Analysis of flow induced texturing in complex fluids with Brillouin Rheology - a proof of concept

The aim of this work is to study the flow induced texturing of molecules (complex fluids) in a shear field. This is done by measuring phonons with photons [1] in a commercially available rotational rheometer. The optical experimental method of transferring momentum and energy between phonons and photons is generally known as Inelastic Light Scattering (ILS) [2]. Brillouin Light Scattering (BLS) [3] has a quite small energy transfer compared to Raman spectroscopy [4] but with a high performance multi-pass Vernier tandem Fabry-Perot interferometer [5] and a sophisticated optical set-up, it is possible to achieve information about the texture (like entanglement of molecules) of complex fluids inside a rheometer gap while shearing. This can be acquired for any position in the sample, in contrast to the rheological experiments were an average over a whole sample is used.
Here we successfully demonstrate an optical set-up that couples a BLS with rotational rheology in order to simultaneously measure the high-frequency longitudinal elastic modulus in a classical rheometer along with the zero-shear viscosity during the flow of complex fluids.
BLS gives the possibility for contactless determination of local elastic properties, while the designed optical set-up is introduced as boundary conditions to control temperature gradients in the sample, and the position and dimension of the scattering volume.
This method was tested for a range of temperatures, and well as for an applied shear field and different radial positions of the scattering volume in the sample using a plate-plate rheometer geometry. Measurements of a dilute polymer system suggest a homogeneous orientation of polymer molecules throughout the sample as soon as a critical shear rate has been reached at one spatial position.