Aqueous foam slip and shear regimes determined by rheometry and multiple light scattering

By using simultaneously rheometry and a multiple light scattering technique, diffusing wave spectroscopy (DWS), we have studied the steady flows of three-dimensional aqueous foams. A number of parameters - the surfactants, the liquid volume fraction, and the roughness of the rheometer surfaces - are widely varied in order to determine which quantities have an impact on the macroscopic flow behaviors. By comparing to previous theoretical and experimental results, we show that flow regimes can either be slip or shear dominated. Two opposite slip regimes are identified; the transition from one to the other is obtained either by changing the surfactant or the liquid fraction, and we quantitately discuss which regime is selected for any given foam properties. Similarly, different shear regimes are also found, and we discuss the link between the macroscopic rheometry measurements, the nature of the flow, and the interfacial microscopic properties. Despite the occurrence of slip, we show how we can recover the actual shear rate by DWS, and how we can quantitatively explain the measured slip velocities. (C) 2008 The Society of Rheology.