Ultrafast imaging of soft materials during shear flow

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.