Optical Rheology

Rheology of Complex Fluids

Shear induced structures and ordered fluid structures are common features of the complex fluids under shear. Shear applied on an elongated micellar solution is particularly interesting because micellar alignment is observed due to the flow birefringence (see Figure 1). Actually we are studying the possibility to understand this alignment induced by shear, which bears some resemblances to the isotropic-nematic phase transition: A shear stress vs. shear rate curve of this type of fluids shows a shear thinning behavior that coincides with the ordered - disordered material coexistence. The shear thinning behavior appears frequently when flow instabilities also appear. Moreover, al low micellar concentration, the fluid presents shear thickening. All those phenomena in a single type of fluid have been a rich field of research since almost twenty years.

We are using a modified Brookfield LV III+ rheometer with a strain controlled cone -plate optical geometry (see Figure 2) and with a transparent glass window that allows us to observe the boundary and the optical properties (birefringence) of the fluids under shear. An inverted microscope with linear polarizers (3 & 5) completes the instrument.




Bohlin Gemini HRnano rheometer.

An advanced rheological characterization is possible using this shear stress - shear rate controlled rheometer. We count with cone-plate, cone-cone and couette geometries. Depending on the test, the material and the geometry, torques as low as 3 nNm and shear rates as low as 0.0001432 1/s are possible to measure.




Optical Couette Rheometer

It was developed in our group for light scattering measurement under shear. The moving cylinder is the external one. So, we can avoid Rayleigh and Taylor instabilities in a huge range of velocities. It rotates over horizontal and vertical air bearings and temperature control is also possible. The most common optical techniques under shear like birefringence and small angle light scattering (SALS) can be added on it. It also has non-developed technical capabilities like stress controlled measurements and mechanical data acquisition.