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Rheo-Microscopy

Observation of sheared individual structures in the micrometer range

Rheological measurements reveal information about macroscopic material properties, which are strongly dependent on the microstructure. In order to understand and customize the behaviour of investigated samples under flow conditions, it is necessary to pay special attention to the structural changes that occur on the microstructural level.

A suitable method for investigating morphology development in the micrometre size is microscopy. The Rheo-Microscope enables observation of individual structural elements in shear flow as well as simultaneous rheological analysis.

Background

Depending on the setup used for rheo-microscopy, the sample within the shear gap can be observed either from below or from the side. The light source integrated into the microscope tube illuminates the sample with visible light. Using a lens-beam-splitter-mirror system allows to focus on the sample as well as record non-distorted magnified images of individual structures within the sample.

The microscope tube can be equipped with polarizers in the light path in order to image the sample under polarized light conditions (parallel or cross polarization). Alternatively, a fluorescence module can be integrated to achieve better contrast between a matrix and fluorescently stained structures in the sample.

Figure 1: Rheo-microscopy working principle

Measurement examples

  1. In the figure below, the viscosity curve and the corresponding microscopy images for a water/oil emulsion are shown. The size and shape of the droplets passing through the field of view are related to the shear rate and shear history. At rest and low shear rates, the droplets have spherical shapes. With an increasing shear rate, the droplets deform and align with the flow direction, resulting in decreasing viscosity. At high shear rates, the large droplets split into smaller ones. The recorded video or pictures permit post-processing data analysis for determination of shape, size, and orientation of droplets along the flow direction.
  2. For additional examples please see the following application reports:

Typical test procedures

  • Flow curves
  • Amplitude sweeps
  • Frequency sweeps
  • Temperature sweeps (melting & crystallization)
  • Shear-induced crystallization at constant temperatures

Measurement equipment

The rheo-microscopy system consists of a microscope tube with exchangeable modules, a long working distance lens, an illumination source, and a charge-coupled device (CCD) camera.

Figure 2: The main components of the microscope

The microscope can be mounted either underneath the measuring plate attached to the temperature control device or sidewise on a holder sustained by a breadboard that is fixed onto the rheometer flange.

More information about this setup can be found at: 

Conclusions

Combining rheological testing with optical methods like microscopy and small-angle light scattering (SALS) creates powerful tools for characterizing complex fluids and solving all kinds of application problems related to the handling of various samples. Rheo-microscopy has proven extremely useful when it comes to study of deformation and orientation of droplets, particles, structures in the flow, particle agglomeration and breakage of agglomerates, stability of emulsions and dispersions, and gelation or crystallization processes. The modularity of the microscope system and the possibility to easily exchange between different optical setups qualifies microscopy as one of the most eligible methods to combine with rheology.