Wide gap Couette flows of dense emulsions : local concentration measurements, and comparison between macroscopic and local constitutive law measurements through MRI

Flows of dense emulsions show many complex features among which long range nonlocal effects pose problem for macroscopic characterization. In order to get round this problem, we study the flows of several dense emulsions, with droplet size ranging from 0.3 to 40 m, in a wide gap Couette geometry. We couple macroscopic rheometric experiments and local velocity measurements through MRI techniques. As concentration heterogeneities are expected in the wide gap Couette flows of multiphase materials, we also designed a new method to measure the local droplet concentration in emulsions with a MRI device. In contrast with dense suspensions of rigid particles where very fast migration occurs under shear in wide gap Couette flows, we show for the first time that no migration takes place in dense emulsions even for strain as large as 100000 in our systems. As a result of the absence of migration and of finite size effect, we are able to determine very precisely the local rheological behavior of several dense emulsions. As the materials are homogeneous, this behavior can also be inferred from purely macroscopic measurements. We thus suggest that properly analyzed purely macroscopic measurements in a wide gap Couette geometry can be used as a tool to study the local constitutive laws of dense emulsions. All behaviors are basically consistent with Herschel-Bulkley laws of index 0.5. The existence of a constitutive law accounting for all flows contrasts with previous results obtained within a microchannel by Goyon et al. [4]: the use of a wide gap Couette geometry is likely to prevent here from nonlocal finite size effects; it also contrasts with the observations of B´ecu et al. [3]. We also evidence the existence of discrepancies between a perfect Herschel-Bulkley behavior and the observed local behavior at the approach of the yield stress due to slow shear flows below the apparent yield stress in the case of a strongly adhesive emulsion.