Particle Motion in Colloidal Dispersions: Microrheology and Microdiffusivity

胶体分散体中的粒子运动：微流变学和微扩散性

• 批准号: 0931418
• 负责人: John Brady
• 金额: $ 30万
• 依托单位: California Institute of Technology
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-09-15 至 2013-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0931418&HistoricalAwards=false
• 关键词: Particle; Motion; Colloidal; Dispersions; Microrheology

0931418 Brady Intellectual Merit: The increased demand for knowledge of small scale behavior has made microrheology a key step in the understanding of biological systems and the design and use of advanced materials and nano scale devices. Most microrheological work to date has focused on linear viscoelastic properties, by correlating the random thermally driven displacements of tracers to the complex modulus through a generalized Stokes Einstein relation, a process which is well understood but which is limited in its scope to equilibrium systems. But many systems of practical interest are driven out of equilibrium and display (indeed, rely upon) nonlinear behaviors. Recently a body of work has emerged focusing on this active, nonlinear microrheology. In such a system, tracer particles undergo displacements due not only to random thermal fluctuations, but also due to the application of an external force applied directly to the tracer. The dispersion is driven out of equilibrium, and as with macrorheology, dynamic responses such as viscosity can be measured. Since the tracer probes the material at its own (micro)scale, much smaller samples are required compared to macrorheology, and localized heterogeneity can be explored. Recent experiments confirm the theory; but in both theory and experiment, the focus thus far has remained on the mean response of the material the viscosity and little focus has been devoted to particle uctuations. Just as the shear flow in macrorheology enhances particle diffusion, an analogous `force induced' diffusivity arises due to the single particle forcing of active microrheology. This diffusive motion is fundamental to the motion of an active microscale particle important both for scientific and technology considerations. The proposed research extends the theory of active microrheology to the force induced diffusive motion of individual particles, as well as normal microstress differences. This work will combine theoretical and computational studies, focusing on colloidal systems because they offer very well characterized materials, allowing for comparisons to macroscale measurements. But the impact of this research extends beyond colloids, as the theoretical foundation and general conclusions are extendable to many complex materials, especially biomaterials. Other issues such as the effect of tracer size on the `continuum approximation', and hydrodynamic interactions between pairs of moving particles leading to structure formation, will be addressed. This work will expose new material capabilities and ultimately provide a validation of microrheology as a sound technique, critical for its continued application and future growth.Broader Impact: Motion control for active microscale particles is a major focus in many fields from biophysics to alternative energy to nanomedicine and it begins with understanding the fluctuations in particle motion. Since this research provides the theoretical foundation for new experimental techniques that have widespread application in science and technology, its impact is both very broad and deep. This research will develop PhD students into experts in colloid physics, rheology, and computational methods, who will become leaders in industry and academia. To aid in the education of future scientists and engineers, a microrheology section for the Caltech chemical engineering laboratory will be created. To disseminate the research widely, a publicly accessible website showcasing research results will accompany publication in technical journals.

0931418 Brady 智力优势：对小尺度行为知识的需求不断增加，使得微流变学成为理解生物系统以及先进材料和纳米级设备的设计和使用的关键一步。迄今为止，大多数微流变学工作都集中在线性粘弹性特性上，通过广义斯托克斯·爱因斯坦关系将示踪剂的随机热驱动位移与复数模量相关联，这个过程很好理解，但其范围仅限于平衡系统。但许多具有实际意义的系统都失去了平衡并表现出（实际上是依赖）非线性行为。最近出现了一系列关注这种活跃的非线性微流变学的工作。在这样的系统中，示踪剂粒子不仅由于随机热波动而发生位移，而且还由于直接施加到示踪剂的外力而发生位移。分散体失去平衡，并且与宏观流变学一样，可以测量粘度等动态响应。由于示踪剂在其自身（微观）尺度上探测材料，因此与宏观流变学相比，所需的样品要小得多，并且可以探索局部异质性。最近的实验证实了这一理论；但在理论和实验中，迄今为止的焦点仍然集中在材料的平均响应和粘度上，而很少关注粒子波动。 正如宏观流变学中的剪切流增强颗粒扩散一样，由于活跃微流变学的单颗粒强迫，也会产生类似的“力诱导”扩散率。 这种扩散运动是活性微尺度粒子运动的基础，对于科学和技术考虑都很重要。 拟议的研究将主动微流变学理论扩展到单个颗粒的力引起的扩散运动以及正常的微应力差异。 这项工作将结合理论和计算研究，重点关注胶体系统，因为它们提供了非常明确的材料，可以与宏观测量进行比较。 但这项研究的影响超越了胶体，因为理论基础和一般结论可扩展到许多复杂材料，尤其是生物材料。 其他问题，例如示踪剂尺寸对“连续体近似”的影响，以及导致结构形成的运动粒子对之间的流体动力学相互作用，也将得到解决。这项工作将揭示新材料的功能，并最终验证微流变学作为一种可靠的技术，对其持续应用和未来发展至关重要。 更广泛的影响：活性微尺度颗粒的运动控制是从生物物理学到替代能源到许多领域的主要焦点纳米医学首先要了解粒子运动的波动。由于这项研究为科学技术中广泛应用的新实验技术提供了理论基础，因此其影响非常广泛和深刻。 这项研究将把博士生培养成胶体物理学、流变学和计算方法方面的专家，他们将成为工业界和学术界的领导者。为了帮助培养未来的科学家和工程师，加州理工学院化学工程实验室将创建一个微流变学部门。为了广泛传播研究成果，技术期刊上将发布一个展示研究成果的公开网站。