Highly Structured Foams from Chitosan Gels

We demonstrate how to generate highly ordered, solidified foam structures from chitosan hydrogels using millifluidic cross-flow techniques In this, aqueous chitosan solutions (containing surfactant) and air are injected simultaneously at constant flow rates in a millimetric T-junction device, in which the gas thread breaks up to form extremely monodisperse bubbles These bubbles travel along a tube in which the cross-linker glyoxal is added at constant flow rate via another T-junction in order to start the gelification of the chitosan The bubbles are collected in appropriate devices, where they form hexagonally close-packed structures under the influence of gravity and/or confinement before the gelification process freezes the foam structure The successful application of this technique requires the optimization of foam stability in conjunction with an appropriate choice of gelification time and the viscosity of the solution, both being strongly dependent on the chitosan concentration Furthermore, reliable generation of monodisperse foams calls for a thorough calibration of the bubbling device for highly viscous and non-Newtonian fluids Both aspects are presented here, relying in particular on rheological investigations, along with some resulting foam structures