Nonlinear microrheology: Bulk stresses versus direct interactions
Nonlinear microrheology: Bulk stresses versus direct interactions
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DOI：
10.1021/la7023692
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发表时间：
2008-02-19
期刊：
影响因子：
3.9
通讯作者：
Squires, Todd M.
中科院分区：
文献类型：
--
作者：
Squires, Todd M.
在被动微观流变学中，复杂流体的线性粘弹性质是由胶体示踪粒子的布朗运动推断出来的。主动（但温和）强迫也可以用来获得这种线性响应信息。更显著的强迫可能会驱使材料显著地脱离平衡，从而潜在地提供进入材料的非线性响应特性的窗口。然而，在离开线性响应制度，被动微观流变学的理论基础是失去了，并出现了各种问题。最普遍的是，什么是可以测量的，以及如何解释这些测量？在这里，我们激励和讨论的各种理论问题所面临的积极微观流变学的解释。首先，在连续极限下，探针周围的不均匀速度场引起流变学不均匀性，因此假定的广义斯托克斯阻力产生探针周围粘度的加权平均值，而不是宏观测量的（均匀）粘度。然后，我们明确地对待材料的微观结构使用模型系统（一个大的胶体探针拉通过小浴颗粒的稀悬浮液）。我们检查了探针粒子上的不同应力源（例如，直接探针浴碰撞以及本体悬浮液内的微观结构变形），并讨论它们在相应的宏观流变系统中的类似物（或缺乏）。我们讨论了几个关键问题的非线性微观流变学的解释：（1）如何解释的不均匀性和nonviscoetric性质的变形场周围的探头，（2）直接和体积应力和它们的反褶积之间的区别，（3）（拉格朗日）随时间变化的性质的应力历史所经历的物质元素，因为他们平流过去的探头。确定了这些问题，我们简要地讨论了适应的基本技术，以恢复本体流变学更忠实。虽然我们专门讨论了模型胶体悬浮液，我们最终设想一种技术能够测量一般材料的非线性流变学。
In passive microrheology, the linearviscoelastic properties of complex fluids are inferred from the Brownian motion of colloidal tracer particles. Active (but gentle) forcing may also be used to obtain such linear-response information. More significant forcing may drive the material significantly out of equilibrium, thus potentially providing a window into the nonlinear response properties of the material. In leaving the linear-response regime, however, the theoretical underpinning for passive microrheology is lost, and a variety of issues arise. Most generally, what exactly can be measured, and how can such measurements be interpreted? Here we motivate and discuss a variety of theoretical issues facing the interpretation of active microrheology. First., in the continuum limit, the inhomogeneous velocity field around the probe gives rise to rheological inhomogeneities, whereupon an assumed generalized Stokes drag yields a weighted average of the viscosities around the probe rather than the (homogeneous) viscosity measured macroscopically. We then explicitly treat the material microstructure using a model system (a large colloidal probe pulled through a dilute suspension of small bath particles). We examine the different sources of stress upon the probe particle (e.g., direct probe-bath collisions as well as microstructural deformations within the bulk suspension) and discuss their analog (or lack thereof) in the corresponding macrorheological system. We discuss several crucial issues for the interpretation of nonlinear microrheology: (1) how to interpret the inhomogeneous and nonviscometric nature of the deformation field around the probe, (2) the distinction between direct and bulk stresses and their deconvolution, and (3) the (Lagrangian) time-dependent nature of the stress histories experienced by material elements as they advect past the probe. Having identified these issues, we briefly discuss adaptations of the basic technique to recover bulk rheology more faithfully. Whereas we specifically discuss a model colloidal suspension, we ultimately envision a technique capable of measuring the nonlinear rheology of general materials.