Foaming of transient polymer hydrogels

Foams made with complex fluids enables to obtain a very good foam stability, however such foams are usually difficult to produce and process : entrapping air bubbles into viscoelastic materials is not an easy task ! We present in this article a way to produce foams from a amphiphilic polymer solution which contains small cross-linking molecules to obtain a viscoelastic foamed gel. We first foam a solution made of a amphiphilic polymer using a mixer. Then we add cross-linker molecules to the foam during the mixing process : these molecules enable to bridge the polymer chains and form the transient polymer gel. We show in our article, to be published in ACS Omega, that the shear stress applied on the bubbles during the mixing process can lead to a complete collapse of the foams once the cross linker molecules are added. We show that the shear induced coalescence of the bubbles is due to the fact that the viscosity of the polymer/cross-linker mixture increases strongly with the shear rate. Consequently the thin liquid films between the bubbles become highly viscous and undergo a viscous to fragile transition : they break as they are stretched during the mixing process. A simple experiment consisting in stretching single thin liquid curved films, named catenoids, enables us to show that when the cross-linker concentration exceeds a critical value, the films break like a fragile material : a fracture propagates in the film and leads to their rupture. Below the critical cross-linker concentration, the films collapse only occurs only when the two curved interfaces meet, which is what is expected for any fluid surfactant film.
Deleurence, Rémi ; Saison, Tamar ; Lequeux François, Monteux, Cécile ;
Foaming of transient polymer hydrogels, to appear in ACS Omega, ao7b01301