate students. Outreach will be achieved via participation in a program aimed at getting junior high and high school girls interested in careers in science. TECHNICAL SUMMARYThis award supports theoretical and computational research, and education to develop computer simulation methods for probe particle microrheology, which involves determining viscoelastic properties of materials by monitoring motion of a microscopic probe particle in the medium. This method has become an established experimental technique for determining viscoelasticity of complex soft matter. Recent advances in experimental techniques have made it possible to apply the technique at the nanoscale. The nanoscale viscoelastic properties of interest are governed by the specific interactions and structure in the system at these length scales. Thus, a simulation technique would be valuable for interpreting probe particle nanorheology experimental results in terms of the molecular properties of the system. In addition to the need for explicitly accounting for the molecular interactions, such an endeavor faces additional challenges at the nanoscale such as availability of an expanded frequency range, and the need to account for the medium and particle inertia when analyzing the particle trajectory. A particulate theory-simulation technique for nanoscale bead rheology will be created in the project that will allow for the investigation of viscoelasticity in complex matter. The inertial extension of the generalized Stokes-Einstein relation (IGSER) will be used to extract medium viscoelasticity from nanoparticle motion. The PIs will focus on three aspects: (1) The frequency range that can be investigated in molecular simulations is severely restricted due to the long-range hydrodynamic interactions between the images of the moving probe particle, which are the result of the periodic boundary conditions that are used in the simulations. A framework will be created to modify the particle trajectory analysis procedure in order to quantitatively account for these hydrodynamic interactions. Such a formalism will significantly extend the frequency range that can be probed by simulations. (2) Probe rheology is presumed to have the ability to determine the local viscoelastic properties of the medium. This hypothesis will be tested by creating a model system with a nanoscale temperature gradient, which in turn, will result in a gradient in viscoelasticity. (3) Particle rheology is usually used to infer viscoelasticity from the knowledge of the probe particle motion. The usage of particle rheology in the reverse direction - predicting the extent of particle motion in the complex medium - given the knowledge of the viscoelastic spectrum of the medium, will be investigated in the project. Accomplishing these tasks will yield a combined theory-simulation technique for the determination of the nanoscale viscoelasticity in complex soft matter. Experimentation at the nanoscale is inherently difficult and very expensive. If successful, the technique developed in this project can help guide the experiments, thus leading to savings in time and cost. The formalism can also be used for designing practical applications such as cancer treatment using nanoparticle based drug delivery. Educational aspects of the work will consist of multidisciplinary training of graduate students. Outreach will be achieved via participation in a program aimed at getting junior high and high school girls interested in careers in science.