Oscillatory shear tests are widely used in rheology to characterize the linear and non-linear mechanical response of complex fluids, including the yielding transition. There is an increasing urge to acquire detailed knowledge of the deformation field that is effectively present across the sample during these tests; at the same time, there is mounting evidence that the macroscopic rheological response depends on the elusive microscopic behavior of the material constituents. Here we employ a strain-controlled shear-cell with transparent walls to visualize and quantify the dynamics of tracers embedded in various cyclically sheared complex fluids, ranging from almost-ideal elastic to yield stress fluids. For each sample, we use image correlation processing to measure the macroscopic deformation field, and echo-differential dynamic microscopy to probe the microscopic irreversible sample dynamics in reciprocal space; finally, we devise a simple scheme to spatially map the rearrangements in the sheared sample, once again without tracking the tracers. For the yield stress sample, we obtain a wave-vector dependent characterization of shear-induced diffusion across the yielding transition, which is accompanied by a three-order-of-magnitude speed-up of the dynamics and by a transition from localized, intermittent rearrangements to a more spatially homogeneous and temporally uniform activity. Our tracking free approach is intrinsically multi-scale, can successfully discriminate between different types of dynamics, and can be automated to minimize user intervention. Applications are many, as it can be translated to other imaging modes, including fluorescence, and can be used with sub-resolution tracers and even without tracers, for samples that provide intrinsic optical contrast.

中文翻译：

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振荡剪切实验中复杂流体的变形剖面和微观动力学

振荡剪切试验广泛用于流变学，以表征复杂流体的线性和非线性机械响应，包括屈服转变。在这些测试过程中，人们越来越渴望获得有效存在于样品中的变形场的详细知识；同时，越来越多的证据表明，宏观流变响应取决于材料成分难以捉摸的微观行为。在这里，我们采用具有透明壁的应变控制剪切单元来可视化和量化嵌入各种循环剪切复杂流体中的示踪剂的动力学，范围从几乎理想的弹性到屈服应力流体。对于每个样本，我们使用图像相关处理来测量宏观变形场，回波微分动态显微技术，探测互易空间中微观不可逆的样品动力学；最后，我们设计了一个简单的方案来空间映射剪切样本中的重排，再次不跟踪示踪剂。对于屈服应力样本，我们获得了跨屈服转变的剪切诱导扩散的波矢量相关表征，伴随着动力学的三个数量级加速和局部、间歇性的转变重新排列为空间更均匀和时间更均匀的活动。我们的无跟踪方法本质上是多尺度的，可以成功区分不同类型的动态，并且可以自动化以最大限度地减少用户干预。应用程序很多，因为它可以转换为其他成像模式，