Microrheology of Complex Fluids: From Colloids to Biomaterials

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、设计和使用先进材料以及现有材料新应用的关键一步。迄今为止，大多数微流变实验和分析都集中在线性粘弹性特性上，通过将小示踪剂的随机热驱动位移与复杂模量通过广义的Stokes-Einstein关系相关联，这是一个相对较好的理解过程，但其范围仅限于平衡系统。许多具有实际意义的系统被驱动出平衡状态并表现出非线性行为，但该领域的微流变学工作很少（传统的宏观流变仪用于测量线性和非线性状态），微观和宏观测量之间的联系尚不清楚。无论是由于材料的稀缺性还是由于系统的尺寸，对微尺度测量的需求和兴趣使得微流变学成为一项重要的技术，但不幸的是，目前在许多方面缺乏基本的理解。拟议的研究通过理论研究检查微观和宏观流变学之间的关系，特别强调“主动”（驱动）和非线性制度。这项研究最好是在胶体分散体的应用方面进行描述，并将重点放在这样的系统上，因为它们提供了非常明确定义和良好表征的材料，允许与宏观尺度测量进行比较。然而，本研究的影响超出了胶体体系，作为理论基础和一般结论可扩展到许多复杂材料，特别是生物材料。具体问题，如微流变学中的剪切增厚，尺寸比（示踪剂尺寸与典型中等长度尺度）对“连续统近似”和微尺度速度波动的影响，以及导致结构形成的运动粒子对之间的相互作用。这是在微流变学背景下分析材料非线性行为的第一次尝试，并提供了微流变学作为一种可靠技术的基本验证，对其继续应用和未来发展至关重要。更广泛的影响：在更广泛的背景下，这项研究将吸引博士生，他们将成为胶体物理和流变学方面的专家，并将继续在工业界和/或学术界担任领导职务。为了帮助培养未来一代的科学家和工程师，加州理工学院将为本科化学工程实验室开设一个微流变学课程。为了尽可能广泛地传播这项研究，除了在传统的技术期刊上发表外，还将建立一个网站，向公众提供研究结果。由于本研究为一种新的实验技术在科学技术上的广泛应用提供了理论基础，因此其影响是非常广泛和深刻的。

Project SummaryThe increased demand for knowledge of small-scale behavior is making microrheology a keystep in the understanding of biological systems, the design and use of advanced materials,and the novel applications of already existing materials. Most microrheological experimentsand analyses to date have focused on linear viscoelastic properties, by correlating therandom thermally-driven displacements of small tracers to the complex modulus through ageneralized Stokes-Einstein relation, a process which is relatively well understood but whichis limited in its scope to equilibrium systems. Many systems of practical interest are drivenout of equilibrium and display nonlinear behaviors, but the microrheology work in thisarea has been scarce (conventional macroscale rheometers are used to measure both linearand nonlinear regimes), and the connection between micro and macroscale measurementsis unclear. The need and interest in microscale measurements, whether due to the scarcityof the material or the size of system, makes microrheology an important technology, butunfortunately one which currently lacks fundamental understanding in many aspects.The proposed research examines the relation between micro and macrorheology through theoretical studies, with a particular and strong emphasis on the `active' (driven) and nonlinear regime. This research is best described in terms of applications to colloidal dispersions, and will focus on such systemsbecause they offer very well-defined and well-characterized materials, allowing for comparisonsto macroscale measurements. However, the impact of this research extends beyondcolloidal systems as the theoretical foundation and general conclusions are extendable tomany complex materials, especially biomaterials. Specific issues such as shear thickeningin microrheology, the effect of the size ratio (tracer size to typical medium length scale) onthe `continuum approximation' and on microscale velocity fluctuations, and the interactionsbetween pairs of moving particles leading to structure formation are addressed. This is the first attempt to analyze the nonlinear behavior of materials within the context of microrheologyand provides a fundamental validation of microrheology as a sound technique, critical forits continued application and future growth.Broader Impact: In a broader context, this research will engage PhD students who willbecome experts in colloidal physics and rheology, and who will go on to positions of leadership in industry and/or academia. To aid in the education of future generations of scientists and engineers, a microrheology section for the undergraduatechemical engineering laboratory at Caltech will be developed. To disseminate the researchas widely as possible, in addition to publication in conventional technical journals, a websitewill be maintained with research results that are accessible to the general public. Since thisresearch provides the theoretical foundation for a new experimental technique that haswidespread application in science and technology, its impact is both very broad and deep.