Particle Motion in Colloidal Dispersions: Microrheology and Microdiffusivity

布拉迪智力优点：对小尺度行为知识的需求增加，使得微流变学成为理解生物系统、设计和使用先进材料和纳米级器件的关键一步。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，通过广义的Stokes - Einstein关系将示踪剂的随机热驱动位移与复杂模量相关联，这一过程被很好地理解，但其范围仅限于平衡系统。但是，许多具有实际意义的系统被赶出了平衡状态，并表现出（实际上，依赖于）非线性行为。最近出现了一组研究这种主动的非线性微流变学的工作。在这样的体系中，示踪粒子不仅由于随机的热波动而发生位移，而且由于直接施加在示踪剂上的外力而发生位移。分散被驱出平衡，并且与宏观流变学一样，可以测量粘度等动态响应。由于示踪剂在自身（微观）尺度上探测材料，与宏观流变学相比，需要更小的样品，并且可以探索局部非均质性。最近的实验证实了这一理论；但在理论和实验中，迄今为止的研究重点都停留在材料黏度的平均响应上，对颗粒波动的研究很少。正如宏观流变学中的剪切流动增强了颗粒扩散一样，由于主动微流变学的单颗粒作用力，类似的“力诱导”扩散率也会产生。这种扩散运动是活跃的微尺度粒子运动的基础，对科学和技术都很重要。该研究将主动微流变学理论扩展到单个颗粒的力诱导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好的表征材料，允许与宏观尺度测量进行比较。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的材料，特别是生物材料。其他问题，如示踪剂尺寸对“连续统近似”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，将被解决。这项工作将揭示新的材料能力，并最终为微流变学作为一种可靠的技术提供验证，这对微流变学的持续应用和未来发展至关重要。更广泛的影响：从生物物理学到替代能源再到纳米医学，主动微尺度粒子的运动控制是许多领域的主要焦点，它始于对粒子运动波动的理解。由于本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，因此其影响是非常广泛和深刻的。这项研究将培养博士生成为胶体物理、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设立一个微流变学部门。为了广泛传播这项研究，在技术期刊上发表研究成果的同时，还将建立一个向公众开放的网站，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.