Foamability and stability of foams obtained with silica/PEI gels

Abstract

In this article we examine the foamability and foam stability of a series of PEI/silica gel suspensions with varying particle concentrations in the presence of a neutral surfactant, C12E23. These suspensions, which were recently developed for 3D printing applications, exhibit a finite yield stress whose value can be tuned by varying the particle concentration and the pH. Our study aims at evaluating whether this system is suitable to produce stable foams that could be further be used as precursors for the 3D printing of porous materials and to evaluate the relation between the yield stress and the foam stability against drainage and ripening. At pH7 where all the suspensions are in the fluid state, the maximum air fraction reaches 80% however the foams are very unstable to drainage. At pH 10.5, the suspensions are in the gel state due to Van der Waals driven aggregation of the particles. In the gel state, the obtained maximum air fraction decreases with the suspension concentration, from ϕair=80% for the 12.8wt% suspension to ϕair=65% for the 15.5wt% suspension. The most concentrated suspension, i.e 19.7wt% could not be foamed. At pH 10.5, the yield stress of the 12.8wt% suspension is 10Pa, which enables to overcome the buoyancy acting on the particles and therefore arrest drainage. However the yield stress is too low to overcome the Laplace pressure in the bubbles, therefore Ostwald ripening is observed. The 15.5wt% suspension exhibits a yield stress of 200Pa, which is high enough to arrest drainage. Ostwald ripening occurs at early times and is arrested when the bubble radius reaches 400μm as the yield stress equals the Laplace pressure.The results obtained in this study show that there is an optimal silica concentration which combines good foamability and large stability to drainage and Ostwald ripening. These materials are promising for the development of 3D printable foams even though the size of the bubbles will limit the spatial resolution of the printing to a few millimeters.