Foaming performance of aqueous albumin and mullite-albumin systems used in cellular ceramic processing

Bovine serum albumin (BSA) solutions (5, 10 and 15 vol%) and stabilized aqueous mullite (40 vol%)–BSA (5, 10 and 15 vol%) suspensions were foamed by mechanical stirring at 2300 rpm. The colloidal and rheological behaviors of suspensions were studied by measuring zeta potential and shear flow properties, respectively. The foaming performance of aqueous systems was evaluated by characterizing the foaming capacity and the foam stability by volume and electrical conductivity measurements. Overrun % was considered as a measure of the foaming capacity, while the variation of this parameter, the foam wetness (ΦFW) and the foam density stability (FDS %) with the stand time and the time to half collapse (t1/2) were evaluated in order to study the stability of foamed systems. Median bubble diameters, bubble size distribution widths and bubble shape factors as a function of time were determined by analyzing images captured by confocal laser-scanning microscopy (CLSM). The results related to the stability of foamed systems were used to evaluate the global destabilization mechanism of these systems, analyzing, in particular, the influence of the solid particles in the protein foam. The obtained results are useful for studying the shaping of cellular mullite green bodies from the thermogelation of foamed aqueous ceramic-protein suspensions.