Foams of Viscoelastic Fluids: Macroscopic Drainage and Single Plateau Border Observations

M. SAFOUANE; A. SAINT-JALMES; H. RITACCO; D. LANGEVIN; V. BERGERON; O. PITOIS

Marne-laValleé, France

Conferencia; EuFoam 2004: 5th European Conference on Foams, emulsions and Applications; 2004

Université de Marne-la-Valleé

We have studied the drainage of foams made of non-Newtonian polymer solutions. Drainage is a matter of hydrodynamics in a skeleton of deformable channels and junctions, and it is important(especially regarding industrial applications) to investigate the possible effects on drainage of the solution complex rheological properties. Macroscopic forced-drainage experiments show that foams made of a surfactant solution containing long flexible Polyethylene Oxide (PEO) molecules counter-intuitively drain faster than foams made of a Newtonian solution having the same shear viscosity. It is known that PEO solutions possess high elongational viscosities: they develop large resistances under elongational strains. Complementary experiments with fluids having all the same shear viscosities but different elongational ones (PEO-based Boger fluids) show that the elastic properties of the PEO solutions are indeed responsible of the faster drainage.We propose an interpretation based on the fact that the foam channels can adapt their cross sections in order to reduce hydrodynamic stresses. In the presence of PEO, the flow and the channels shape change to minimize the cross section variations (smaller elongation stresses), thus leading to faster speeds (in analogy with the elongational-thickening effects which stabilize elastic liquid jets).High-speed video camera studies and pressure drop measurements on a single channel, held on frames or confined between plates, allow us to visualize the way a channel opens or closes undervarious flow conditions, and to investigate the effects of the PEO and of the elongational stresses at this scale of a Plateau Border.In the case of shear-thinning solutions, the velocity of drainage is found to correspond to a viscosity equal to the one at the local shear rate in the channels. For any solutions, when the viscosity is increased, a change in drainage regime occurs, resulting from a coupling between flow in the channel surfaces and within the Plateau Borders, which can effectively be varied via the bulk viscosity.