Particle Motion in Colloidal Dispersions: Microrheology and Microdiffusivity

0931418布雷迪智力优点：对小尺度行为知识的需求增加，使微观流变学成为理解生物系统和设计的关键一步，并使用先进的材料和纳米尺度的设备。迄今为止，大多数微观流变学工作都集中在线性粘弹性性质上，通过将示踪剂的随机热驱动位移与复模量通过广义斯托克斯-爱因斯坦关系相关联，这是一个很好理解的过程，但其范围限于平衡系统。但是，许多具有实际意义的系统都是被驱离平衡的，并表现出（实际上，依赖于）非线性行为。最近出现了一个机构的工作集中在这个积极的，非线性的微观流变学。在这样的系统中，示踪剂颗粒不仅由于随机的热波动，而且由于直接施加到示踪剂的外力的施加而经历位移。分散体被驱离平衡，并且与宏观流变学一样，可以测量动态响应，例如粘度。由于示踪剂探测材料在其自身的（微）规模，更小的样品相比，宏观流变学，需要和局部异质性可以探索。最近的实验证实了这一理论，但在理论和实验中，迄今为止的重点仍然是对材料粘度的平均响应，而很少关注颗粒波动。 正如宏观流变学中的剪切流增强了颗粒扩散一样，由于活性微观流变学的单颗粒强迫，出现了类似的“力诱导”扩散。 这种扩散运动对于活性微尺度粒子的运动是基本的，这对于科学和技术考虑都是重要的。 拟议的研究扩展了主动微观流变学理论的力引起的单个颗粒的扩散运动，以及正常的微观应力差异。 这项工作将结合联合收割机理论和计算研究，重点是胶体系统，因为它们提供了非常好的表征材料，允许与宏观测量进行比较。 但这项研究的影响超出了胶体，因为理论基础和一般结论可以扩展到许多复杂材料，特别是生物材料。 其他问题，如示踪剂的大小对“连续近似”的影响，以及导致结构形成的移动粒子对之间的流体动力学相互作用，将得到解决。这项工作将揭示新材料的能力，并最终提供一个验证的微观流变学作为一个健全的技术，其持续应用和未来的growth.Broader影响的关键：运动控制的活性微尺度粒子是一个主要的焦点，在许多领域从生物物理学的替代能源纳米医学，它开始了解粒子运动的波动。由于这项研究为在科学和技术中具有广泛应用的新实验技术提供了理论基础，因此其影响既广泛又深刻。 这项研究将培养博士生成为胶体物理学，流变学和计算方法的专家，他们将成为工业和学术界的领导者。为了帮助未来的科学家和工程师的教育，加州理工学院化学工程实验室将创建一个微观流变学部分。为了广泛传播研究成果，将在技术期刊上发表研究成果，同时建立一个可公开访问的网站，展示研究成果。

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.