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

G. Ovarlez, S. Rodts, A. Ragouilliaux, P. Coussot, J. Goyon, A. Colin — Phys. Rev. E vol 78 (2008)

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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.