  We probe the complex rheology of nearly ideal 3d foam by flowing through a
narrow column. The foams we investigate have large bubble size, to minimize the
effects of coarsening, and are very dry. Foams of this type cannot be studied
via conventional rheometry. The foam flows upward through a vertical
rectangular column with a 4:1 cross-sectional aspect ratio, by bubbling gas
through a soapy solution at the base of our apparatus. At the column's narrow
surfaces are sticky boundaries, which create shear due to the zero velocity
boundary condition. As expected, the flow profile between the adjacent slippery
broad faces is flat, however the profile between the narrow, sticky faces
exhibits a curved velocity profile that is dependent on gas flow rate. We are
able to analyze a 2d velocity profile from a 3d bulk system. We employ particle
image velocimetry to measure the strain rate, and compute the stress from the
pressure drop along the channel, to investigate the local stress-strain
relationships in a flowing foam. We find these dry foams to have a
Hershel-Bulkley exponent of 0.21, which is significantly lower (more shear
thinning) than other results shown in the literature for much wetter foams.
