The direct imaging of flow induced microstructural changes in complex fluids can have advantages over the use of scattering methods, since localized phenomena can be observed directly and more mechanistic insights can be obtained. This is useful in particular for materials with hierarchical or multiscale structures such as aggregated dispersions. Rheoconfocal instruments are ideally suited for this purpose but were, as yet, limited to relatively low imaging rates. In the present work, a stress-controlled rheometer was coupled to a fast scanning, instant structured illumination confocal microscope which uses a multi-array illumination and detection scheme. A second motor is integrated in a custom-made rheoconfocal instrument to achieve the counter-rotation of the lower glass plate. The resulting stagnation plane can be moved within the shearing gap in real time and allows the stable imaging of micro-structural features under steady shear. Velocity profiles were measured to validate the performance of the mechanical components, using particle image velocimetry on a sterically stabilized suspension. Structured illumination optics yielded an excellent inplane spatial resolution, while the multipoint scanning allows speeds as high as 1000 frames per second at full frame resolution. However, for rheological studies the 3D structure should ideally be resolved. The mechanical refocusing using a fast piezo stage at high speeds led to deformations of the lower thin glass plate. To circumvent this bottleneck, a focus-tunable lens was incorporated in the setup to acquire 3D image volumes at video rates. The excellent combination of temporal and spatial resolution under flow is demonstrated here using selected results from aggregated colloidal dispersions. The microstructure of a model depletion gel is studied over a broad range of shear rates under strong to moderate flow conditions. The ability to measure rheological properties while imaging the time-dependent microstructure is demonstrated with particles dispersed in a more viscous PDMS matrix. Transient rheology is reported simultaneously with high resolution imaging of the microscopic structural recovery. This novel tool enables the direct imaging of rheologically complex materials under conditions relevant to processing, to elucidate the physical phenomena underlying nonlinear rheology and thixotropy.

中文翻译：

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剪切流期间对软材料的超快成像

与使用散射方法相比，对复杂流体中的流动引起的微结构变化进行直接成像可能具有优势，因为可以直接观察到局部现象，并且可以获得更多的机械原理。这对于具有分层或多尺度结构的材料（例如聚集的分散体）尤其有用。流浆镜仪器非常适合于此目的，但迄今为止仅限于相对较低的成像率。在目前的工作中，将应力控制的流变仪与使用多阵列照明和检测方案的快速扫描，即时结构化照明共聚焦显微镜耦合。第二台电动机集成在定制的流凸仪中，以实现下玻璃板的反向旋转。生成的停滞平面可以实时在剪切间隙内移动，并允许在稳定剪切下对微结构特征进行稳定成像。使用在空间稳定的悬浮液上的粒子图像测速仪，测量速度曲线以验证机械组件的性能。结构化照明光学器件产生了出色的平面内空间分辨率，而多点扫描在全帧分辨率下的速度高达每秒1000帧。但是，对于流变学研究，理想情况下应解析3D结构。使用快速压电平台高速进行机械重新聚焦导致下部薄玻璃板变形。为了克服这一瓶颈，在设备中加入了可调焦镜头，以视频速率获取3D图像量。使用聚集的胶体分散体的选定结果，可以证明流动条件下时间和空间分辨率的完美结合。在强流量到中等流量条件下，在较宽的剪切速率范围内研究了模型耗竭凝胶的微观结构。用分散在更粘的PDMS基质中的颗粒证明了在对随时间变化的微观结构进行成像时测量流变性质的能力。瞬时流变学与微观结构恢复的高分辨率成像同时报道。这种新颖的工具能够在与加工相关的条件下直接对流变复杂的材料进行成像，以阐明非线性流变学和触变性的物理现象。