Shear induced drainage of a foamy yield stress fluid is investigated using
MRI techniques. Whereas the yield stress of the interstitial fluid stabilizes
the system at rest, a fast drainage is observed when a horizontal shear is
imposed. It is shown that the sheared interstitial material behaves as a
viscous fluid in the direction of gravity, the effective viscosity of which is
controlled by shear in transient foam films between bubbles. Results provided
for several bubble sizes are not captured by the R^2 scaling classically
observed for liquid flow in particulate systems, such as foams and thus
constitute a remarkable demonstration of the strong coupling of drainage flow
and shear induced interstitial flow. Furthermore, foam films are found to be
responsible for the unexpected arrest of drainage, thus trapping irreversibly a
significant amount of interstitial liquid.Comment: Published in Physical Review Letters.
http://prl.aps.org/abstract/PRL/v104/i12/e12830
