Chaotic mixing of complex fluids: from yield stress fluids to blending of wet granular materials – RHEOMEL

Our approach is based on (i) the experimental device, a mixer with a model egg-beater-like geometry and (ii) the use of materials that will be all well-characterized independently (calibrated PMMA or glass bead, surface condition, rheology of the fluids). The project relies on the combination of imaging techniques, at the scale of the mixer as well as at smaller scale (index matching, X-ray tomography for the granular media).

Measurements of mixing rate for different complex fluids.

Applications in different industrial domains are expected, since mixing different complex fluids is ubiquitous, such as in powders, granulation, cement, concrete, or molten polymers. Proceedings prescriptions on the role of yield stress on the mixing quality and rate will help to determine the range of applicable fluids for a given mixer.

Oral présentation in an international congress

A number of processes of the food engineering or construction industries include a mixing step aiming at homogenizing viscous fluids or granular pastes. From a rheological viewpoint, most of these materials may be viewed as yield-stress fluids to a first approximation. The initial ingredients of such processes bear in general different physical or chemical natures – as in the case of grains and a liquid –, and have to be mixed together. Little fundamental knowledge exists about the mixing mechanisms of such materials.

This project aims at providing answers about the rate of mixing of complex fluids, and the patterns that appear in the course of the mixing of constituents with different rheologies. The behavior of yield-stress fluids and granular media will be compared. Our approach is based on (i) the experimental device, a mixer with a model egg-beater-like geometry and (ii) the use of materials that will be all well-characterized independently (calibrated PMMA or glass bead, surface condition, rheology of the fluids). The project relies on the combination of imaging techniques, at the scale of the mixer as well as at smaller scale (index matching, X-ray tomography for the granular media). We will characterize experimentally the propertis of chaotic mixing in such materials, as a function of the rheology of the different fluids. Then we will mix together different fluids, and we will aim in particular at a fundamental understanding of the mixing-induced blending of liquid into granular matter. The analysis of mixing patterns and their fluctuations will be inspired by the recent and general idea that some zones with slow stretching slow down and control the speed of mixing. However, we will also strive to identify mechanisms that are specific to our model materials, such as the influence of transiently unyielded zones, or the evolution of wet and dry zones in granular media.

The members of Rheomel bring in complementary skills and competences to the project, both on the experimenal and modelling sides. Pierre Jop has a recognised experience on the behavior of granular materials and yield-stress fluids. E. Gouillart brings in her knowledge of chaotic mixing in Newtonian fluids, as well as image processing of mixing patterns and tomography images. Besides, two technicians will participate to the design and realization of the experimental devices. In addition, a Cifre graduate student already working on wet granulation and supervised by Pierre Jop will participate actively to the experiments involving the incorporation of liquid, and their further analysis. No ANR funding is requested for the three latter members of the project . A funding of 191400 euros is asked for in order to realize this project. The following points amount to the latter sum. First, a post-doc will be funded during two years and work on the exploitation of experiments on the mixing of different constituents (50 %). 38% of the budget correspond to the realization of the experiments, with 24% for X-ray tomography (sub-contracted to the Navier Institute, plus first experiments at the ESRF) and 14% for the set-up and usage of dedicated experiments. Finally, 13% of the budget will be allocated to missions and the valorization of the results.