Rheology of nearly ideal 3d foams

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.